0960ndash308503$2350+000 Institution of Chemical Engineers

wwwingentaselectcom=titles=09603085htm Trans IChemE Vol 81 Part C March 2003

AN OVERVIEW OF STERILIZATION METHODS FORPACKAGING MATERIALS USED IN ASEPTIC

PACKAGING SYSTEMS

MD I A ANSARI and A K DATTAAgricultural and Food Engineering Department Indian Institute of Technology Kharagpur West Bengal India

I n aseptic packaging systems packaging materials are sterilized by various methods inorder to kill microorganisms contained in the packages during forming and transportthrough the machine prior to lling Experimental data as well as theoretical results from

several years of research in the area of sterilization methods for effective inactivation ofmicroorganisms on surfaces of aseptic packaging materials are compiled and presented in orderto choose the right method of sterilization by the food processing industry for a successfuloperation Hydrogen peroxide with concentrations up to 30 temperatures of up to 80macrC andcontact times up to 15 s with or without wetting agent has been found to be successful for in-line aseptic packaging The nal product must not contain greater than 05 ppm H2O2Economic considerations and non-uniform dose delivery to pre-formed containers inhibitcommercial adoption of ionizing radiation sterilization in-line with aseptic packaging systems

Keywords aseptic packaging sterilization thermal processes radiation light-pulse chemicalmethods

INTRODUCTION

Packaging plays an important role in the food manufacturingprocess It makes food more convenient and gives the foodgreater safety assurance from microorganisms and biologi-cal and chemical changes such that the packed foods canhave longer shelf life In order to meet the huge demand ofprocessed food with longer shelf life various new methodsof packaging are being used in the food processing industryAseptic packaging is one of them Aseptic packaginginvolves the lling and sealing of microbiologically stable(ie commercially sterile) product into sterilized containersunder conditions that prevent microbial recontamination ofthe product the containers and their closures (ie underaseptic condition) It is a technology of continuous foodsterilization and packaging that is of considerable currentinterest to the US and European food industries A typical ow diagram is illustrated in Figure 1 In aseptic packagingraw or unprocessed product is heated sterilized by holdingat high temperature for a predetermined amount of timethen cooled and delivered to a packaging unit for packagingwhile packaging material and equipment surface may besterilized by various methods such as heat hydrogenperoxide irradiation infrared light etc and combinationsof methods Some of these methods are given in Table 1Aseptic packaging can be used to package a wide rangeof products such as milk juice drinks concentrates winetea mineral water nutritional beverages sauces dairy andtomato products Aseptic packaging offers advantages tothe consumer as well as to distribution channels (lowerdistribution and storage costs extended shelf life relief of

pressure on chilled cabinet cost effectiveness and freedomfrom additives)

Sterilization of packaging material is a critical step in theaseptic packaging system Therefore the sterilizationprocess should meet the following requirements forsterilization of packaging materials

deg rapid microbicidal activitydeg compatibility with surfaces treated especially packaging

material and equipmentdeg easily removed from surface minimum residuedeg present no health hazard to the consumerdeg no adverse effect on product quality in the case of

unavoidable residue or erroneous high concentrationdeg present no health hazard to operation personnel around

the packaging equipmentdeg compatibility with environmentdeg non-corrosive to surfaces treateddeg reliable and economical

When using sterilants that do not leave any residue on thefood contact surface the Food and Drug Administration(FDA) considers sterilization as a process which is regu-lated only when used on low acid foods (Code of FederalRegulations 1986 Title 21 Part 113) However whenplastic packaging materials and chemical sterilants areused the process is regulated as an indirect additive tofood (Code of Federal Regulations 1986 Title 21 Part174) The process of obtaining FDA approval for the use ofchemical sterilants in food packaging has eased consider-ably since the concept was introduced in the early 1970sScienti c evidence for process adequacy must be led by

57

processors before a system is placed into commercialoperation For successful operation of aseptic packagingsystems parameters for process adequacy can be effectivelydesigned A major concern in aseptic packaging of low-acidfoods (pH gt 46) is the assurance of microbial safety in the nal product which supports the growth of Clostridiumbotulinum

Hydrogen peroxide is now the only chemical sterilant forsterilization of packaging materials that has been proved tobe acceptable in the USA The FDA regulations specify thata maximum concentration of 35 H2O2 may be used forsterilizing food contact surfaces In a properly designedaseptic packaging system a good microbicidal effect using

hydrogen peroxide can be achieved and the level of residuecan be effectively controlled to within permissible limitsThe nal product must not contain greater than 05 ppmH2O2 Initially the use of hydrogen peroxide as a sterilantfor packaging material that directly contacts food wasapproved only for polyethylene [Federal Register 198146(6) 2341] The approval was extended to include allpolyole ns in March 1984 (Code of Federal Regulations1984 Title 21 Part 178 1005) and in 1985 approval wasextended to include polystyrene modi ed polystyreneionomeric resins ethylene methyl acrylate copolymerresin ethylene vinyl acetate copolymer resin and polyethy-lene tetrapthalate (Code of Federal Regulations 1986 Title21 Part 178 1005) In January 1987 approval was extendedto include ethylene acrylic acid copolymers [Food ChemicalNews 1987 28(44) 28] The rst successful aseptic llingsystem for cartoning the aseptic Tetra Pak of 1961 used acombination of hydrogen peroxide and heat for the steriliza-tion of the surface of container material (Burton 1988)

Development of acceptable sterilization methods suitablefor different types of packaging materials and consumerdemand for good quality microbiologically safe shelf-stablefood encouraged commercial adoption of aseptic packagingsystems The present work is a critical review of literature oncurrently used and new methods for in-line sterilization ofpackaging materials mentioned in the scienti c and patentliteratures The limited body of literature directly related tothe effects of sterilization methods on product quality andstorage stability also has been discussed

STERILIZATION METHODS FOR PACKAGINGMATERIALS AND EQUIPMENT

Various methods for the sterilization of packaging mate-rials are currently used in aseptic packaging systems Thesterilization process employed should be established interms of numbers of log cycle reductions of the mostresistant organisms Packaging material is usually sterilizedeither inside the packaging machine or externally andintroduced aseptically into the aseptic zone of the packagingmachine Many aseptic packaging systems utilize plastics

Figure 1 Schematic ow diagram of aseptic packaging system

Table 1 Methods for sterilizing aseptic packages

Methods Application Advantages=disadvantages Reference

Superheated steam Metal containers High temperature at atmosphericpressure Microorganisms are moreresistant than in saturated steam

Collier and Townsend (1956)

Dry hot air Metal or composite juiceand beverage containers

High temperature at atmosphericpressure Microorganisms are moreresistant than in saturated steam

Denny and Mathys (1975)

Hot hydrogen peroxide Plastic containerslaminated foil

Fast and ef cient method Denny et al (1974)

Hydrogen peroxide=UVlight combination

Plastic containers(pre-formed cartons)

UV increases effectiveness ofhydrogen peroxide

Bayliss and Waites (1982)

Ethylene oxide Glass and plastic containers Cannot be used where chloridesare present or where residualswould remain

Blake and Stumbo (1970)

Heat from co-extrusionprocess

Plastic containers No chemicals used mdash

Radiation Heat-sensitive plasticcontainers

Can be used to sterilize heat-sensitivepackaging materials ExpensiveProblems with location of radiationsource

mdash

58 ANSARI and DATTA

Trans IChemE Vol 81 Part C March 2003

paperboard or foil containers to pack the sterilized productand sterilize by physical methods and chemical reagent

Thermal Processes

Microorganism inactivation has traditionally been accom-plished by heating Microorganisms especially spores showgreater thermal resistance when exposed to dry heat thanmoist heat Spores of Bacillus stearothermophilus are extre-mely resistant to heat (Russell 1982) Spores of Bacillussubtilis ATCC 9372 are used as indicators in dry heat(Anonymous 1995) and these spores are used commerciallyin sterility testing of aseptic llers (eg the spore-strip kit ofNorth American Science Associates Northwood OHUSA) D-values (the time required to reduce the numberof microorganisms by one log cycle) and Z-values (thetemperature rise necessary to reduce the D-value by afactor of 10) are signi cantly higher for dry heat sterilizationthan for steam sterilization Dry heat produces microbialdeath as a result of dehydration followed by protein oxida-tion Death by moist heat is caused by denaturation andcoagulation of essential cell proteins (Parrott 1970) Reac-tion velocity for their destruction depends on how rapidlythe heat from the thermal carrier can be transferred to thecell When heat is used the nature of the surface must beconsidered Plastics or carton packaging with their lowconductivity are more dif cult to thermally sterilize thanare metal containers In addition plastic materials generallyhave a low thermal stability and can be permanentlydeformed by the time=temperature sequences necessary toachieve sterilization Thermal processes do not deposit anyhazardous or undesirable residues on the surface beingtreated and do not present environmental hazards

Saturated steamSaturated steam is a form of moist heat Sterilization of

metal cans and lids by saturated steam under pressure wasused as early as 1920 in the USA (Reuter 1988) and is usedtoday for sterilizing thermostable plastic cups To attainadequate destruction in the short time available in high-speed packaging units the surface temperature of the mate-rial must reach 135macrC The technological problem is toobtain a suf ciently high surface temperature to achievethe required sterility in a time consistent with high produc-tion rates and to avoid softening and deformation of thematerial Moulded polystyrene cups and foil lids aresubjected to saturated steam at 165macrC and 6 bars immediatelyafter deep drawing At the same time the external cup surfaceis cooled to limit the effect of the short time heat applicationon the material It should be noted that Bacillus subtilis is notgenerally recommended for validation of steam sterilization(Larousse and Brown 1997) Polypropylene cups that havehigher thermal tolerance can be treated on surfaces at lowertemperatures and longer times 140ndash147macrC for 4ndash6 s in apressure chamber Lid foil can be treated continuously bypassage through a pressure lock Moist heat could causeblistering or delamination of paper-based packagingmaterials and impair the heat-sealing characteristics ofplastics Atmospheric steam can only be used on non-paper-based pre-formed containers Saturated steam is apreferred treatment method for sterilizing metal food contactsurfaces downstream from the hold tube including sterile

hold tank homogenizers llers and the aseptic packagingzone Equipment sterilization can be attained by exposure ofthe surfaces to an appropriate time=temperature sequence(eg 30 min at a surface temperature of at least 121macrC) bysuperheated water saturated steam superheated steam orother appropriate treatments

Superheated steam and hot airDry heat delivered by superheated steam or hot air is not

as effective as moist heat at the same temperature forsterilization therefore higher temperatures are requiredThe MartinndashDole process continuously sterilizes tinplatecans by passage through 220ndash256macrC-superheated steams atnormal pressure for 45 s (Larousse and Brown 1997) Whenpaper-based packaging materials are used hot air ispreferred as a sterilant over superheated steam Combi-cans of cardboard laminates (ie aluminium and paper)are sterilized in hot air at 145macrC for 3 min (Reuter 1988)This method has only been found suitable for products thathave a pH of lt46

When sterilizing by hot air or superheated steam there isno effect on packaging material quality Furthermore thetemperature within the entire aseptic lling chamber may bemaintained at a level lethal to microorganisms thus assuringasepticity The high temperature of the headspace gas at thetime of sealing also reduces the level of oxygen in the sealedpackage and can contribute towards prolonging product shelflife Hot air sterilization units are commercially available forjuice and beverages Dry heat sterilization is slow and notsuitable for heat-sensitive materials like many plastic items

Heating by extrusionAseptic packaging system utilizing heat coextrusion

process for sterilization is a formndash llndashseal packagingsystem that relies on the temperatures reached by thermo-plastic resin during the co-extrusion process used toproduce multi-layer packaging material to produce a sterileproduct contact surface Temperatures of 180ndash230macrC can bereached during the extrusion of granulated plastics as apretreatment for a subsequent blow moulding operation Thehigh temperature produced by extrusion produces sterilesurfaces During production the multi-layer package mate-rial is fed into the machine where it is delaminated understerile conditions This removes a layer of material andexposes the sterile product contact surface The containermaterial is then thermoformed into cups The lid materialthat is also delaminated is then sealed onto the cup after lling The sterility of the forming lling and sealing areasis maintained by sterile air under positive pressures Reportsas to the ef cacy of this heating method (Reuter 1988) varyconsiderably Therefore aseptic lling into extrudedcontainers appears only suitable for acidic products withpH less than 46 For products with pH greater than 46extruded containers should be post-sterilized with hydrogenperoxide or with mixtures of peracetic acid It would beprudent to validate any application of this method on acase-by-case basis

Radiation

When neither heat nor chemicals can be used to sterilizea given material radiation is considered as an option

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STERILIZATION METHODS FOR PACKAGING MATERIALS 59

Electromagnetic radiation characterized by a frequency awavelength penetrating power and an energy range areinfrared ultraviolet g rays etc The dosage and type ofradiation sterilization is a function of the type and amount ofthe microbial load that has to be removed from a givenmaterials and equipment As irradiation does not leave anyresidue on the treated surface nor affect the immediateenvironment it appears at rst sight to be a perfect solutionfor sterilization of packaging material The products usedfor packaging in radiation sterilization recommended by theInternational Atomic Energy Agency (IAEA) are polyethy-lene polyester polypropylene nylon PVC etc Irradiationmay indirectly affect chemical or biological propertiesleading to interaction with the packaged product changingits ef cacy or usability

UV rayUV irradiation has been successfully applied to the

sterilization of air and of water which is free from suspendedmatter With the development of more powerful sources ofUV radiation (Bachmann 1975) attempts have been madeto use it for surface sterilization The microbicidal effects ofUV rays lies in the range of 200ndash315 nm with an optimaleffect being between 250 and 280 nm the so-called UV-Crange UV light at a wavelength of 2537 nm is an effectivegermicide against yeast moulds bacteria viruses and algaeThe lethal irradiation dose is de ned as the product ofirradiation intensity and time and expressed in terms ofmilli Watts second per square centimeter (mW s cmiexcl2)High intensity sources of UV radiation are very effectivein killing unprotected microorganisms suspended in air oron the surface of materials and products Data published byBachmann (1975) on dose levels required to inactivatespores of moulds and bacteria indicated excellent potentialof the process for sterilizing aseptic packaging materialsMaunder (1977) suggested the use of high-intensity UV foraseptic packaging material sterilization Both vegetativecells and spores are sensitive to UV radiation Microbialresistance to UV radiation increases in the order vegetativebacterial cells frac12 yeasts ltbacterial spores lt mould sporesGram-positive bacteria require twice the dosage of Gram-negative bacteria and 5ndash10 times for bacterial sporesMould spores particularly those that have dark colours(eg Aspergillus niger) are especially resistant and requiredosage levels 20ndash100 times higher (Wallhauser 1978)Covered shaded or shielded organisms will not be affectedDust particles present on the surfaces reduce the effective-ness of UV irradiation for sterilization of aseptic packagingmaterials The penetration ability of UV rays is very limitedas shown in Table 2

A typical UV radiation intensity is between 05 and15 W cmiexcl2 at a distance of 10 cm Laboratory tests using

an energy density of 30 mW cmiexcl2 on arti cially contami-nated smooth even dust-free surfaces and an irradiationtime of 4ndash6 s showed a 4ndash5 log cycle reduction(Cerny 1977) Kodera (1983) obtained a patent for boththe method (UV light plus organic acid or hot water) andapparatus for sterilizing packaging lm The reportedadvantage of this method is that organic acids and waterare not toxic UV irradiation and a citric acid solutionresulted in approximately a six-log cycle inactivation ofBacillus subtilis spores within 5 s Doyen (1973) describedan aseptic packaging machine that used both an alcohol bathand high intensity UV radiation for sterilization of exibleplastic pouches The lm was washed in 95 ethanol for20 s and then passed under a ow of 10 mW cmiexcl2 of UV for30 s The rate of microbial reduction in the sterilization ofsmooth surface plastic cups and lids by UV irradiation at aconstant radiator output depends upon a number of factors(Cerny 1977) UV irradiation under practical conditionsonly always count reduction of 2ndash3 log and mould sporescan be particularly resistant This low count reduction isinsuf cient for aseptic packaging the process is thereforemerely suitable for a lling and packaging process with arelatively low contamination count

Combination of hydrogen peroxide with UVThe ability of UV and hydrogen peroxide to inactivate

bacteria has been well known for a long time The process ofsimultaneously applying hydrogen peroxide solution andUV will provide adequate sporicidal activity to relativelylow concentrations of hydrogen peroxide solution minimiz-ing the problem of removal of residues after sterilizationThe Liquid-Pak System uses this effect with a cartonforming lling and closing system in a sterile chamberMany researchers have considered sterilization of packagingmaterials using peroxide and UV irradiation It has beenfound that the lethal action of peroxide solutions with andwithout the application of heat is increased by simultaneousUV irradiation and that the overall lethal effect is greaterthan the sum of the effects of the peroxide and irradiationalone (Bayliss and Waites 1979a 1982) They subsequentlydemonstrated the same type of synergy with a variety ofBacillus and Clostridium (Bayliss and Waites 1979b) butfound that mild heat was also needed for effective killing ofmore resistant spores tested Higher count reduction can beobtained by prior treatment of the surface with coldhydrogen peroxide and by ensuring that the surface is dustfree (Bernard 1983 Cerny 1977 Ito and Stevenson 1984Wainess 1982) The effect is optimum at a relatively lowperoxide concentration between 05 and 5 and at higherconcentration the peroxide appears to have a protectiveeffect and the greater the UV intensity the higher is theoptimum peroxide concentration Four decimal reductionsof Bacillus subtilis spores of a strain very resistant to H2O2

were easily obtained by irradiation with an UV dose of18 W cmiexcl2 in 25 peroxide followed by a short heattreatment (Bayliss and Waites 1982) Subsequent heatingto 80macrC increased the kill further Maunder (1977) used aconventional UV lamp to sterilize pouches also with theadvent of high intensity UV-C lamp (Brandli 1975) veryhigh lethal effects have been reported up to 5 decimalreductions on at board (Cerny 1977) and 7 decimalreduction for Bacillus spores on inoculated strips placedinto food packaging cartons (Sturm and Gilliand 1974)

Table 2 Penetration of UV light

Substrate Distance that UV will penetrate (cm)

Air 300ndash500Water 30Window glass lt01Transparent plastic foil lt001Fruit juice 01Fluid milk 001

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60 ANSARI and DATTA

Decontamination of pre-formed cartons for aseptic packa-ging poses problems which are not encountered with atsurfaces Therefore UV irradiation of a pre-formed cartonmust be considerably more intense than for at boards toensure that all surfaces are effectively irradiated (Bachmann1975) Experiments with cartons arti cially contaminatedwith Bacillus subtilis spores sprayed with 1 H2O2 andthen irradiated for 10 s with a high-intensity UV sourceabove the carton showed 5 decimal reductions with poly-ethylene lined material and 35 decimal reductions with apolyethylene=aluminium foil laminate (Stannard et al1983) No heat was applied This combination of hydrogenperoxide with UV has now been applied commercially tocarton sterilization during aseptic lling

Infrared rayInfrared rays in the waveband 08ndash15 pound 10iexcl6 are easily

generated On an absorbent surface they are converted intosensible heat producing an increased temperature Theenergy impact on the surface that is convertible into heatis dependent on the same geometric conditions as thoseoutlined for UV rays Thus infrared irradiation is bestapplied to a smooth even surface with vertical radiationincline This method has been used to treat the interior ofaluminium lids with a plastic coating on the exterior surfaceAs the temperature rise can cause a softening of the plasticthe maximum temperature and exposure time are limitedCount reductions on the same order as those for UVirradiation has been found

Ionizing rayIonizing radiation is an excellent sterilizing agent and

penetrates deep into objects Ionizing radiation of primaryinterest includes a-rays X-rays b-rays electron beams andg-rays Bags used in bag-in-box aseptic packaging arecurrently sterilized with g-rays from a 60Co source Radia-tion kills bacteria in the same logarithmic fashion as heatand D-values for many organisms have been reported Adose of 47 Mrad is considered a 12D level for Clostridiumbotulinum which are the most radiation-resistant spores ofpublic health and spoilage signi cance (Urbain 1978) Theeffect of water activity on irradiation by 60Co g-rays wasinvestigated by Harmulv and Snygg (1973) Dosage levelsto inactivate 90 of Bacillus subtilis spores increased from023 to 042 Mrad as the water activity (aw) decreased from100 to 000 For a 999 reduction (3 log) of Bacillusstearothermophilus spores the dosage increased from 17Mrad at an aw of 100 to 300 Mrad at 000

Gamma irradiation has been used to sterilize plastic bagsfor bulk packaging acid foods using an aseptic bag-in-boxsystem (Nelson 1984) The bag made of plastic laminatesare treated in a specialized irradiation plant and given aradiation dose of 25 kGy (25 Mrad) or more which issuf cient to ensure sterility However the dif culty ofmaintaining sterility during the transfer operation to the lling spout of an aseptic lling machine raises somedoubts as to the practicality of gamma irradiation sterilizedpackaging materials for use on low-acid foods The heavyshielding necessary to prevent radiation leakage fromgamma ray source makes the system impractical to use forin-line sterilization Gamma radiation has not yet beenwidely employed in the USA because of the cost andconcerns about the effects of g-radiation on food

Electron beam sterilization of packaging material is aproven state-of-the art technology which outperforms anylegislative and customer requirements Studies have shownthat depending on the level of microorganisms a dose of5ndash7 kGy was found to be effective against yeast mould andspores and no viable microorganism was found after treat-ment Unfavourable effects of electron beam treatment werenot observed High-energy electron beams are not as pene-trating as g-rays and therefore do not require as muchshielding as a g-ray sources The irradiation process takesjust a few seconds and does not leave any type of residueHowever with pre-formed containers the geometry of thedelivery system makes it dif cult to apply a uniform dose onthe whole container In-line processing heavily demandedby industry requires a careful process design Technologicaland economic optimization of electron beam source andshielding are the most important tasks to be accomplishedbefore in-line electron decontamination of food packagingmay be applied on a large scale A major impediment toadoption of this technology in the food industry is the highinitial and operating cost of the units

Care must be used when using ionizing radiation forsterilization of aseptic packaging materials Although alumi-nium foil is not affected by irradiation paper and plasticcould be affected with results ranging from discolourationto loss of pliability and mechanical strength While radiationcan be quite effective the equipment is expensive as is theprotection for workers and the environment Workers are notusually willing to work in such an environment It willprobably be more extensively employed in the future

Light-pulseThe technology of utilizing short pulses of light is an

attractive alternative for sterilizing packaging materials andprocessing equipment in aseptic packaging The spectrum oflight used for sterilization purposes includes wavelengths inthe ultraviolet to wavelengths in the near-infrared regionThe material to be sterilized is exposed to at least one pulseof light having an energy density in the range of about 001ndash50 J cmiexcl2 at the surface The duration (Anonymous 1994)of pulses ranges from 1 ms to 01 s The packaging materialsare typically exposed to between 1 and 20 pulses of light-intensity short-duration light A few ashes applied at arate of 1ndash10 per second provide very high microbial killlevels making the system ideal for continuous in-linesterilization It is desirable for certain aseptic processesthat packaging material be treated with pulses having arelatively high UV content to minimize the total uence(J cmiexcl2) necessary to achieve the desired reduction inmicrobial population (Dunn et al 1991) Comparison ofthe antimicrobial effects obtained using pulsed light withthose obtained using non-pulsed or continuous waveconventional UV sources shows a signi cantly higher inac-tivation for pulsed light More than 7 log cycles of Asper-gillus niger spore inactivation result with few pulsed light ashes A variety of microorganisms including Bacillussubtilis are inactivated by using between 1 and 35 pulsesof light with intensity ranging between about 1 and12 J cmiexcl2 Greater inactivation is obtained when full-spectrum light rather than glass- ltered spectrum light isused Thus the UV component of light is needed to inacti-vate microorganisms (Dunn et al 1991) Spores of Bacillus

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STERILIZATION METHODS FOR PACKAGING MATERIALS 61

subtilis Bacillus pumilus Bacillus stearothermophilus andAspergillus niger were inactivated completely from 6ndash8 logsof colony forming units (CFU) with 1ndash3 pulses (Bushnellet al 1998) Bacillus subtilis for example is sterilized(99999) by about 42600mW s cmiexcl2 of UV while requir-ing a dose of only 4500mW s cmiexcl2 under pulsed lightMicroorganisms are inactivated by a combination of photo-chemical and photothermal mechanisms

Light pulses apparently do not affect the nutrient retentionin food although a detailed study is not available Pulsedlight treatment costs are very favourable (Dunn et al 1995)Packaging materials compatible with this process will trans-mit light over the broad spectrum employed LLDPELDPE nylon Aclar HDPE and PP have all been usedPackage geometry should not allow any shadowing on theproduct or the light exposure may not be suf cient

Chemical Methods

Thermal destruction is by far the most common andef cient mode of sterilization Dry or moist heat for ster-ilization is also not always feasible Chemical methods use awide variety of chemicals in the form of liquids and gases todisinfect and sterilize equipment and packaging materials

Hydrogen peroxideHydrogen peroxide is one of the most widely used

sterilants used for sterilizing packaging materials The rstsuccessful aseptic lling system for cartoning the asepticTetra Pak of 1961 used a combination of hydrogen peroxideand heat for the sterilization of the surface of containermaterial (Burton 1988) Many aseptic packaging systemsuse hydrogen peroxide at concentrations from 30 to 35 asa sterilant for packaging materials and other food contactsurfaces followed by hot air (60ndash125macrC) to augment thesterilizing effect and to dissipate residual hydrogen perox-ide Sterilizing performance increases with both peroxideconcentration and temperature At ambient environmentaltemperatures the activity of hydrogen peroxide is relativelyslow however this can be increased considerably by raisingthe temperature to 85ndash90macrC and=or increasing concentration(Smith and Brown 1980 Swartling and Lindgren 1968)Therefore for effective sporicidal action in food processingenvironment treated with hydrogen peroxide (at 30) isfollowed by application of hot air (Yokoyama 1990)Swartling and Lindgren (1968) found that four decimalreductions were obtained after suspension in 20 at 80macrCfor 15 s Ito and Stevenson (1984) state that the challengeorganisms of choice are Bacillus stearothermophilus foraseptic packaging systems that use heat sterilization andstrains of Bacillus subtilis A or Bacillus subtilis var globigiifor ller surfaces that sterilize with hydrogen peroxide andheated air

The following relationship holds for destruction of sporesof Bacillusstearothermophilus(Shapton and Shapton 1998)

log D ˆ 17 iexcl 13 curren log ( H2O2)

where D is the decimal reduction time in minutesSpores resistant to hydrogen peroxide have been reviewed

by Ito (1973) and Stevenson and Shafer (1983) Toledo et al(1973) reported that dry spores of Bacillus subtilis varglobigii (NCIB 8058) were less resistant than the wet

spores Consistent with observations for heat and irradi-ation dry spores are more resistant than wet spores Sincesterilization processes used on aseptic packaging systemsfor low-acid foods must have documentation for adequacyto inactivate pathogenic and spoilage microorganisms a unitmust be tested using an acceptably resistant organism beforeit can be placed in commercial production In commercialaseptic packaging equipment which uses roll stock thepackaging material is immersed in hydrogen peroxidesolution followed by heating to vaporize the peroxidebefore the packages are lled Contact time with the solu-tions which contain a wetting agent is often less than 1 minA large amount of the sterilizing liquid is removed mechani-cally eg by rollers or air blasts and the remainder isgenerally removed by drying with hot or sterile air or radiantheat The nascent oxygen thus produced due to breakdownof peroxide reacts with the oxidizable cell components ofthe microorganisms and causes bactericidal effect Commer-cial packaging systems employ hydrogen peroxide atconcentrations of 10ndash35 at either room or elevatedtemperature (Von Bockelmann 1974 Von Bockelmannand Von Bockelmann 1972) in the form of a spray orimmersion bath The sporicidal action of hydrogen peroxidealso has been reviewed by Von Bockelmann and VonBockelman (1972) Hydrogen peroxide kills a wide varietyof organisms (Wang and Toledo 1986) and viruii (Mentaland Schmidt 1973) Both linear (Swartling and Lindgren1968 Toledo et al 1973) and nonlinear (Cerf and Hermier1972) relationships between log number of survivors andtime of exposure to different concentrations of hydrogenperoxide have been reported and Toledo et alrsquos (1973) datashow that 1 min in 30 hydrogen peroxide at ambienttemperature produced only 25 decimal reductions ofmould spores and bacterial spores were not affected at allThus in current commercial practice the microbicidalaction is primarily due to the heated lm of hydrogenperoxide at the start of the drying phase of the sterilizationprocess For this reason wetting of the packaging materialand the presence of a uniform lm of liquid on the materialsurface are critical factors

When sterilizing pre-formed containers hydrogenperoxide is sprayed or atomized into the container Ameasured amount of hydrogen peroxide is metred intoeach nozzle which delivers the solution into each containerto ensure that a uniform lm coats the inside surface of thepackage For acid food products relatively low microbiallethality required makes it easy to achieve the desired levelof microbial inactivation and meet the residue tolerancelevel However for low-acid foods a system must becarefully designed such that both criteria of commercialsterility and adequate sterilant removal are achieved

Hydrogen peroxide-sterilized aseptic packaging systemsthat have been approved for commercial packaging of low-acid foods have been proven to produce an acceptable levelof residues The Food and Drug administration of the USAhas ruled that the level of hydrogen peroxide that may bepresent in product packaged in material sterilized byhydrogen peroxide must not be greater than 05 ppm(Reuter 1988) An improperly designed system may resultin more incidence of non-compliance with regulationspertaining to residues Prolonged contact between liquidhydrogen peroxide and the packaging material alsoincreased the dif culty of removing residues

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62 ANSARI and DATTA

Hydrogen peroxide is frequently used to sterilize thepackaging material as it decomposes to a product whichwill neither taint the food nor render it toxic It is reportedthat the traces of residual of hydrogen peroxide left in foodis less than 025ppm and normally this amount does nothave any harmful effect when consumed (Hedrick 1973Smith and Brown 1980) Residual hydrogen peroxide andthat trapped in the package headspace at the time of sealinghas an adverse effect on product stability particularly onascorbic acid degradation in fruit juices (Toledo 1986)When the concentration of hydrogen peroxide at the time ofpackaging exceeded 01 mg=mliexcl1 a marked increase inascorbic acid degradation in bottled orange juice andorange juice concentrates was observed by Toledo (19751986) Aseptic packaging of foods containing ascorbic acidin a system that uses hydrogen peroxide for sterilization ofmaterial is not desirable unless some means is used toremove hydrogen peroxide vapours from the package headspace and packaging material is completely free fromhydrogen peroxide residues

There are three commercial methods for applyinghydrogen peroxide to packaging materials dippingspraying and rinsing

Dipping the packaging material (ie plastic laminateswith cardboard lms of thermoformable plastics and lami-nates) are taken from a reel and dipped into a bath ofaqueous 30ndash33 hydrogen peroxide (Reuter 1988) Countreductions on the order of 4ndash5 logs are claimed Wettingagents are added to ensure uniform wetting of the surfacesExcess solution is removed by squeeze rolls or air jets afterremoval of the material from the bath which leaves a thin lm of solution that is then dried by the application of hotair To increase the ef cacy especially in the case of dusty orslightly soiled material prior treatment of the material withrotating brushes sterile compressed air jets or ultrasoundapplied to the bath may be added

Spraying this is the method (Reuter 1988) for preformedcontainers in which the hydrogen peroxide is sprayed in theform of small dispersed droplets into the container Sprayingdoes not result in a cohesive lm due to the hydrophobiccharacteristics of plastics rather only 30ndash40 of the innersurfaces of the container are covered and thereby it does notensure good sterility of lm A conventional spray givesdrops of over 30 mm diameters on the surface and only30ndash40 of the surface area is covered An ultrasonic systemcan be used to give particle sizes of only 3 mm diameterwhich will give an average surface cover of about 60The droplet size distribution between 2 and 80 mm and thedistribution of droplets on the inner surfaces of the variousgeometric shapes of the containers is problematical Theef cacy is dependent upon the volume of solution sprayedhowever the larger the volume the longer the drying timeThe drying must be carried out with hot sterile air Improvedaerosol sprayers can limit the droplet size to 2ndash4 mm andincrease the coverage to 60 while permitting reducedvolume of peroxide solution to be applied hence thedrying time This method is being replaced by the use ofa mixture of hot air and vapourized peroxide Sterilizationby hydrogen peroxide vapour would be a cost-effectivealternative as the least amount of hydrogen peroxide isused Furthermore the amount of hydrogen peroxideadsorbed on the treated surface from the vapour phase willbe several orders of magnitude smaller than a liquid

lm Therefore ushing the vapour-treated surface withlow-temperature sterile air free of hydrogen peroxidevapours can effectively eliminate residues

Low levels of hydrogen peroxide which remain in thevapour phase in air at near ambient temperature has beenshown to have sporicidal activity suitable for in-line ster-ilization of aseptic packaging materials (Wang and Toledo1986) Air saturated with hydrogen peroxide vapour carryrelatively low concentration of hydrogen peroxide(76 mg liexcl1 at 70macrC) but can induce 6 decimal reductionsof spores of Bacillus subtilis var niger in 12 min Incomparison the same organism requires 12 min to achieveone decimal reduction in hot air alone at 150macrC Since theamount of hydrogen peroxide vapourized from solution isless than that from water the solution is enriched duringprolonged operation and a system must be devised to addmake-up water to maintain the desired concentration

Rinsing prefabricated intricately shaped containers forwhich the spraying process is unsuitablecan be rinsed with anaqueous 30ndash33 H2O2 solution For ambient temperaturesterilization this can be combined with peracetic acid Thecontainers are drained allowed to drip dry and then com-pletely dried by hot sterile air Glass containers metal cansand blow-moulded plastic bottles are treated in this manner

Ethylene oxideImplementation of ethylene oxide sterilization has made

possible the use of sterile low-cost disposable thermoplas-tic devices for industrial application It has been used to pre-sterilize paperboard cartons (Hedrick 1973) and plasticpackaging materials (Alguire 1973) However no commer-cial system uses ethylene oxide as a sterilizing agent inaseptic packaging of low acid foods for ambient temperaturestorage and distribution The vapour form is ammable andexplosive (Merck 1989) and hence employed as a mixturewith an inert gas such as dichlodi uoromethane Alguire(1973) reported that 100 ethylene oxide has better micro-bicidal properties than an ethylene oxide inert gas mixtureAn important consideration in bulk sterilization of pre-formed packaging materials is ethylene oxidersquos ability topermeate and contact all surfaces that need to be sterilizedThe use of lower concentration of ethylene oxide is saferthan use of 100 ethylene oxide from standpoint ofpotential for injury to personnel The D-value for Clostri-dium botulinum 62 A require a minimum of 72 min for a12D process thus making it impractical to use ethyleneoxide for in-line sterilization of aseptic packaging materialsin high-speed packaging lines Ethylene oxide is effectiveagainst bacteria moulds yeasts as well as viruses The mostprobable activity is alkylation of nucleic acids and othercellular components (Phillips 1977) It is a particularlyeffective sterilizing agent because it rapidly penetratespackaging material The ethylene oxide concentrationhumidity and temperature in uence the rate of sterilizationA clean object can be sterilized if treated for 5ndash8 h at 38macrCor 3ndash4 h at 54macrC when the relative humidity is maintained at40ndash50 and the EtO concentration at 700 mg liexcl1 Extensiveaeration of the sterilized materials is necessary to removeresidual EtO because it is so toxic However one disadvan-tage of it is its comparatively slow rate of action againstmicroorganisms Spores of Bacillus subtilis var niger ATCC9372 are used as indicators in dry heat and ethylene oxidesterilization (Anonymous 1995 1999)

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STERILIZATION METHODS FOR PACKAGING MATERIALS 63

Other sterilization methodsThere are many other chemicals such as peracetic acid

beta propiolactone alcohol chlorine and its oxide andozone etc that have been suggested as having potentialfor use in sterilizing aseptic packaging materials Peraceticacid is a peroxide of acetic acid and on decomposition itforms acetic acid and water The low pH and oxidizingproperties make it an excellent sporicidal agent It has all theadvantages of hydrogen peroxide and is unaffected bycatalase and peroxidases Peracetic acid is a liquid sterilantwhich is particularly effective against spores of aerobic andanaerobic bacteria and is effective at lower temperaturesthan hydrogen peroxide In practice it is used in a solutionwhich also contains hydrogen peroxide The solutioncontaining peracetic acid and hydrogen peroxide is effectiveagainst resistant bacterial spores even at 20macrC for example a1 solution will eliminate 10iexcl7ndash10iexcl8 of most resistantspore stains in 5 min at 20macrC and the most resistant strainsin 60 min The maximum usable temperature is 40macrC whenthe sterilization times are about 5 times shorter In spite ofits sporicidal properties peracetic acid is not an approvedsterilant for use on aseptic packaging materials Its vapour isvery pungent and irritating Therefore a system that utilizesthis compound must be airtight to prevent environmentalrelease Unlike hydrogen peroxide vapours for which atolerance is allowed inside sealed packages no such toler-ance is allowed for peracetic acid and vapours in the head-space can cause disagreeable vinegar like off- avour insome food products In spite of these problems thiscompound deserves attention as a possible sterilant foraseptic packaging materials because of its effectiveness asa sporicidal agent The presence of small amounts of aceticacid may not be considered off- avours At ambienttemperature and halogen concentration of 100 ppm expo-sure times for effective sterilization would be on the order ofhours Under these conditions it would be impractical to usethese compounds as in-line sterilants in aseptic packagingOne major problem with the use of strong chlorine prepara-tions for sterilization is that available chlorine reacts withorganic material Reaction between chlorine and the materialbeing sterilized may produce off- avours

Beta propiolactone is bactericidal sporicidal fungicidaland virucidal It lacks the penetrating power of the ethyleneoxide but it is considerably more active against microorgan-isms Only 2ndash5 mg of beta propiolactone per litre is requiredfor sterilization compared with 400ndash800 mg of ethyleneoxide per litre However its poor penetration of materialsand its cancer-inducing properties have restricted its use as apractical sterilizing agent

Ethanol is the only industrially signi cant alcohol used asbactericide and fungicide but not for sporicide Theirmechanism of action appears to involve protein denaturationand membrane lipids dissolution Ethanol liquid and vapourhave been suggested (Doyen 1973 Mita 1985) Howeverethanol is effective only against vegetative cells and notagainst fungal conidia or bacterial spores therefore its use inpackaging of foods is limited to extension of shelf lifeof packaged foods which are normally stored underrefrigeration The two most popular alcohol germicides areethanol and isopropanol usually used in about 70ndash80concentration

Chlorine and its oxide and ozone are used mainly forwater disinfection Ozone can be used as a sterilizing agent

because of its high oxidizing capability Death of almost allmicroorganisms usually occurs within 30 min Ozone wasrecommended recently as an alternative to chlorine (Kim1998) and hydrogen peroxide (Khadre and Yousef 2001)They found that hydrogen peroxide at 10000-fold higherconcentration was less effective than ozone against Bacillusspores Antimicrobial power of ozone increases as tempera-ture decreases below ambient (Herbold et al 1989)Comparatively low concentration is needed to eliminatelarge population of spores at ambient temperature in shorttime period and this makes ozone best suited for anindustrial setting

CONCLUSIONS

Sterilization of packaging materials in-line of processingposes certain dif culties Based on the review presented it isevident that hydrogen peroxide sterilization followed by hotair appears to have the most potential for use as in-linesterilants for packaging materials This combination is time-tested and no adverse report seems to have put any doubtover the methodrsquos acceptability Sterilization of packagingmaterials using hydrogen peroxide and followed by ultra-violet irradiation also has been accepted for industrialapplication Dry heat saturated steam and superheatedsteam can be effective sterilants but the degree of heatdamages many packaging materials so they nd limitedapplication Infrared rays cannot be applied as there is atemperature rise of the packaging material due to infraredapplication and this results in softening of the plasticsIonizing rays are not accepted as they have harmful effecton personnel Light pulses have not been adequately studiedso far as their effect on food material is concerned Ethyleneoxide requires a very long time which precludes its use forin-line applications Peracetic acid produces an off- avourin food if residual deposit is enclosed in the container Betapropiolactone lacks the penetrating power of ethylene oxideand ethanol is not effective against spores

REFERENCES

Alguire DE 1973 Ethylene oxide gas sterilization of packaging materialsFood Technol 27(9) 64

Anonymous 1994 The Pure Bright Process Pure Pulse ProprietaryInformation

Anonymous 1995 Biological indicators for dry heat sterilization controland ethylene oxide sterilization control in United States Pharmaco-peia=National Formulary (US Pharmacopeial Convention RockvilleMD) pp 200 202

Anonymous AE 1999 Biological indicators of sterilization in BritishPharmacopeia Vol II (British Pharmacopeia Commission London) ppA281ndashA283

Bachmann R 1975 Sterilization by intense ultraviolet radiation BrownBoveri Rev 62 206ndash209

Bayliss CE and Waites WM 1979a The synergistic killing of spores ofBacillius subtilis by hydrogen peroxide and ultraviolet irradiation FEMSMicrobiol Lett 5 331ndash333

Bayliss CE and Waites WM 1979b The combine effect of hydrogenperoxide and ultraviolet irradiation on bacterial spores J Appl Bacteriol47(2) 263ndash269

Bayliss CE and Waites WM 1982 Effect of simultaneous high intensityultraviolet irradiation and hydrogen peroxide on bacterial spores J FoodTechnol 17 467ndash470

Bernard D 1983 Microbiological considerations of testing asepticprocessing and packaging systems in Processing of National FoodProcessors Association Conference Capitalizing on Aseptic (FPIWashington DC) pp 13ndash14

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64 ANSARI and DATTA

paperboard or foil containers to pack the sterilized productand sterilize by physical methods and chemical reagent

Thermal Processes

Microorganism inactivation has traditionally been accom-plished by heating Microorganisms especially spores showgreater thermal resistance when exposed to dry heat thanmoist heat Spores of Bacillus stearothermophilus are extre-mely resistant to heat (Russell 1982) Spores of Bacillussubtilis ATCC 9372 are used as indicators in dry heat(Anonymous 1995) and these spores are used commerciallyin sterility testing of aseptic llers (eg the spore-strip kit ofNorth American Science Associates Northwood OHUSA) D-values (the time required to reduce the numberof microorganisms by one log cycle) and Z-values (thetemperature rise necessary to reduce the D-value by afactor of 10) are signi cantly higher for dry heat sterilizationthan for steam sterilization Dry heat produces microbialdeath as a result of dehydration followed by protein oxida-tion Death by moist heat is caused by denaturation andcoagulation of essential cell proteins (Parrott 1970) Reac-tion velocity for their destruction depends on how rapidlythe heat from the thermal carrier can be transferred to thecell When heat is used the nature of the surface must beconsidered Plastics or carton packaging with their lowconductivity are more dif cult to thermally sterilize thanare metal containers In addition plastic materials generallyhave a low thermal stability and can be permanentlydeformed by the time=temperature sequences necessary toachieve sterilization Thermal processes do not deposit anyhazardous or undesirable residues on the surface beingtreated and do not present environmental hazards

Saturated steamSaturated steam is a form of moist heat Sterilization of

metal cans and lids by saturated steam under pressure wasused as early as 1920 in the USA (Reuter 1988) and is usedtoday for sterilizing thermostable plastic cups To attainadequate destruction in the short time available in high-speed packaging units the surface temperature of the mate-rial must reach 135macrC The technological problem is toobtain a suf ciently high surface temperature to achievethe required sterility in a time consistent with high produc-tion rates and to avoid softening and deformation of thematerial Moulded polystyrene cups and foil lids aresubjected to saturated steam at 165macrC and 6 bars immediatelyafter deep drawing At the same time the external cup surfaceis cooled to limit the effect of the short time heat applicationon the material It should be noted that Bacillus subtilis is notgenerally recommended for validation of steam sterilization(Larousse and Brown 1997) Polypropylene cups that havehigher thermal tolerance can be treated on surfaces at lowertemperatures and longer times 140ndash147macrC for 4ndash6 s in apressure chamber Lid foil can be treated continuously bypassage through a pressure lock Moist heat could causeblistering or delamination of paper-based packagingmaterials and impair the heat-sealing characteristics ofplastics Atmospheric steam can only be used on non-paper-based pre-formed containers Saturated steam is apreferred treatment method for sterilizing metal food contactsurfaces downstream from the hold tube including sterile

hold tank homogenizers llers and the aseptic packagingzone Equipment sterilization can be attained by exposure ofthe surfaces to an appropriate time=temperature sequence(eg 30 min at a surface temperature of at least 121macrC) bysuperheated water saturated steam superheated steam orother appropriate treatments

Superheated steam and hot airDry heat delivered by superheated steam or hot air is not

as effective as moist heat at the same temperature forsterilization therefore higher temperatures are requiredThe MartinndashDole process continuously sterilizes tinplatecans by passage through 220ndash256macrC-superheated steams atnormal pressure for 45 s (Larousse and Brown 1997) Whenpaper-based packaging materials are used hot air ispreferred as a sterilant over superheated steam Combi-cans of cardboard laminates (ie aluminium and paper)are sterilized in hot air at 145macrC for 3 min (Reuter 1988)This method has only been found suitable for products thathave a pH of lt46

When sterilizing by hot air or superheated steam there isno effect on packaging material quality Furthermore thetemperature within the entire aseptic lling chamber may bemaintained at a level lethal to microorganisms thus assuringasepticity The high temperature of the headspace gas at thetime of sealing also reduces the level of oxygen in the sealedpackage and can contribute towards prolonging product shelflife Hot air sterilization units are commercially available forjuice and beverages Dry heat sterilization is slow and notsuitable for heat-sensitive materials like many plastic items

Heating by extrusionAseptic packaging system utilizing heat coextrusion

process for sterilization is a formndash llndashseal packagingsystem that relies on the temperatures reached by thermo-plastic resin during the co-extrusion process used toproduce multi-layer packaging material to produce a sterileproduct contact surface Temperatures of 180ndash230macrC can bereached during the extrusion of granulated plastics as apretreatment for a subsequent blow moulding operation Thehigh temperature produced by extrusion produces sterilesurfaces During production the multi-layer package mate-rial is fed into the machine where it is delaminated understerile conditions This removes a layer of material andexposes the sterile product contact surface The containermaterial is then thermoformed into cups The lid materialthat is also delaminated is then sealed onto the cup after lling The sterility of the forming lling and sealing areasis maintained by sterile air under positive pressures Reportsas to the ef cacy of this heating method (Reuter 1988) varyconsiderably Therefore aseptic lling into extrudedcontainers appears only suitable for acidic products withpH less than 46 For products with pH greater than 46extruded containers should be post-sterilized with hydrogenperoxide or with mixtures of peracetic acid It would beprudent to validate any application of this method on acase-by-case basis

Radiation

When neither heat nor chemicals can be used to sterilizea given material radiation is considered as an option

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STERILIZATION METHODS FOR PACKAGING MATERIALS 59

Electromagnetic radiation characterized by a frequency awavelength penetrating power and an energy range areinfrared ultraviolet g rays etc The dosage and type ofradiation sterilization is a function of the type and amount ofthe microbial load that has to be removed from a givenmaterials and equipment As irradiation does not leave anyresidue on the treated surface nor affect the immediateenvironment it appears at rst sight to be a perfect solutionfor sterilization of packaging material The products usedfor packaging in radiation sterilization recommended by theInternational Atomic Energy Agency (IAEA) are polyethy-lene polyester polypropylene nylon PVC etc Irradiationmay indirectly affect chemical or biological propertiesleading to interaction with the packaged product changingits ef cacy or usability

UV rayUV irradiation has been successfully applied to the

sterilization of air and of water which is free from suspendedmatter With the development of more powerful sources ofUV radiation (Bachmann 1975) attempts have been madeto use it for surface sterilization The microbicidal effects ofUV rays lies in the range of 200ndash315 nm with an optimaleffect being between 250 and 280 nm the so-called UV-Crange UV light at a wavelength of 2537 nm is an effectivegermicide against yeast moulds bacteria viruses and algaeThe lethal irradiation dose is de ned as the product ofirradiation intensity and time and expressed in terms ofmilli Watts second per square centimeter (mW s cmiexcl2)High intensity sources of UV radiation are very effectivein killing unprotected microorganisms suspended in air oron the surface of materials and products Data published byBachmann (1975) on dose levels required to inactivatespores of moulds and bacteria indicated excellent potentialof the process for sterilizing aseptic packaging materialsMaunder (1977) suggested the use of high-intensity UV foraseptic packaging material sterilization Both vegetativecells and spores are sensitive to UV radiation Microbialresistance to UV radiation increases in the order vegetativebacterial cells frac12 yeasts ltbacterial spores lt mould sporesGram-positive bacteria require twice the dosage of Gram-negative bacteria and 5ndash10 times for bacterial sporesMould spores particularly those that have dark colours(eg Aspergillus niger) are especially resistant and requiredosage levels 20ndash100 times higher (Wallhauser 1978)Covered shaded or shielded organisms will not be affectedDust particles present on the surfaces reduce the effective-ness of UV irradiation for sterilization of aseptic packagingmaterials The penetration ability of UV rays is very limitedas shown in Table 2

A typical UV radiation intensity is between 05 and15 W cmiexcl2 at a distance of 10 cm Laboratory tests using

an energy density of 30 mW cmiexcl2 on arti cially contami-nated smooth even dust-free surfaces and an irradiationtime of 4ndash6 s showed a 4ndash5 log cycle reduction(Cerny 1977) Kodera (1983) obtained a patent for boththe method (UV light plus organic acid or hot water) andapparatus for sterilizing packaging lm The reportedadvantage of this method is that organic acids and waterare not toxic UV irradiation and a citric acid solutionresulted in approximately a six-log cycle inactivation ofBacillus subtilis spores within 5 s Doyen (1973) describedan aseptic packaging machine that used both an alcohol bathand high intensity UV radiation for sterilization of exibleplastic pouches The lm was washed in 95 ethanol for20 s and then passed under a ow of 10 mW cmiexcl2 of UV for30 s The rate of microbial reduction in the sterilization ofsmooth surface plastic cups and lids by UV irradiation at aconstant radiator output depends upon a number of factors(Cerny 1977) UV irradiation under practical conditionsonly always count reduction of 2ndash3 log and mould sporescan be particularly resistant This low count reduction isinsuf cient for aseptic packaging the process is thereforemerely suitable for a lling and packaging process with arelatively low contamination count

Combination of hydrogen peroxide with UVThe ability of UV and hydrogen peroxide to inactivate

bacteria has been well known for a long time The process ofsimultaneously applying hydrogen peroxide solution andUV will provide adequate sporicidal activity to relativelylow concentrations of hydrogen peroxide solution minimiz-ing the problem of removal of residues after sterilizationThe Liquid-Pak System uses this effect with a cartonforming lling and closing system in a sterile chamberMany researchers have considered sterilization of packagingmaterials using peroxide and UV irradiation It has beenfound that the lethal action of peroxide solutions with andwithout the application of heat is increased by simultaneousUV irradiation and that the overall lethal effect is greaterthan the sum of the effects of the peroxide and irradiationalone (Bayliss and Waites 1979a 1982) They subsequentlydemonstrated the same type of synergy with a variety ofBacillus and Clostridium (Bayliss and Waites 1979b) butfound that mild heat was also needed for effective killing ofmore resistant spores tested Higher count reduction can beobtained by prior treatment of the surface with coldhydrogen peroxide and by ensuring that the surface is dustfree (Bernard 1983 Cerny 1977 Ito and Stevenson 1984Wainess 1982) The effect is optimum at a relatively lowperoxide concentration between 05 and 5 and at higherconcentration the peroxide appears to have a protectiveeffect and the greater the UV intensity the higher is theoptimum peroxide concentration Four decimal reductionsof Bacillus subtilis spores of a strain very resistant to H2O2

were easily obtained by irradiation with an UV dose of18 W cmiexcl2 in 25 peroxide followed by a short heattreatment (Bayliss and Waites 1982) Subsequent heatingto 80macrC increased the kill further Maunder (1977) used aconventional UV lamp to sterilize pouches also with theadvent of high intensity UV-C lamp (Brandli 1975) veryhigh lethal effects have been reported up to 5 decimalreductions on at board (Cerny 1977) and 7 decimalreduction for Bacillus spores on inoculated strips placedinto food packaging cartons (Sturm and Gilliand 1974)

Table 2 Penetration of UV light

Substrate Distance that UV will penetrate (cm)

Air 300ndash500Water 30Window glass lt01Transparent plastic foil lt001Fruit juice 01Fluid milk 001

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60 ANSARI and DATTA

Decontamination of pre-formed cartons for aseptic packa-ging poses problems which are not encountered with atsurfaces Therefore UV irradiation of a pre-formed cartonmust be considerably more intense than for at boards toensure that all surfaces are effectively irradiated (Bachmann1975) Experiments with cartons arti cially contaminatedwith Bacillus subtilis spores sprayed with 1 H2O2 andthen irradiated for 10 s with a high-intensity UV sourceabove the carton showed 5 decimal reductions with poly-ethylene lined material and 35 decimal reductions with apolyethylene=aluminium foil laminate (Stannard et al1983) No heat was applied This combination of hydrogenperoxide with UV has now been applied commercially tocarton sterilization during aseptic lling

Infrared rayInfrared rays in the waveband 08ndash15 pound 10iexcl6 are easily

generated On an absorbent surface they are converted intosensible heat producing an increased temperature Theenergy impact on the surface that is convertible into heatis dependent on the same geometric conditions as thoseoutlined for UV rays Thus infrared irradiation is bestapplied to a smooth even surface with vertical radiationincline This method has been used to treat the interior ofaluminium lids with a plastic coating on the exterior surfaceAs the temperature rise can cause a softening of the plasticthe maximum temperature and exposure time are limitedCount reductions on the same order as those for UVirradiation has been found

Ionizing rayIonizing radiation is an excellent sterilizing agent and

penetrates deep into objects Ionizing radiation of primaryinterest includes a-rays X-rays b-rays electron beams andg-rays Bags used in bag-in-box aseptic packaging arecurrently sterilized with g-rays from a 60Co source Radia-tion kills bacteria in the same logarithmic fashion as heatand D-values for many organisms have been reported Adose of 47 Mrad is considered a 12D level for Clostridiumbotulinum which are the most radiation-resistant spores ofpublic health and spoilage signi cance (Urbain 1978) Theeffect of water activity on irradiation by 60Co g-rays wasinvestigated by Harmulv and Snygg (1973) Dosage levelsto inactivate 90 of Bacillus subtilis spores increased from023 to 042 Mrad as the water activity (aw) decreased from100 to 000 For a 999 reduction (3 log) of Bacillusstearothermophilus spores the dosage increased from 17Mrad at an aw of 100 to 300 Mrad at 000

Gamma irradiation has been used to sterilize plastic bagsfor bulk packaging acid foods using an aseptic bag-in-boxsystem (Nelson 1984) The bag made of plastic laminatesare treated in a specialized irradiation plant and given aradiation dose of 25 kGy (25 Mrad) or more which issuf cient to ensure sterility However the dif culty ofmaintaining sterility during the transfer operation to the lling spout of an aseptic lling machine raises somedoubts as to the practicality of gamma irradiation sterilizedpackaging materials for use on low-acid foods The heavyshielding necessary to prevent radiation leakage fromgamma ray source makes the system impractical to use forin-line sterilization Gamma radiation has not yet beenwidely employed in the USA because of the cost andconcerns about the effects of g-radiation on food

Electron beam sterilization of packaging material is aproven state-of-the art technology which outperforms anylegislative and customer requirements Studies have shownthat depending on the level of microorganisms a dose of5ndash7 kGy was found to be effective against yeast mould andspores and no viable microorganism was found after treat-ment Unfavourable effects of electron beam treatment werenot observed High-energy electron beams are not as pene-trating as g-rays and therefore do not require as muchshielding as a g-ray sources The irradiation process takesjust a few seconds and does not leave any type of residueHowever with pre-formed containers the geometry of thedelivery system makes it dif cult to apply a uniform dose onthe whole container In-line processing heavily demandedby industry requires a careful process design Technologicaland economic optimization of electron beam source andshielding are the most important tasks to be accomplishedbefore in-line electron decontamination of food packagingmay be applied on a large scale A major impediment toadoption of this technology in the food industry is the highinitial and operating cost of the units

Care must be used when using ionizing radiation forsterilization of aseptic packaging materials Although alumi-nium foil is not affected by irradiation paper and plasticcould be affected with results ranging from discolourationto loss of pliability and mechanical strength While radiationcan be quite effective the equipment is expensive as is theprotection for workers and the environment Workers are notusually willing to work in such an environment It willprobably be more extensively employed in the future

Light-pulseThe technology of utilizing short pulses of light is an

attractive alternative for sterilizing packaging materials andprocessing equipment in aseptic packaging The spectrum oflight used for sterilization purposes includes wavelengths inthe ultraviolet to wavelengths in the near-infrared regionThe material to be sterilized is exposed to at least one pulseof light having an energy density in the range of about 001ndash50 J cmiexcl2 at the surface The duration (Anonymous 1994)of pulses ranges from 1 ms to 01 s The packaging materialsare typically exposed to between 1 and 20 pulses of light-intensity short-duration light A few ashes applied at arate of 1ndash10 per second provide very high microbial killlevels making the system ideal for continuous in-linesterilization It is desirable for certain aseptic processesthat packaging material be treated with pulses having arelatively high UV content to minimize the total uence(J cmiexcl2) necessary to achieve the desired reduction inmicrobial population (Dunn et al 1991) Comparison ofthe antimicrobial effects obtained using pulsed light withthose obtained using non-pulsed or continuous waveconventional UV sources shows a signi cantly higher inac-tivation for pulsed light More than 7 log cycles of Asper-gillus niger spore inactivation result with few pulsed light ashes A variety of microorganisms including Bacillussubtilis are inactivated by using between 1 and 35 pulsesof light with intensity ranging between about 1 and12 J cmiexcl2 Greater inactivation is obtained when full-spectrum light rather than glass- ltered spectrum light isused Thus the UV component of light is needed to inacti-vate microorganisms (Dunn et al 1991) Spores of Bacillus

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STERILIZATION METHODS FOR PACKAGING MATERIALS 61

subtilis Bacillus pumilus Bacillus stearothermophilus andAspergillus niger were inactivated completely from 6ndash8 logsof colony forming units (CFU) with 1ndash3 pulses (Bushnellet al 1998) Bacillus subtilis for example is sterilized(99999) by about 42600mW s cmiexcl2 of UV while requir-ing a dose of only 4500mW s cmiexcl2 under pulsed lightMicroorganisms are inactivated by a combination of photo-chemical and photothermal mechanisms

Light pulses apparently do not affect the nutrient retentionin food although a detailed study is not available Pulsedlight treatment costs are very favourable (Dunn et al 1995)Packaging materials compatible with this process will trans-mit light over the broad spectrum employed LLDPELDPE nylon Aclar HDPE and PP have all been usedPackage geometry should not allow any shadowing on theproduct or the light exposure may not be suf cient

Chemical Methods

Thermal destruction is by far the most common andef cient mode of sterilization Dry or moist heat for ster-ilization is also not always feasible Chemical methods use awide variety of chemicals in the form of liquids and gases todisinfect and sterilize equipment and packaging materials

Hydrogen peroxideHydrogen peroxide is one of the most widely used

sterilants used for sterilizing packaging materials The rstsuccessful aseptic lling system for cartoning the asepticTetra Pak of 1961 used a combination of hydrogen peroxideand heat for the sterilization of the surface of containermaterial (Burton 1988) Many aseptic packaging systemsuse hydrogen peroxide at concentrations from 30 to 35 asa sterilant for packaging materials and other food contactsurfaces followed by hot air (60ndash125macrC) to augment thesterilizing effect and to dissipate residual hydrogen perox-ide Sterilizing performance increases with both peroxideconcentration and temperature At ambient environmentaltemperatures the activity of hydrogen peroxide is relativelyslow however this can be increased considerably by raisingthe temperature to 85ndash90macrC and=or increasing concentration(Smith and Brown 1980 Swartling and Lindgren 1968)Therefore for effective sporicidal action in food processingenvironment treated with hydrogen peroxide (at 30) isfollowed by application of hot air (Yokoyama 1990)Swartling and Lindgren (1968) found that four decimalreductions were obtained after suspension in 20 at 80macrCfor 15 s Ito and Stevenson (1984) state that the challengeorganisms of choice are Bacillus stearothermophilus foraseptic packaging systems that use heat sterilization andstrains of Bacillus subtilis A or Bacillus subtilis var globigiifor ller surfaces that sterilize with hydrogen peroxide andheated air

The following relationship holds for destruction of sporesof Bacillusstearothermophilus(Shapton and Shapton 1998)

log D ˆ 17 iexcl 13 curren log ( H2O2)

where D is the decimal reduction time in minutesSpores resistant to hydrogen peroxide have been reviewed

by Ito (1973) and Stevenson and Shafer (1983) Toledo et al(1973) reported that dry spores of Bacillus subtilis varglobigii (NCIB 8058) were less resistant than the wet

spores Consistent with observations for heat and irradi-ation dry spores are more resistant than wet spores Sincesterilization processes used on aseptic packaging systemsfor low-acid foods must have documentation for adequacyto inactivate pathogenic and spoilage microorganisms a unitmust be tested using an acceptably resistant organism beforeit can be placed in commercial production In commercialaseptic packaging equipment which uses roll stock thepackaging material is immersed in hydrogen peroxidesolution followed by heating to vaporize the peroxidebefore the packages are lled Contact time with the solu-tions which contain a wetting agent is often less than 1 minA large amount of the sterilizing liquid is removed mechani-cally eg by rollers or air blasts and the remainder isgenerally removed by drying with hot or sterile air or radiantheat The nascent oxygen thus produced due to breakdownof peroxide reacts with the oxidizable cell components ofthe microorganisms and causes bactericidal effect Commer-cial packaging systems employ hydrogen peroxide atconcentrations of 10ndash35 at either room or elevatedtemperature (Von Bockelmann 1974 Von Bockelmannand Von Bockelmann 1972) in the form of a spray orimmersion bath The sporicidal action of hydrogen peroxidealso has been reviewed by Von Bockelmann and VonBockelman (1972) Hydrogen peroxide kills a wide varietyof organisms (Wang and Toledo 1986) and viruii (Mentaland Schmidt 1973) Both linear (Swartling and Lindgren1968 Toledo et al 1973) and nonlinear (Cerf and Hermier1972) relationships between log number of survivors andtime of exposure to different concentrations of hydrogenperoxide have been reported and Toledo et alrsquos (1973) datashow that 1 min in 30 hydrogen peroxide at ambienttemperature produced only 25 decimal reductions ofmould spores and bacterial spores were not affected at allThus in current commercial practice the microbicidalaction is primarily due to the heated lm of hydrogenperoxide at the start of the drying phase of the sterilizationprocess For this reason wetting of the packaging materialand the presence of a uniform lm of liquid on the materialsurface are critical factors

When sterilizing pre-formed containers hydrogenperoxide is sprayed or atomized into the container Ameasured amount of hydrogen peroxide is metred intoeach nozzle which delivers the solution into each containerto ensure that a uniform lm coats the inside surface of thepackage For acid food products relatively low microbiallethality required makes it easy to achieve the desired levelof microbial inactivation and meet the residue tolerancelevel However for low-acid foods a system must becarefully designed such that both criteria of commercialsterility and adequate sterilant removal are achieved

Hydrogen peroxide-sterilized aseptic packaging systemsthat have been approved for commercial packaging of low-acid foods have been proven to produce an acceptable levelof residues The Food and Drug administration of the USAhas ruled that the level of hydrogen peroxide that may bepresent in product packaged in material sterilized byhydrogen peroxide must not be greater than 05 ppm(Reuter 1988) An improperly designed system may resultin more incidence of non-compliance with regulationspertaining to residues Prolonged contact between liquidhydrogen peroxide and the packaging material alsoincreased the dif culty of removing residues

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62 ANSARI and DATTA

Hydrogen peroxide is frequently used to sterilize thepackaging material as it decomposes to a product whichwill neither taint the food nor render it toxic It is reportedthat the traces of residual of hydrogen peroxide left in foodis less than 025ppm and normally this amount does nothave any harmful effect when consumed (Hedrick 1973Smith and Brown 1980) Residual hydrogen peroxide andthat trapped in the package headspace at the time of sealinghas an adverse effect on product stability particularly onascorbic acid degradation in fruit juices (Toledo 1986)When the concentration of hydrogen peroxide at the time ofpackaging exceeded 01 mg=mliexcl1 a marked increase inascorbic acid degradation in bottled orange juice andorange juice concentrates was observed by Toledo (19751986) Aseptic packaging of foods containing ascorbic acidin a system that uses hydrogen peroxide for sterilization ofmaterial is not desirable unless some means is used toremove hydrogen peroxide vapours from the package headspace and packaging material is completely free fromhydrogen peroxide residues

There are three commercial methods for applyinghydrogen peroxide to packaging materials dippingspraying and rinsing

Dipping the packaging material (ie plastic laminateswith cardboard lms of thermoformable plastics and lami-nates) are taken from a reel and dipped into a bath ofaqueous 30ndash33 hydrogen peroxide (Reuter 1988) Countreductions on the order of 4ndash5 logs are claimed Wettingagents are added to ensure uniform wetting of the surfacesExcess solution is removed by squeeze rolls or air jets afterremoval of the material from the bath which leaves a thin lm of solution that is then dried by the application of hotair To increase the ef cacy especially in the case of dusty orslightly soiled material prior treatment of the material withrotating brushes sterile compressed air jets or ultrasoundapplied to the bath may be added

Spraying this is the method (Reuter 1988) for preformedcontainers in which the hydrogen peroxide is sprayed in theform of small dispersed droplets into the container Sprayingdoes not result in a cohesive lm due to the hydrophobiccharacteristics of plastics rather only 30ndash40 of the innersurfaces of the container are covered and thereby it does notensure good sterility of lm A conventional spray givesdrops of over 30 mm diameters on the surface and only30ndash40 of the surface area is covered An ultrasonic systemcan be used to give particle sizes of only 3 mm diameterwhich will give an average surface cover of about 60The droplet size distribution between 2 and 80 mm and thedistribution of droplets on the inner surfaces of the variousgeometric shapes of the containers is problematical Theef cacy is dependent upon the volume of solution sprayedhowever the larger the volume the longer the drying timeThe drying must be carried out with hot sterile air Improvedaerosol sprayers can limit the droplet size to 2ndash4 mm andincrease the coverage to 60 while permitting reducedvolume of peroxide solution to be applied hence thedrying time This method is being replaced by the use ofa mixture of hot air and vapourized peroxide Sterilizationby hydrogen peroxide vapour would be a cost-effectivealternative as the least amount of hydrogen peroxide isused Furthermore the amount of hydrogen peroxideadsorbed on the treated surface from the vapour phase willbe several orders of magnitude smaller than a liquid

lm Therefore ushing the vapour-treated surface withlow-temperature sterile air free of hydrogen peroxidevapours can effectively eliminate residues

Low levels of hydrogen peroxide which remain in thevapour phase in air at near ambient temperature has beenshown to have sporicidal activity suitable for in-line ster-ilization of aseptic packaging materials (Wang and Toledo1986) Air saturated with hydrogen peroxide vapour carryrelatively low concentration of hydrogen peroxide(76 mg liexcl1 at 70macrC) but can induce 6 decimal reductionsof spores of Bacillus subtilis var niger in 12 min Incomparison the same organism requires 12 min to achieveone decimal reduction in hot air alone at 150macrC Since theamount of hydrogen peroxide vapourized from solution isless than that from water the solution is enriched duringprolonged operation and a system must be devised to addmake-up water to maintain the desired concentration

Rinsing prefabricated intricately shaped containers forwhich the spraying process is unsuitablecan be rinsed with anaqueous 30ndash33 H2O2 solution For ambient temperaturesterilization this can be combined with peracetic acid Thecontainers are drained allowed to drip dry and then com-pletely dried by hot sterile air Glass containers metal cansand blow-moulded plastic bottles are treated in this manner

Ethylene oxideImplementation of ethylene oxide sterilization has made

possible the use of sterile low-cost disposable thermoplas-tic devices for industrial application It has been used to pre-sterilize paperboard cartons (Hedrick 1973) and plasticpackaging materials (Alguire 1973) However no commer-cial system uses ethylene oxide as a sterilizing agent inaseptic packaging of low acid foods for ambient temperaturestorage and distribution The vapour form is ammable andexplosive (Merck 1989) and hence employed as a mixturewith an inert gas such as dichlodi uoromethane Alguire(1973) reported that 100 ethylene oxide has better micro-bicidal properties than an ethylene oxide inert gas mixtureAn important consideration in bulk sterilization of pre-formed packaging materials is ethylene oxidersquos ability topermeate and contact all surfaces that need to be sterilizedThe use of lower concentration of ethylene oxide is saferthan use of 100 ethylene oxide from standpoint ofpotential for injury to personnel The D-value for Clostri-dium botulinum 62 A require a minimum of 72 min for a12D process thus making it impractical to use ethyleneoxide for in-line sterilization of aseptic packaging materialsin high-speed packaging lines Ethylene oxide is effectiveagainst bacteria moulds yeasts as well as viruses The mostprobable activity is alkylation of nucleic acids and othercellular components (Phillips 1977) It is a particularlyeffective sterilizing agent because it rapidly penetratespackaging material The ethylene oxide concentrationhumidity and temperature in uence the rate of sterilizationA clean object can be sterilized if treated for 5ndash8 h at 38macrCor 3ndash4 h at 54macrC when the relative humidity is maintained at40ndash50 and the EtO concentration at 700 mg liexcl1 Extensiveaeration of the sterilized materials is necessary to removeresidual EtO because it is so toxic However one disadvan-tage of it is its comparatively slow rate of action againstmicroorganisms Spores of Bacillus subtilis var niger ATCC9372 are used as indicators in dry heat and ethylene oxidesterilization (Anonymous 1995 1999)

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STERILIZATION METHODS FOR PACKAGING MATERIALS 63

Other sterilization methodsThere are many other chemicals such as peracetic acid

beta propiolactone alcohol chlorine and its oxide andozone etc that have been suggested as having potentialfor use in sterilizing aseptic packaging materials Peraceticacid is a peroxide of acetic acid and on decomposition itforms acetic acid and water The low pH and oxidizingproperties make it an excellent sporicidal agent It has all theadvantages of hydrogen peroxide and is unaffected bycatalase and peroxidases Peracetic acid is a liquid sterilantwhich is particularly effective against spores of aerobic andanaerobic bacteria and is effective at lower temperaturesthan hydrogen peroxide In practice it is used in a solutionwhich also contains hydrogen peroxide The solutioncontaining peracetic acid and hydrogen peroxide is effectiveagainst resistant bacterial spores even at 20macrC for example a1 solution will eliminate 10iexcl7ndash10iexcl8 of most resistantspore stains in 5 min at 20macrC and the most resistant strainsin 60 min The maximum usable temperature is 40macrC whenthe sterilization times are about 5 times shorter In spite ofits sporicidal properties peracetic acid is not an approvedsterilant for use on aseptic packaging materials Its vapour isvery pungent and irritating Therefore a system that utilizesthis compound must be airtight to prevent environmentalrelease Unlike hydrogen peroxide vapours for which atolerance is allowed inside sealed packages no such toler-ance is allowed for peracetic acid and vapours in the head-space can cause disagreeable vinegar like off- avour insome food products In spite of these problems thiscompound deserves attention as a possible sterilant foraseptic packaging materials because of its effectiveness asa sporicidal agent The presence of small amounts of aceticacid may not be considered off- avours At ambienttemperature and halogen concentration of 100 ppm expo-sure times for effective sterilization would be on the order ofhours Under these conditions it would be impractical to usethese compounds as in-line sterilants in aseptic packagingOne major problem with the use of strong chlorine prepara-tions for sterilization is that available chlorine reacts withorganic material Reaction between chlorine and the materialbeing sterilized may produce off- avours

Beta propiolactone is bactericidal sporicidal fungicidaland virucidal It lacks the penetrating power of the ethyleneoxide but it is considerably more active against microorgan-isms Only 2ndash5 mg of beta propiolactone per litre is requiredfor sterilization compared with 400ndash800 mg of ethyleneoxide per litre However its poor penetration of materialsand its cancer-inducing properties have restricted its use as apractical sterilizing agent

Ethanol is the only industrially signi cant alcohol used asbactericide and fungicide but not for sporicide Theirmechanism of action appears to involve protein denaturationand membrane lipids dissolution Ethanol liquid and vapourhave been suggested (Doyen 1973 Mita 1985) Howeverethanol is effective only against vegetative cells and notagainst fungal conidia or bacterial spores therefore its use inpackaging of foods is limited to extension of shelf lifeof packaged foods which are normally stored underrefrigeration The two most popular alcohol germicides areethanol and isopropanol usually used in about 70ndash80concentration

Chlorine and its oxide and ozone are used mainly forwater disinfection Ozone can be used as a sterilizing agent

because of its high oxidizing capability Death of almost allmicroorganisms usually occurs within 30 min Ozone wasrecommended recently as an alternative to chlorine (Kim1998) and hydrogen peroxide (Khadre and Yousef 2001)They found that hydrogen peroxide at 10000-fold higherconcentration was less effective than ozone against Bacillusspores Antimicrobial power of ozone increases as tempera-ture decreases below ambient (Herbold et al 1989)Comparatively low concentration is needed to eliminatelarge population of spores at ambient temperature in shorttime period and this makes ozone best suited for anindustrial setting

CONCLUSIONS

Sterilization of packaging materials in-line of processingposes certain dif culties Based on the review presented it isevident that hydrogen peroxide sterilization followed by hotair appears to have the most potential for use as in-linesterilants for packaging materials This combination is time-tested and no adverse report seems to have put any doubtover the methodrsquos acceptability Sterilization of packagingmaterials using hydrogen peroxide and followed by ultra-violet irradiation also has been accepted for industrialapplication Dry heat saturated steam and superheatedsteam can be effective sterilants but the degree of heatdamages many packaging materials so they nd limitedapplication Infrared rays cannot be applied as there is atemperature rise of the packaging material due to infraredapplication and this results in softening of the plasticsIonizing rays are not accepted as they have harmful effecton personnel Light pulses have not been adequately studiedso far as their effect on food material is concerned Ethyleneoxide requires a very long time which precludes its use forin-line applications Peracetic acid produces an off- avourin food if residual deposit is enclosed in the container Betapropiolactone lacks the penetrating power of ethylene oxideand ethanol is not effective against spores

REFERENCES

Alguire DE 1973 Ethylene oxide gas sterilization of packaging materialsFood Technol 27(9) 64

Anonymous 1994 The Pure Bright Process Pure Pulse ProprietaryInformation

Anonymous 1995 Biological indicators for dry heat sterilization controland ethylene oxide sterilization control in United States Pharmaco-peia=National Formulary (US Pharmacopeial Convention RockvilleMD) pp 200 202

Anonymous AE 1999 Biological indicators of sterilization in BritishPharmacopeia Vol II (British Pharmacopeia Commission London) ppA281ndashA283

Bachmann R 1975 Sterilization by intense ultraviolet radiation BrownBoveri Rev 62 206ndash209

Bayliss CE and Waites WM 1979a The synergistic killing of spores ofBacillius subtilis by hydrogen peroxide and ultraviolet irradiation FEMSMicrobiol Lett 5 331ndash333

Bayliss CE and Waites WM 1979b The combine effect of hydrogenperoxide and ultraviolet irradiation on bacterial spores J Appl Bacteriol47(2) 263ndash269

Bayliss CE and Waites WM 1982 Effect of simultaneous high intensityultraviolet irradiation and hydrogen peroxide on bacterial spores J FoodTechnol 17 467ndash470

Bernard D 1983 Microbiological considerations of testing asepticprocessing and packaging systems in Processing of National FoodProcessors Association Conference Capitalizing on Aseptic (FPIWashington DC) pp 13ndash14

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64 ANSARI and DATTA

Electromagnetic radiation characterized by a frequency awavelength penetrating power and an energy range areinfrared ultraviolet g rays etc The dosage and type ofradiation sterilization is a function of the type and amount ofthe microbial load that has to be removed from a givenmaterials and equipment As irradiation does not leave anyresidue on the treated surface nor affect the immediateenvironment it appears at rst sight to be a perfect solutionfor sterilization of packaging material The products usedfor packaging in radiation sterilization recommended by theInternational Atomic Energy Agency (IAEA) are polyethy-lene polyester polypropylene nylon PVC etc Irradiationmay indirectly affect chemical or biological propertiesleading to interaction with the packaged product changingits ef cacy or usability

UV rayUV irradiation has been successfully applied to the

sterilization of air and of water which is free from suspendedmatter With the development of more powerful sources ofUV radiation (Bachmann 1975) attempts have been madeto use it for surface sterilization The microbicidal effects ofUV rays lies in the range of 200ndash315 nm with an optimaleffect being between 250 and 280 nm the so-called UV-Crange UV light at a wavelength of 2537 nm is an effectivegermicide against yeast moulds bacteria viruses and algaeThe lethal irradiation dose is de ned as the product ofirradiation intensity and time and expressed in terms ofmilli Watts second per square centimeter (mW s cmiexcl2)High intensity sources of UV radiation are very effectivein killing unprotected microorganisms suspended in air oron the surface of materials and products Data published byBachmann (1975) on dose levels required to inactivatespores of moulds and bacteria indicated excellent potentialof the process for sterilizing aseptic packaging materialsMaunder (1977) suggested the use of high-intensity UV foraseptic packaging material sterilization Both vegetativecells and spores are sensitive to UV radiation Microbialresistance to UV radiation increases in the order vegetativebacterial cells frac12 yeasts ltbacterial spores lt mould sporesGram-positive bacteria require twice the dosage of Gram-negative bacteria and 5ndash10 times for bacterial sporesMould spores particularly those that have dark colours(eg Aspergillus niger) are especially resistant and requiredosage levels 20ndash100 times higher (Wallhauser 1978)Covered shaded or shielded organisms will not be affectedDust particles present on the surfaces reduce the effective-ness of UV irradiation for sterilization of aseptic packagingmaterials The penetration ability of UV rays is very limitedas shown in Table 2

A typical UV radiation intensity is between 05 and15 W cmiexcl2 at a distance of 10 cm Laboratory tests using

an energy density of 30 mW cmiexcl2 on arti cially contami-nated smooth even dust-free surfaces and an irradiationtime of 4ndash6 s showed a 4ndash5 log cycle reduction(Cerny 1977) Kodera (1983) obtained a patent for boththe method (UV light plus organic acid or hot water) andapparatus for sterilizing packaging lm The reportedadvantage of this method is that organic acids and waterare not toxic UV irradiation and a citric acid solutionresulted in approximately a six-log cycle inactivation ofBacillus subtilis spores within 5 s Doyen (1973) describedan aseptic packaging machine that used both an alcohol bathand high intensity UV radiation for sterilization of exibleplastic pouches The lm was washed in 95 ethanol for20 s and then passed under a ow of 10 mW cmiexcl2 of UV for30 s The rate of microbial reduction in the sterilization ofsmooth surface plastic cups and lids by UV irradiation at aconstant radiator output depends upon a number of factors(Cerny 1977) UV irradiation under practical conditionsonly always count reduction of 2ndash3 log and mould sporescan be particularly resistant This low count reduction isinsuf cient for aseptic packaging the process is thereforemerely suitable for a lling and packaging process with arelatively low contamination count

Combination of hydrogen peroxide with UVThe ability of UV and hydrogen peroxide to inactivate

bacteria has been well known for a long time The process ofsimultaneously applying hydrogen peroxide solution andUV will provide adequate sporicidal activity to relativelylow concentrations of hydrogen peroxide solution minimiz-ing the problem of removal of residues after sterilizationThe Liquid-Pak System uses this effect with a cartonforming lling and closing system in a sterile chamberMany researchers have considered sterilization of packagingmaterials using peroxide and UV irradiation It has beenfound that the lethal action of peroxide solutions with andwithout the application of heat is increased by simultaneousUV irradiation and that the overall lethal effect is greaterthan the sum of the effects of the peroxide and irradiationalone (Bayliss and Waites 1979a 1982) They subsequentlydemonstrated the same type of synergy with a variety ofBacillus and Clostridium (Bayliss and Waites 1979b) butfound that mild heat was also needed for effective killing ofmore resistant spores tested Higher count reduction can beobtained by prior treatment of the surface with coldhydrogen peroxide and by ensuring that the surface is dustfree (Bernard 1983 Cerny 1977 Ito and Stevenson 1984Wainess 1982) The effect is optimum at a relatively lowperoxide concentration between 05 and 5 and at higherconcentration the peroxide appears to have a protectiveeffect and the greater the UV intensity the higher is theoptimum peroxide concentration Four decimal reductionsof Bacillus subtilis spores of a strain very resistant to H2O2

were easily obtained by irradiation with an UV dose of18 W cmiexcl2 in 25 peroxide followed by a short heattreatment (Bayliss and Waites 1982) Subsequent heatingto 80macrC increased the kill further Maunder (1977) used aconventional UV lamp to sterilize pouches also with theadvent of high intensity UV-C lamp (Brandli 1975) veryhigh lethal effects have been reported up to 5 decimalreductions on at board (Cerny 1977) and 7 decimalreduction for Bacillus spores on inoculated strips placedinto food packaging cartons (Sturm and Gilliand 1974)

Table 2 Penetration of UV light

Substrate Distance that UV will penetrate (cm)

Air 300ndash500Water 30Window glass lt01Transparent plastic foil lt001Fruit juice 01Fluid milk 001

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60 ANSARI and DATTA

Decontamination of pre-formed cartons for aseptic packa-ging poses problems which are not encountered with atsurfaces Therefore UV irradiation of a pre-formed cartonmust be considerably more intense than for at boards toensure that all surfaces are effectively irradiated (Bachmann1975) Experiments with cartons arti cially contaminatedwith Bacillus subtilis spores sprayed with 1 H2O2 andthen irradiated for 10 s with a high-intensity UV sourceabove the carton showed 5 decimal reductions with poly-ethylene lined material and 35 decimal reductions with apolyethylene=aluminium foil laminate (Stannard et al1983) No heat was applied This combination of hydrogenperoxide with UV has now been applied commercially tocarton sterilization during aseptic lling

Infrared rayInfrared rays in the waveband 08ndash15 pound 10iexcl6 are easily

generated On an absorbent surface they are converted intosensible heat producing an increased temperature Theenergy impact on the surface that is convertible into heatis dependent on the same geometric conditions as thoseoutlined for UV rays Thus infrared irradiation is bestapplied to a smooth even surface with vertical radiationincline This method has been used to treat the interior ofaluminium lids with a plastic coating on the exterior surfaceAs the temperature rise can cause a softening of the plasticthe maximum temperature and exposure time are limitedCount reductions on the same order as those for UVirradiation has been found

Ionizing rayIonizing radiation is an excellent sterilizing agent and

penetrates deep into objects Ionizing radiation of primaryinterest includes a-rays X-rays b-rays electron beams andg-rays Bags used in bag-in-box aseptic packaging arecurrently sterilized with g-rays from a 60Co source Radia-tion kills bacteria in the same logarithmic fashion as heatand D-values for many organisms have been reported Adose of 47 Mrad is considered a 12D level for Clostridiumbotulinum which are the most radiation-resistant spores ofpublic health and spoilage signi cance (Urbain 1978) Theeffect of water activity on irradiation by 60Co g-rays wasinvestigated by Harmulv and Snygg (1973) Dosage levelsto inactivate 90 of Bacillus subtilis spores increased from023 to 042 Mrad as the water activity (aw) decreased from100 to 000 For a 999 reduction (3 log) of Bacillusstearothermophilus spores the dosage increased from 17Mrad at an aw of 100 to 300 Mrad at 000

Gamma irradiation has been used to sterilize plastic bagsfor bulk packaging acid foods using an aseptic bag-in-boxsystem (Nelson 1984) The bag made of plastic laminatesare treated in a specialized irradiation plant and given aradiation dose of 25 kGy (25 Mrad) or more which issuf cient to ensure sterility However the dif culty ofmaintaining sterility during the transfer operation to the lling spout of an aseptic lling machine raises somedoubts as to the practicality of gamma irradiation sterilizedpackaging materials for use on low-acid foods The heavyshielding necessary to prevent radiation leakage fromgamma ray source makes the system impractical to use forin-line sterilization Gamma radiation has not yet beenwidely employed in the USA because of the cost andconcerns about the effects of g-radiation on food

Electron beam sterilization of packaging material is aproven state-of-the art technology which outperforms anylegislative and customer requirements Studies have shownthat depending on the level of microorganisms a dose of5ndash7 kGy was found to be effective against yeast mould andspores and no viable microorganism was found after treat-ment Unfavourable effects of electron beam treatment werenot observed High-energy electron beams are not as pene-trating as g-rays and therefore do not require as muchshielding as a g-ray sources The irradiation process takesjust a few seconds and does not leave any type of residueHowever with pre-formed containers the geometry of thedelivery system makes it dif cult to apply a uniform dose onthe whole container In-line processing heavily demandedby industry requires a careful process design Technologicaland economic optimization of electron beam source andshielding are the most important tasks to be accomplishedbefore in-line electron decontamination of food packagingmay be applied on a large scale A major impediment toadoption of this technology in the food industry is the highinitial and operating cost of the units

Care must be used when using ionizing radiation forsterilization of aseptic packaging materials Although alumi-nium foil is not affected by irradiation paper and plasticcould be affected with results ranging from discolourationto loss of pliability and mechanical strength While radiationcan be quite effective the equipment is expensive as is theprotection for workers and the environment Workers are notusually willing to work in such an environment It willprobably be more extensively employed in the future

Light-pulseThe technology of utilizing short pulses of light is an

attractive alternative for sterilizing packaging materials andprocessing equipment in aseptic packaging The spectrum oflight used for sterilization purposes includes wavelengths inthe ultraviolet to wavelengths in the near-infrared regionThe material to be sterilized is exposed to at least one pulseof light having an energy density in the range of about 001ndash50 J cmiexcl2 at the surface The duration (Anonymous 1994)of pulses ranges from 1 ms to 01 s The packaging materialsare typically exposed to between 1 and 20 pulses of light-intensity short-duration light A few ashes applied at arate of 1ndash10 per second provide very high microbial killlevels making the system ideal for continuous in-linesterilization It is desirable for certain aseptic processesthat packaging material be treated with pulses having arelatively high UV content to minimize the total uence(J cmiexcl2) necessary to achieve the desired reduction inmicrobial population (Dunn et al 1991) Comparison ofthe antimicrobial effects obtained using pulsed light withthose obtained using non-pulsed or continuous waveconventional UV sources shows a signi cantly higher inac-tivation for pulsed light More than 7 log cycles of Asper-gillus niger spore inactivation result with few pulsed light ashes A variety of microorganisms including Bacillussubtilis are inactivated by using between 1 and 35 pulsesof light with intensity ranging between about 1 and12 J cmiexcl2 Greater inactivation is obtained when full-spectrum light rather than glass- ltered spectrum light isused Thus the UV component of light is needed to inacti-vate microorganisms (Dunn et al 1991) Spores of Bacillus

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STERILIZATION METHODS FOR PACKAGING MATERIALS 61

subtilis Bacillus pumilus Bacillus stearothermophilus andAspergillus niger were inactivated completely from 6ndash8 logsof colony forming units (CFU) with 1ndash3 pulses (Bushnellet al 1998) Bacillus subtilis for example is sterilized(99999) by about 42600mW s cmiexcl2 of UV while requir-ing a dose of only 4500mW s cmiexcl2 under pulsed lightMicroorganisms are inactivated by a combination of photo-chemical and photothermal mechanisms

Light pulses apparently do not affect the nutrient retentionin food although a detailed study is not available Pulsedlight treatment costs are very favourable (Dunn et al 1995)Packaging materials compatible with this process will trans-mit light over the broad spectrum employed LLDPELDPE nylon Aclar HDPE and PP have all been usedPackage geometry should not allow any shadowing on theproduct or the light exposure may not be suf cient

Chemical Methods

Thermal destruction is by far the most common andef cient mode of sterilization Dry or moist heat for ster-ilization is also not always feasible Chemical methods use awide variety of chemicals in the form of liquids and gases todisinfect and sterilize equipment and packaging materials

Hydrogen peroxideHydrogen peroxide is one of the most widely used

sterilants used for sterilizing packaging materials The rstsuccessful aseptic lling system for cartoning the asepticTetra Pak of 1961 used a combination of hydrogen peroxideand heat for the sterilization of the surface of containermaterial (Burton 1988) Many aseptic packaging systemsuse hydrogen peroxide at concentrations from 30 to 35 asa sterilant for packaging materials and other food contactsurfaces followed by hot air (60ndash125macrC) to augment thesterilizing effect and to dissipate residual hydrogen perox-ide Sterilizing performance increases with both peroxideconcentration and temperature At ambient environmentaltemperatures the activity of hydrogen peroxide is relativelyslow however this can be increased considerably by raisingthe temperature to 85ndash90macrC and=or increasing concentration(Smith and Brown 1980 Swartling and Lindgren 1968)Therefore for effective sporicidal action in food processingenvironment treated with hydrogen peroxide (at 30) isfollowed by application of hot air (Yokoyama 1990)Swartling and Lindgren (1968) found that four decimalreductions were obtained after suspension in 20 at 80macrCfor 15 s Ito and Stevenson (1984) state that the challengeorganisms of choice are Bacillus stearothermophilus foraseptic packaging systems that use heat sterilization andstrains of Bacillus subtilis A or Bacillus subtilis var globigiifor ller surfaces that sterilize with hydrogen peroxide andheated air

The following relationship holds for destruction of sporesof Bacillusstearothermophilus(Shapton and Shapton 1998)

log D ˆ 17 iexcl 13 curren log ( H2O2)

where D is the decimal reduction time in minutesSpores resistant to hydrogen peroxide have been reviewed

by Ito (1973) and Stevenson and Shafer (1983) Toledo et al(1973) reported that dry spores of Bacillus subtilis varglobigii (NCIB 8058) were less resistant than the wet

spores Consistent with observations for heat and irradi-ation dry spores are more resistant than wet spores Sincesterilization processes used on aseptic packaging systemsfor low-acid foods must have documentation for adequacyto inactivate pathogenic and spoilage microorganisms a unitmust be tested using an acceptably resistant organism beforeit can be placed in commercial production In commercialaseptic packaging equipment which uses roll stock thepackaging material is immersed in hydrogen peroxidesolution followed by heating to vaporize the peroxidebefore the packages are lled Contact time with the solu-tions which contain a wetting agent is often less than 1 minA large amount of the sterilizing liquid is removed mechani-cally eg by rollers or air blasts and the remainder isgenerally removed by drying with hot or sterile air or radiantheat The nascent oxygen thus produced due to breakdownof peroxide reacts with the oxidizable cell components ofthe microorganisms and causes bactericidal effect Commer-cial packaging systems employ hydrogen peroxide atconcentrations of 10ndash35 at either room or elevatedtemperature (Von Bockelmann 1974 Von Bockelmannand Von Bockelmann 1972) in the form of a spray orimmersion bath The sporicidal action of hydrogen peroxidealso has been reviewed by Von Bockelmann and VonBockelman (1972) Hydrogen peroxide kills a wide varietyof organisms (Wang and Toledo 1986) and viruii (Mentaland Schmidt 1973) Both linear (Swartling and Lindgren1968 Toledo et al 1973) and nonlinear (Cerf and Hermier1972) relationships between log number of survivors andtime of exposure to different concentrations of hydrogenperoxide have been reported and Toledo et alrsquos (1973) datashow that 1 min in 30 hydrogen peroxide at ambienttemperature produced only 25 decimal reductions ofmould spores and bacterial spores were not affected at allThus in current commercial practice the microbicidalaction is primarily due to the heated lm of hydrogenperoxide at the start of the drying phase of the sterilizationprocess For this reason wetting of the packaging materialand the presence of a uniform lm of liquid on the materialsurface are critical factors

When sterilizing pre-formed containers hydrogenperoxide is sprayed or atomized into the container Ameasured amount of hydrogen peroxide is metred intoeach nozzle which delivers the solution into each containerto ensure that a uniform lm coats the inside surface of thepackage For acid food products relatively low microbiallethality required makes it easy to achieve the desired levelof microbial inactivation and meet the residue tolerancelevel However for low-acid foods a system must becarefully designed such that both criteria of commercialsterility and adequate sterilant removal are achieved

Hydrogen peroxide-sterilized aseptic packaging systemsthat have been approved for commercial packaging of low-acid foods have been proven to produce an acceptable levelof residues The Food and Drug administration of the USAhas ruled that the level of hydrogen peroxide that may bepresent in product packaged in material sterilized byhydrogen peroxide must not be greater than 05 ppm(Reuter 1988) An improperly designed system may resultin more incidence of non-compliance with regulationspertaining to residues Prolonged contact between liquidhydrogen peroxide and the packaging material alsoincreased the dif culty of removing residues

Trans IChemE Vol 81 Part C March 2003

62 ANSARI and DATTA

Hydrogen peroxide is frequently used to sterilize thepackaging material as it decomposes to a product whichwill neither taint the food nor render it toxic It is reportedthat the traces of residual of hydrogen peroxide left in foodis less than 025ppm and normally this amount does nothave any harmful effect when consumed (Hedrick 1973Smith and Brown 1980) Residual hydrogen peroxide andthat trapped in the package headspace at the time of sealinghas an adverse effect on product stability particularly onascorbic acid degradation in fruit juices (Toledo 1986)When the concentration of hydrogen peroxide at the time ofpackaging exceeded 01 mg=mliexcl1 a marked increase inascorbic acid degradation in bottled orange juice andorange juice concentrates was observed by Toledo (19751986) Aseptic packaging of foods containing ascorbic acidin a system that uses hydrogen peroxide for sterilization ofmaterial is not desirable unless some means is used toremove hydrogen peroxide vapours from the package headspace and packaging material is completely free fromhydrogen peroxide residues

There are three commercial methods for applyinghydrogen peroxide to packaging materials dippingspraying and rinsing

Dipping the packaging material (ie plastic laminateswith cardboard lms of thermoformable plastics and lami-nates) are taken from a reel and dipped into a bath ofaqueous 30ndash33 hydrogen peroxide (Reuter 1988) Countreductions on the order of 4ndash5 logs are claimed Wettingagents are added to ensure uniform wetting of the surfacesExcess solution is removed by squeeze rolls or air jets afterremoval of the material from the bath which leaves a thin lm of solution that is then dried by the application of hotair To increase the ef cacy especially in the case of dusty orslightly soiled material prior treatment of the material withrotating brushes sterile compressed air jets or ultrasoundapplied to the bath may be added

Spraying this is the method (Reuter 1988) for preformedcontainers in which the hydrogen peroxide is sprayed in theform of small dispersed droplets into the container Sprayingdoes not result in a cohesive lm due to the hydrophobiccharacteristics of plastics rather only 30ndash40 of the innersurfaces of the container are covered and thereby it does notensure good sterility of lm A conventional spray givesdrops of over 30 mm diameters on the surface and only30ndash40 of the surface area is covered An ultrasonic systemcan be used to give particle sizes of only 3 mm diameterwhich will give an average surface cover of about 60The droplet size distribution between 2 and 80 mm and thedistribution of droplets on the inner surfaces of the variousgeometric shapes of the containers is problematical Theef cacy is dependent upon the volume of solution sprayedhowever the larger the volume the longer the drying timeThe drying must be carried out with hot sterile air Improvedaerosol sprayers can limit the droplet size to 2ndash4 mm andincrease the coverage to 60 while permitting reducedvolume of peroxide solution to be applied hence thedrying time This method is being replaced by the use ofa mixture of hot air and vapourized peroxide Sterilizationby hydrogen peroxide vapour would be a cost-effectivealternative as the least amount of hydrogen peroxide isused Furthermore the amount of hydrogen peroxideadsorbed on the treated surface from the vapour phase willbe several orders of magnitude smaller than a liquid

lm Therefore ushing the vapour-treated surface withlow-temperature sterile air free of hydrogen peroxidevapours can effectively eliminate residues

Low levels of hydrogen peroxide which remain in thevapour phase in air at near ambient temperature has beenshown to have sporicidal activity suitable for in-line ster-ilization of aseptic packaging materials (Wang and Toledo1986) Air saturated with hydrogen peroxide vapour carryrelatively low concentration of hydrogen peroxide(76 mg liexcl1 at 70macrC) but can induce 6 decimal reductionsof spores of Bacillus subtilis var niger in 12 min Incomparison the same organism requires 12 min to achieveone decimal reduction in hot air alone at 150macrC Since theamount of hydrogen peroxide vapourized from solution isless than that from water the solution is enriched duringprolonged operation and a system must be devised to addmake-up water to maintain the desired concentration

Rinsing prefabricated intricately shaped containers forwhich the spraying process is unsuitablecan be rinsed with anaqueous 30ndash33 H2O2 solution For ambient temperaturesterilization this can be combined with peracetic acid Thecontainers are drained allowed to drip dry and then com-pletely dried by hot sterile air Glass containers metal cansand blow-moulded plastic bottles are treated in this manner

Ethylene oxideImplementation of ethylene oxide sterilization has made

possible the use of sterile low-cost disposable thermoplas-tic devices for industrial application It has been used to pre-sterilize paperboard cartons (Hedrick 1973) and plasticpackaging materials (Alguire 1973) However no commer-cial system uses ethylene oxide as a sterilizing agent inaseptic packaging of low acid foods for ambient temperaturestorage and distribution The vapour form is ammable andexplosive (Merck 1989) and hence employed as a mixturewith an inert gas such as dichlodi uoromethane Alguire(1973) reported that 100 ethylene oxide has better micro-bicidal properties than an ethylene oxide inert gas mixtureAn important consideration in bulk sterilization of pre-formed packaging materials is ethylene oxidersquos ability topermeate and contact all surfaces that need to be sterilizedThe use of lower concentration of ethylene oxide is saferthan use of 100 ethylene oxide from standpoint ofpotential for injury to personnel The D-value for Clostri-dium botulinum 62 A require a minimum of 72 min for a12D process thus making it impractical to use ethyleneoxide for in-line sterilization of aseptic packaging materialsin high-speed packaging lines Ethylene oxide is effectiveagainst bacteria moulds yeasts as well as viruses The mostprobable activity is alkylation of nucleic acids and othercellular components (Phillips 1977) It is a particularlyeffective sterilizing agent because it rapidly penetratespackaging material The ethylene oxide concentrationhumidity and temperature in uence the rate of sterilizationA clean object can be sterilized if treated for 5ndash8 h at 38macrCor 3ndash4 h at 54macrC when the relative humidity is maintained at40ndash50 and the EtO concentration at 700 mg liexcl1 Extensiveaeration of the sterilized materials is necessary to removeresidual EtO because it is so toxic However one disadvan-tage of it is its comparatively slow rate of action againstmicroorganisms Spores of Bacillus subtilis var niger ATCC9372 are used as indicators in dry heat and ethylene oxidesterilization (Anonymous 1995 1999)

Trans IChemE Vol 81 Part C March 2003

STERILIZATION METHODS FOR PACKAGING MATERIALS 63

Other sterilization methodsThere are many other chemicals such as peracetic acid

beta propiolactone alcohol chlorine and its oxide andozone etc that have been suggested as having potentialfor use in sterilizing aseptic packaging materials Peraceticacid is a peroxide of acetic acid and on decomposition itforms acetic acid and water The low pH and oxidizingproperties make it an excellent sporicidal agent It has all theadvantages of hydrogen peroxide and is unaffected bycatalase and peroxidases Peracetic acid is a liquid sterilantwhich is particularly effective against spores of aerobic andanaerobic bacteria and is effective at lower temperaturesthan hydrogen peroxide In practice it is used in a solutionwhich also contains hydrogen peroxide The solutioncontaining peracetic acid and hydrogen peroxide is effectiveagainst resistant bacterial spores even at 20macrC for example a1 solution will eliminate 10iexcl7ndash10iexcl8 of most resistantspore stains in 5 min at 20macrC and the most resistant strainsin 60 min The maximum usable temperature is 40macrC whenthe sterilization times are about 5 times shorter In spite ofits sporicidal properties peracetic acid is not an approvedsterilant for use on aseptic packaging materials Its vapour isvery pungent and irritating Therefore a system that utilizesthis compound must be airtight to prevent environmentalrelease Unlike hydrogen peroxide vapours for which atolerance is allowed inside sealed packages no such toler-ance is allowed for peracetic acid and vapours in the head-space can cause disagreeable vinegar like off- avour insome food products In spite of these problems thiscompound deserves attention as a possible sterilant foraseptic packaging materials because of its effectiveness asa sporicidal agent The presence of small amounts of aceticacid may not be considered off- avours At ambienttemperature and halogen concentration of 100 ppm expo-sure times for effective sterilization would be on the order ofhours Under these conditions it would be impractical to usethese compounds as in-line sterilants in aseptic packagingOne major problem with the use of strong chlorine prepara-tions for sterilization is that available chlorine reacts withorganic material Reaction between chlorine and the materialbeing sterilized may produce off- avours

Beta propiolactone is bactericidal sporicidal fungicidaland virucidal It lacks the penetrating power of the ethyleneoxide but it is considerably more active against microorgan-isms Only 2ndash5 mg of beta propiolactone per litre is requiredfor sterilization compared with 400ndash800 mg of ethyleneoxide per litre However its poor penetration of materialsand its cancer-inducing properties have restricted its use as apractical sterilizing agent

Ethanol is the only industrially signi cant alcohol used asbactericide and fungicide but not for sporicide Theirmechanism of action appears to involve protein denaturationand membrane lipids dissolution Ethanol liquid and vapourhave been suggested (Doyen 1973 Mita 1985) Howeverethanol is effective only against vegetative cells and notagainst fungal conidia or bacterial spores therefore its use inpackaging of foods is limited to extension of shelf lifeof packaged foods which are normally stored underrefrigeration The two most popular alcohol germicides areethanol and isopropanol usually used in about 70ndash80concentration

Chlorine and its oxide and ozone are used mainly forwater disinfection Ozone can be used as a sterilizing agent

because of its high oxidizing capability Death of almost allmicroorganisms usually occurs within 30 min Ozone wasrecommended recently as an alternative to chlorine (Kim1998) and hydrogen peroxide (Khadre and Yousef 2001)They found that hydrogen peroxide at 10000-fold higherconcentration was less effective than ozone against Bacillusspores Antimicrobial power of ozone increases as tempera-ture decreases below ambient (Herbold et al 1989)Comparatively low concentration is needed to eliminatelarge population of spores at ambient temperature in shorttime period and this makes ozone best suited for anindustrial setting

CONCLUSIONS

Sterilization of packaging materials in-line of processingposes certain dif culties Based on the review presented it isevident that hydrogen peroxide sterilization followed by hotair appears to have the most potential for use as in-linesterilants for packaging materials This combination is time-tested and no adverse report seems to have put any doubtover the methodrsquos acceptability Sterilization of packagingmaterials using hydrogen peroxide and followed by ultra-violet irradiation also has been accepted for industrialapplication Dry heat saturated steam and superheatedsteam can be effective sterilants but the degree of heatdamages many packaging materials so they nd limitedapplication Infrared rays cannot be applied as there is atemperature rise of the packaging material due to infraredapplication and this results in softening of the plasticsIonizing rays are not accepted as they have harmful effecton personnel Light pulses have not been adequately studiedso far as their effect on food material is concerned Ethyleneoxide requires a very long time which precludes its use forin-line applications Peracetic acid produces an off- avourin food if residual deposit is enclosed in the container Betapropiolactone lacks the penetrating power of ethylene oxideand ethanol is not effective against spores

REFERENCES

Alguire DE 1973 Ethylene oxide gas sterilization of packaging materialsFood Technol 27(9) 64

Anonymous 1994 The Pure Bright Process Pure Pulse ProprietaryInformation

Anonymous 1995 Biological indicators for dry heat sterilization controland ethylene oxide sterilization control in United States Pharmaco-peia=National Formulary (US Pharmacopeial Convention RockvilleMD) pp 200 202

Anonymous AE 1999 Biological indicators of sterilization in BritishPharmacopeia Vol II (British Pharmacopeia Commission London) ppA281ndashA283

Bachmann R 1975 Sterilization by intense ultraviolet radiation BrownBoveri Rev 62 206ndash209

Bayliss CE and Waites WM 1979a The synergistic killing of spores ofBacillius subtilis by hydrogen peroxide and ultraviolet irradiation FEMSMicrobiol Lett 5 331ndash333

Bayliss CE and Waites WM 1979b The combine effect of hydrogenperoxide and ultraviolet irradiation on bacterial spores J Appl Bacteriol47(2) 263ndash269

Bayliss CE and Waites WM 1982 Effect of simultaneous high intensityultraviolet irradiation and hydrogen peroxide on bacterial spores J FoodTechnol 17 467ndash470

Bernard D 1983 Microbiological considerations of testing asepticprocessing and packaging systems in Processing of National FoodProcessors Association Conference Capitalizing on Aseptic (FPIWashington DC) pp 13ndash14

Trans IChemE Vol 81 Part C March 2003

64 ANSARI and DATTA

Decontamination of pre-formed cartons for aseptic packa-ging poses problems which are not encountered with atsurfaces Therefore UV irradiation of a pre-formed cartonmust be considerably more intense than for at boards toensure that all surfaces are effectively irradiated (Bachmann1975) Experiments with cartons arti cially contaminatedwith Bacillus subtilis spores sprayed with 1 H2O2 andthen irradiated for 10 s with a high-intensity UV sourceabove the carton showed 5 decimal reductions with poly-ethylene lined material and 35 decimal reductions with apolyethylene=aluminium foil laminate (Stannard et al1983) No heat was applied This combination of hydrogenperoxide with UV has now been applied commercially tocarton sterilization during aseptic lling

Infrared rayInfrared rays in the waveband 08ndash15 pound 10iexcl6 are easily

generated On an absorbent surface they are converted intosensible heat producing an increased temperature Theenergy impact on the surface that is convertible into heatis dependent on the same geometric conditions as thoseoutlined for UV rays Thus infrared irradiation is bestapplied to a smooth even surface with vertical radiationincline This method has been used to treat the interior ofaluminium lids with a plastic coating on the exterior surfaceAs the temperature rise can cause a softening of the plasticthe maximum temperature and exposure time are limitedCount reductions on the same order as those for UVirradiation has been found

Ionizing rayIonizing radiation is an excellent sterilizing agent and

penetrates deep into objects Ionizing radiation of primaryinterest includes a-rays X-rays b-rays electron beams andg-rays Bags used in bag-in-box aseptic packaging arecurrently sterilized with g-rays from a 60Co source Radia-tion kills bacteria in the same logarithmic fashion as heatand D-values for many organisms have been reported Adose of 47 Mrad is considered a 12D level for Clostridiumbotulinum which are the most radiation-resistant spores ofpublic health and spoilage signi cance (Urbain 1978) Theeffect of water activity on irradiation by 60Co g-rays wasinvestigated by Harmulv and Snygg (1973) Dosage levelsto inactivate 90 of Bacillus subtilis spores increased from023 to 042 Mrad as the water activity (aw) decreased from100 to 000 For a 999 reduction (3 log) of Bacillusstearothermophilus spores the dosage increased from 17Mrad at an aw of 100 to 300 Mrad at 000

Gamma irradiation has been used to sterilize plastic bagsfor bulk packaging acid foods using an aseptic bag-in-boxsystem (Nelson 1984) The bag made of plastic laminatesare treated in a specialized irradiation plant and given aradiation dose of 25 kGy (25 Mrad) or more which issuf cient to ensure sterility However the dif culty ofmaintaining sterility during the transfer operation to the lling spout of an aseptic lling machine raises somedoubts as to the practicality of gamma irradiation sterilizedpackaging materials for use on low-acid foods The heavyshielding necessary to prevent radiation leakage fromgamma ray source makes the system impractical to use forin-line sterilization Gamma radiation has not yet beenwidely employed in the USA because of the cost andconcerns about the effects of g-radiation on food

Electron beam sterilization of packaging material is aproven state-of-the art technology which outperforms anylegislative and customer requirements Studies have shownthat depending on the level of microorganisms a dose of5ndash7 kGy was found to be effective against yeast mould andspores and no viable microorganism was found after treat-ment Unfavourable effects of electron beam treatment werenot observed High-energy electron beams are not as pene-trating as g-rays and therefore do not require as muchshielding as a g-ray sources The irradiation process takesjust a few seconds and does not leave any type of residueHowever with pre-formed containers the geometry of thedelivery system makes it dif cult to apply a uniform dose onthe whole container In-line processing heavily demandedby industry requires a careful process design Technologicaland economic optimization of electron beam source andshielding are the most important tasks to be accomplishedbefore in-line electron decontamination of food packagingmay be applied on a large scale A major impediment toadoption of this technology in the food industry is the highinitial and operating cost of the units

Care must be used when using ionizing radiation forsterilization of aseptic packaging materials Although alumi-nium foil is not affected by irradiation paper and plasticcould be affected with results ranging from discolourationto loss of pliability and mechanical strength While radiationcan be quite effective the equipment is expensive as is theprotection for workers and the environment Workers are notusually willing to work in such an environment It willprobably be more extensively employed in the future

Light-pulseThe technology of utilizing short pulses of light is an

attractive alternative for sterilizing packaging materials andprocessing equipment in aseptic packaging The spectrum oflight used for sterilization purposes includes wavelengths inthe ultraviolet to wavelengths in the near-infrared regionThe material to be sterilized is exposed to at least one pulseof light having an energy density in the range of about 001ndash50 J cmiexcl2 at the surface The duration (Anonymous 1994)of pulses ranges from 1 ms to 01 s The packaging materialsare typically exposed to between 1 and 20 pulses of light-intensity short-duration light A few ashes applied at arate of 1ndash10 per second provide very high microbial killlevels making the system ideal for continuous in-linesterilization It is desirable for certain aseptic processesthat packaging material be treated with pulses having arelatively high UV content to minimize the total uence(J cmiexcl2) necessary to achieve the desired reduction inmicrobial population (Dunn et al 1991) Comparison ofthe antimicrobial effects obtained using pulsed light withthose obtained using non-pulsed or continuous waveconventional UV sources shows a signi cantly higher inac-tivation for pulsed light More than 7 log cycles of Asper-gillus niger spore inactivation result with few pulsed light ashes A variety of microorganisms including Bacillussubtilis are inactivated by using between 1 and 35 pulsesof light with intensity ranging between about 1 and12 J cmiexcl2 Greater inactivation is obtained when full-spectrum light rather than glass- ltered spectrum light isused Thus the UV component of light is needed to inacti-vate microorganisms (Dunn et al 1991) Spores of Bacillus

Trans IChemE Vol 81 Part C March 2003

STERILIZATION METHODS FOR PACKAGING MATERIALS 61

subtilis Bacillus pumilus Bacillus stearothermophilus andAspergillus niger were inactivated completely from 6ndash8 logsof colony forming units (CFU) with 1ndash3 pulses (Bushnellet al 1998) Bacillus subtilis for example is sterilized(99999) by about 42600mW s cmiexcl2 of UV while requir-ing a dose of only 4500mW s cmiexcl2 under pulsed lightMicroorganisms are inactivated by a combination of photo-chemical and photothermal mechanisms

Light pulses apparently do not affect the nutrient retentionin food although a detailed study is not available Pulsedlight treatment costs are very favourable (Dunn et al 1995)Packaging materials compatible with this process will trans-mit light over the broad spectrum employed LLDPELDPE nylon Aclar HDPE and PP have all been usedPackage geometry should not allow any shadowing on theproduct or the light exposure may not be suf cient

Chemical Methods

Thermal destruction is by far the most common andef cient mode of sterilization Dry or moist heat for ster-ilization is also not always feasible Chemical methods use awide variety of chemicals in the form of liquids and gases todisinfect and sterilize equipment and packaging materials

Hydrogen peroxideHydrogen peroxide is one of the most widely used

sterilants used for sterilizing packaging materials The rstsuccessful aseptic lling system for cartoning the asepticTetra Pak of 1961 used a combination of hydrogen peroxideand heat for the sterilization of the surface of containermaterial (Burton 1988) Many aseptic packaging systemsuse hydrogen peroxide at concentrations from 30 to 35 asa sterilant for packaging materials and other food contactsurfaces followed by hot air (60ndash125macrC) to augment thesterilizing effect and to dissipate residual hydrogen perox-ide Sterilizing performance increases with both peroxideconcentration and temperature At ambient environmentaltemperatures the activity of hydrogen peroxide is relativelyslow however this can be increased considerably by raisingthe temperature to 85ndash90macrC and=or increasing concentration(Smith and Brown 1980 Swartling and Lindgren 1968)Therefore for effective sporicidal action in food processingenvironment treated with hydrogen peroxide (at 30) isfollowed by application of hot air (Yokoyama 1990)Swartling and Lindgren (1968) found that four decimalreductions were obtained after suspension in 20 at 80macrCfor 15 s Ito and Stevenson (1984) state that the challengeorganisms of choice are Bacillus stearothermophilus foraseptic packaging systems that use heat sterilization andstrains of Bacillus subtilis A or Bacillus subtilis var globigiifor ller surfaces that sterilize with hydrogen peroxide andheated air

The following relationship holds for destruction of sporesof Bacillusstearothermophilus(Shapton and Shapton 1998)

log D ˆ 17 iexcl 13 curren log ( H2O2)

where D is the decimal reduction time in minutesSpores resistant to hydrogen peroxide have been reviewed

by Ito (1973) and Stevenson and Shafer (1983) Toledo et al(1973) reported that dry spores of Bacillus subtilis varglobigii (NCIB 8058) were less resistant than the wet

spores Consistent with observations for heat and irradi-ation dry spores are more resistant than wet spores Sincesterilization processes used on aseptic packaging systemsfor low-acid foods must have documentation for adequacyto inactivate pathogenic and spoilage microorganisms a unitmust be tested using an acceptably resistant organism beforeit can be placed in commercial production In commercialaseptic packaging equipment which uses roll stock thepackaging material is immersed in hydrogen peroxidesolution followed by heating to vaporize the peroxidebefore the packages are lled Contact time with the solu-tions which contain a wetting agent is often less than 1 minA large amount of the sterilizing liquid is removed mechani-cally eg by rollers or air blasts and the remainder isgenerally removed by drying with hot or sterile air or radiantheat The nascent oxygen thus produced due to breakdownof peroxide reacts with the oxidizable cell components ofthe microorganisms and causes bactericidal effect Commer-cial packaging systems employ hydrogen peroxide atconcentrations of 10ndash35 at either room or elevatedtemperature (Von Bockelmann 1974 Von Bockelmannand Von Bockelmann 1972) in the form of a spray orimmersion bath The sporicidal action of hydrogen peroxidealso has been reviewed by Von Bockelmann and VonBockelman (1972) Hydrogen peroxide kills a wide varietyof organisms (Wang and Toledo 1986) and viruii (Mentaland Schmidt 1973) Both linear (Swartling and Lindgren1968 Toledo et al 1973) and nonlinear (Cerf and Hermier1972) relationships between log number of survivors andtime of exposure to different concentrations of hydrogenperoxide have been reported and Toledo et alrsquos (1973) datashow that 1 min in 30 hydrogen peroxide at ambienttemperature produced only 25 decimal reductions ofmould spores and bacterial spores were not affected at allThus in current commercial practice the microbicidalaction is primarily due to the heated lm of hydrogenperoxide at the start of the drying phase of the sterilizationprocess For this reason wetting of the packaging materialand the presence of a uniform lm of liquid on the materialsurface are critical factors

When sterilizing pre-formed containers hydrogenperoxide is sprayed or atomized into the container Ameasured amount of hydrogen peroxide is metred intoeach nozzle which delivers the solution into each containerto ensure that a uniform lm coats the inside surface of thepackage For acid food products relatively low microbiallethality required makes it easy to achieve the desired levelof microbial inactivation and meet the residue tolerancelevel However for low-acid foods a system must becarefully designed such that both criteria of commercialsterility and adequate sterilant removal are achieved

Hydrogen peroxide-sterilized aseptic packaging systemsthat have been approved for commercial packaging of low-acid foods have been proven to produce an acceptable levelof residues The Food and Drug administration of the USAhas ruled that the level of hydrogen peroxide that may bepresent in product packaged in material sterilized byhydrogen peroxide must not be greater than 05 ppm(Reuter 1988) An improperly designed system may resultin more incidence of non-compliance with regulationspertaining to residues Prolonged contact between liquidhydrogen peroxide and the packaging material alsoincreased the dif culty of removing residues

Trans IChemE Vol 81 Part C March 2003

62 ANSARI and DATTA

Hydrogen peroxide is frequently used to sterilize thepackaging material as it decomposes to a product whichwill neither taint the food nor render it toxic It is reportedthat the traces of residual of hydrogen peroxide left in foodis less than 025ppm and normally this amount does nothave any harmful effect when consumed (Hedrick 1973Smith and Brown 1980) Residual hydrogen peroxide andthat trapped in the package headspace at the time of sealinghas an adverse effect on product stability particularly onascorbic acid degradation in fruit juices (Toledo 1986)When the concentration of hydrogen peroxide at the time ofpackaging exceeded 01 mg=mliexcl1 a marked increase inascorbic acid degradation in bottled orange juice andorange juice concentrates was observed by Toledo (19751986) Aseptic packaging of foods containing ascorbic acidin a system that uses hydrogen peroxide for sterilization ofmaterial is not desirable unless some means is used toremove hydrogen peroxide vapours from the package headspace and packaging material is completely free fromhydrogen peroxide residues

There are three commercial methods for applyinghydrogen peroxide to packaging materials dippingspraying and rinsing

Dipping the packaging material (ie plastic laminateswith cardboard lms of thermoformable plastics and lami-nates) are taken from a reel and dipped into a bath ofaqueous 30ndash33 hydrogen peroxide (Reuter 1988) Countreductions on the order of 4ndash5 logs are claimed Wettingagents are added to ensure uniform wetting of the surfacesExcess solution is removed by squeeze rolls or air jets afterremoval of the material from the bath which leaves a thin lm of solution that is then dried by the application of hotair To increase the ef cacy especially in the case of dusty orslightly soiled material prior treatment of the material withrotating brushes sterile compressed air jets or ultrasoundapplied to the bath may be added

Spraying this is the method (Reuter 1988) for preformedcontainers in which the hydrogen peroxide is sprayed in theform of small dispersed droplets into the container Sprayingdoes not result in a cohesive lm due to the hydrophobiccharacteristics of plastics rather only 30ndash40 of the innersurfaces of the container are covered and thereby it does notensure good sterility of lm A conventional spray givesdrops of over 30 mm diameters on the surface and only30ndash40 of the surface area is covered An ultrasonic systemcan be used to give particle sizes of only 3 mm diameterwhich will give an average surface cover of about 60The droplet size distribution between 2 and 80 mm and thedistribution of droplets on the inner surfaces of the variousgeometric shapes of the containers is problematical Theef cacy is dependent upon the volume of solution sprayedhowever the larger the volume the longer the drying timeThe drying must be carried out with hot sterile air Improvedaerosol sprayers can limit the droplet size to 2ndash4 mm andincrease the coverage to 60 while permitting reducedvolume of peroxide solution to be applied hence thedrying time This method is being replaced by the use ofa mixture of hot air and vapourized peroxide Sterilizationby hydrogen peroxide vapour would be a cost-effectivealternative as the least amount of hydrogen peroxide isused Furthermore the amount of hydrogen peroxideadsorbed on the treated surface from the vapour phase willbe several orders of magnitude smaller than a liquid

lm Therefore ushing the vapour-treated surface withlow-temperature sterile air free of hydrogen peroxidevapours can effectively eliminate residues

Low levels of hydrogen peroxide which remain in thevapour phase in air at near ambient temperature has beenshown to have sporicidal activity suitable for in-line ster-ilization of aseptic packaging materials (Wang and Toledo1986) Air saturated with hydrogen peroxide vapour carryrelatively low concentration of hydrogen peroxide(76 mg liexcl1 at 70macrC) but can induce 6 decimal reductionsof spores of Bacillus subtilis var niger in 12 min Incomparison the same organism requires 12 min to achieveone decimal reduction in hot air alone at 150macrC Since theamount of hydrogen peroxide vapourized from solution isless than that from water the solution is enriched duringprolonged operation and a system must be devised to addmake-up water to maintain the desired concentration

Rinsing prefabricated intricately shaped containers forwhich the spraying process is unsuitablecan be rinsed with anaqueous 30ndash33 H2O2 solution For ambient temperaturesterilization this can be combined with peracetic acid Thecontainers are drained allowed to drip dry and then com-pletely dried by hot sterile air Glass containers metal cansand blow-moulded plastic bottles are treated in this manner

Ethylene oxideImplementation of ethylene oxide sterilization has made

possible the use of sterile low-cost disposable thermoplas-tic devices for industrial application It has been used to pre-sterilize paperboard cartons (Hedrick 1973) and plasticpackaging materials (Alguire 1973) However no commer-cial system uses ethylene oxide as a sterilizing agent inaseptic packaging of low acid foods for ambient temperaturestorage and distribution The vapour form is ammable andexplosive (Merck 1989) and hence employed as a mixturewith an inert gas such as dichlodi uoromethane Alguire(1973) reported that 100 ethylene oxide has better micro-bicidal properties than an ethylene oxide inert gas mixtureAn important consideration in bulk sterilization of pre-formed packaging materials is ethylene oxidersquos ability topermeate and contact all surfaces that need to be sterilizedThe use of lower concentration of ethylene oxide is saferthan use of 100 ethylene oxide from standpoint ofpotential for injury to personnel The D-value for Clostri-dium botulinum 62 A require a minimum of 72 min for a12D process thus making it impractical to use ethyleneoxide for in-line sterilization of aseptic packaging materialsin high-speed packaging lines Ethylene oxide is effectiveagainst bacteria moulds yeasts as well as viruses The mostprobable activity is alkylation of nucleic acids and othercellular components (Phillips 1977) It is a particularlyeffective sterilizing agent because it rapidly penetratespackaging material The ethylene oxide concentrationhumidity and temperature in uence the rate of sterilizationA clean object can be sterilized if treated for 5ndash8 h at 38macrCor 3ndash4 h at 54macrC when the relative humidity is maintained at40ndash50 and the EtO concentration at 700 mg liexcl1 Extensiveaeration of the sterilized materials is necessary to removeresidual EtO because it is so toxic However one disadvan-tage of it is its comparatively slow rate of action againstmicroorganisms Spores of Bacillus subtilis var niger ATCC9372 are used as indicators in dry heat and ethylene oxidesterilization (Anonymous 1995 1999)

Trans IChemE Vol 81 Part C March 2003

STERILIZATION METHODS FOR PACKAGING MATERIALS 63

Other sterilization methodsThere are many other chemicals such as peracetic acid

beta propiolactone alcohol chlorine and its oxide andozone etc that have been suggested as having potentialfor use in sterilizing aseptic packaging materials Peraceticacid is a peroxide of acetic acid and on decomposition itforms acetic acid and water The low pH and oxidizingproperties make it an excellent sporicidal agent It has all theadvantages of hydrogen peroxide and is unaffected bycatalase and peroxidases Peracetic acid is a liquid sterilantwhich is particularly effective against spores of aerobic andanaerobic bacteria and is effective at lower temperaturesthan hydrogen peroxide In practice it is used in a solutionwhich also contains hydrogen peroxide The solutioncontaining peracetic acid and hydrogen peroxide is effectiveagainst resistant bacterial spores even at 20macrC for example a1 solution will eliminate 10iexcl7ndash10iexcl8 of most resistantspore stains in 5 min at 20macrC and the most resistant strainsin 60 min The maximum usable temperature is 40macrC whenthe sterilization times are about 5 times shorter In spite ofits sporicidal properties peracetic acid is not an approvedsterilant for use on aseptic packaging materials Its vapour isvery pungent and irritating Therefore a system that utilizesthis compound must be airtight to prevent environmentalrelease Unlike hydrogen peroxide vapours for which atolerance is allowed inside sealed packages no such toler-ance is allowed for peracetic acid and vapours in the head-space can cause disagreeable vinegar like off- avour insome food products In spite of these problems thiscompound deserves attention as a possible sterilant foraseptic packaging materials because of its effectiveness asa sporicidal agent The presence of small amounts of aceticacid may not be considered off- avours At ambienttemperature and halogen concentration of 100 ppm expo-sure times for effective sterilization would be on the order ofhours Under these conditions it would be impractical to usethese compounds as in-line sterilants in aseptic packagingOne major problem with the use of strong chlorine prepara-tions for sterilization is that available chlorine reacts withorganic material Reaction between chlorine and the materialbeing sterilized may produce off- avours

Beta propiolactone is bactericidal sporicidal fungicidaland virucidal It lacks the penetrating power of the ethyleneoxide but it is considerably more active against microorgan-isms Only 2ndash5 mg of beta propiolactone per litre is requiredfor sterilization compared with 400ndash800 mg of ethyleneoxide per litre However its poor penetration of materialsand its cancer-inducing properties have restricted its use as apractical sterilizing agent

Ethanol is the only industrially signi cant alcohol used asbactericide and fungicide but not for sporicide Theirmechanism of action appears to involve protein denaturationand membrane lipids dissolution Ethanol liquid and vapourhave been suggested (Doyen 1973 Mita 1985) Howeverethanol is effective only against vegetative cells and notagainst fungal conidia or bacterial spores therefore its use inpackaging of foods is limited to extension of shelf lifeof packaged foods which are normally stored underrefrigeration The two most popular alcohol germicides areethanol and isopropanol usually used in about 70ndash80concentration

Chlorine and its oxide and ozone are used mainly forwater disinfection Ozone can be used as a sterilizing agent

because of its high oxidizing capability Death of almost allmicroorganisms usually occurs within 30 min Ozone wasrecommended recently as an alternative to chlorine (Kim1998) and hydrogen peroxide (Khadre and Yousef 2001)They found that hydrogen peroxide at 10000-fold higherconcentration was less effective than ozone against Bacillusspores Antimicrobial power of ozone increases as tempera-ture decreases below ambient (Herbold et al 1989)Comparatively low concentration is needed to eliminatelarge population of spores at ambient temperature in shorttime period and this makes ozone best suited for anindustrial setting

CONCLUSIONS

Sterilization of packaging materials in-line of processingposes certain dif culties Based on the review presented it isevident that hydrogen peroxide sterilization followed by hotair appears to have the most potential for use as in-linesterilants for packaging materials This combination is time-tested and no adverse report seems to have put any doubtover the methodrsquos acceptability Sterilization of packagingmaterials using hydrogen peroxide and followed by ultra-violet irradiation also has been accepted for industrialapplication Dry heat saturated steam and superheatedsteam can be effective sterilants but the degree of heatdamages many packaging materials so they nd limitedapplication Infrared rays cannot be applied as there is atemperature rise of the packaging material due to infraredapplication and this results in softening of the plasticsIonizing rays are not accepted as they have harmful effecton personnel Light pulses have not been adequately studiedso far as their effect on food material is concerned Ethyleneoxide requires a very long time which precludes its use forin-line applications Peracetic acid produces an off- avourin food if residual deposit is enclosed in the container Betapropiolactone lacks the penetrating power of ethylene oxideand ethanol is not effective against spores

REFERENCES

Alguire DE 1973 Ethylene oxide gas sterilization of packaging materialsFood Technol 27(9) 64

Anonymous 1994 The Pure Bright Process Pure Pulse ProprietaryInformation

Anonymous 1995 Biological indicators for dry heat sterilization controland ethylene oxide sterilization control in United States Pharmaco-peia=National Formulary (US Pharmacopeial Convention RockvilleMD) pp 200 202

Anonymous AE 1999 Biological indicators of sterilization in BritishPharmacopeia Vol II (British Pharmacopeia Commission London) ppA281ndashA283

Bachmann R 1975 Sterilization by intense ultraviolet radiation BrownBoveri Rev 62 206ndash209

Bayliss CE and Waites WM 1979a The synergistic killing of spores ofBacillius subtilis by hydrogen peroxide and ultraviolet irradiation FEMSMicrobiol Lett 5 331ndash333

Bayliss CE and Waites WM 1979b The combine effect of hydrogenperoxide and ultraviolet irradiation on bacterial spores J Appl Bacteriol47(2) 263ndash269

Bayliss CE and Waites WM 1982 Effect of simultaneous high intensityultraviolet irradiation and hydrogen peroxide on bacterial spores J FoodTechnol 17 467ndash470

Bernard D 1983 Microbiological considerations of testing asepticprocessing and packaging systems in Processing of National FoodProcessors Association Conference Capitalizing on Aseptic (FPIWashington DC) pp 13ndash14

Trans IChemE Vol 81 Part C March 2003

64 ANSARI and DATTA

subtilis Bacillus pumilus Bacillus stearothermophilus andAspergillus niger were inactivated completely from 6ndash8 logsof colony forming units (CFU) with 1ndash3 pulses (Bushnellet al 1998) Bacillus subtilis for example is sterilized(99999) by about 42600mW s cmiexcl2 of UV while requir-ing a dose of only 4500mW s cmiexcl2 under pulsed lightMicroorganisms are inactivated by a combination of photo-chemical and photothermal mechanisms

Light pulses apparently do not affect the nutrient retentionin food although a detailed study is not available Pulsedlight treatment costs are very favourable (Dunn et al 1995)Packaging materials compatible with this process will trans-mit light over the broad spectrum employed LLDPELDPE nylon Aclar HDPE and PP have all been usedPackage geometry should not allow any shadowing on theproduct or the light exposure may not be suf cient

Chemical Methods

Thermal destruction is by far the most common andef cient mode of sterilization Dry or moist heat for ster-ilization is also not always feasible Chemical methods use awide variety of chemicals in the form of liquids and gases todisinfect and sterilize equipment and packaging materials

Hydrogen peroxideHydrogen peroxide is one of the most widely used

sterilants used for sterilizing packaging materials The rstsuccessful aseptic lling system for cartoning the asepticTetra Pak of 1961 used a combination of hydrogen peroxideand heat for the sterilization of the surface of containermaterial (Burton 1988) Many aseptic packaging systemsuse hydrogen peroxide at concentrations from 30 to 35 asa sterilant for packaging materials and other food contactsurfaces followed by hot air (60ndash125macrC) to augment thesterilizing effect and to dissipate residual hydrogen perox-ide Sterilizing performance increases with both peroxideconcentration and temperature At ambient environmentaltemperatures the activity of hydrogen peroxide is relativelyslow however this can be increased considerably by raisingthe temperature to 85ndash90macrC and=or increasing concentration(Smith and Brown 1980 Swartling and Lindgren 1968)Therefore for effective sporicidal action in food processingenvironment treated with hydrogen peroxide (at 30) isfollowed by application of hot air (Yokoyama 1990)Swartling and Lindgren (1968) found that four decimalreductions were obtained after suspension in 20 at 80macrCfor 15 s Ito and Stevenson (1984) state that the challengeorganisms of choice are Bacillus stearothermophilus foraseptic packaging systems that use heat sterilization andstrains of Bacillus subtilis A or Bacillus subtilis var globigiifor ller surfaces that sterilize with hydrogen peroxide andheated air

The following relationship holds for destruction of sporesof Bacillusstearothermophilus(Shapton and Shapton 1998)

log D ˆ 17 iexcl 13 curren log ( H2O2)

where D is the decimal reduction time in minutesSpores resistant to hydrogen peroxide have been reviewed

by Ito (1973) and Stevenson and Shafer (1983) Toledo et al(1973) reported that dry spores of Bacillus subtilis varglobigii (NCIB 8058) were less resistant than the wet

spores Consistent with observations for heat and irradi-ation dry spores are more resistant than wet spores Sincesterilization processes used on aseptic packaging systemsfor low-acid foods must have documentation for adequacyto inactivate pathogenic and spoilage microorganisms a unitmust be tested using an acceptably resistant organism beforeit can be placed in commercial production In commercialaseptic packaging equipment which uses roll stock thepackaging material is immersed in hydrogen peroxidesolution followed by heating to vaporize the peroxidebefore the packages are lled Contact time with the solu-tions which contain a wetting agent is often less than 1 minA large amount of the sterilizing liquid is removed mechani-cally eg by rollers or air blasts and the remainder isgenerally removed by drying with hot or sterile air or radiantheat The nascent oxygen thus produced due to breakdownof peroxide reacts with the oxidizable cell components ofthe microorganisms and causes bactericidal effect Commer-cial packaging systems employ hydrogen peroxide atconcentrations of 10ndash35 at either room or elevatedtemperature (Von Bockelmann 1974 Von Bockelmannand Von Bockelmann 1972) in the form of a spray orimmersion bath The sporicidal action of hydrogen peroxidealso has been reviewed by Von Bockelmann and VonBockelman (1972) Hydrogen peroxide kills a wide varietyof organisms (Wang and Toledo 1986) and viruii (Mentaland Schmidt 1973) Both linear (Swartling and Lindgren1968 Toledo et al 1973) and nonlinear (Cerf and Hermier1972) relationships between log number of survivors andtime of exposure to different concentrations of hydrogenperoxide have been reported and Toledo et alrsquos (1973) datashow that 1 min in 30 hydrogen peroxide at ambienttemperature produced only 25 decimal reductions ofmould spores and bacterial spores were not affected at allThus in current commercial practice the microbicidalaction is primarily due to the heated lm of hydrogenperoxide at the start of the drying phase of the sterilizationprocess For this reason wetting of the packaging materialand the presence of a uniform lm of liquid on the materialsurface are critical factors

When sterilizing pre-formed containers hydrogenperoxide is sprayed or atomized into the container Ameasured amount of hydrogen peroxide is metred intoeach nozzle which delivers the solution into each containerto ensure that a uniform lm coats the inside surface of thepackage For acid food products relatively low microbiallethality required makes it easy to achieve the desired levelof microbial inactivation and meet the residue tolerancelevel However for low-acid foods a system must becarefully designed such that both criteria of commercialsterility and adequate sterilant removal are achieved

Hydrogen peroxide-sterilized aseptic packaging systemsthat have been approved for commercial packaging of low-acid foods have been proven to produce an acceptable levelof residues The Food and Drug administration of the USAhas ruled that the level of hydrogen peroxide that may bepresent in product packaged in material sterilized byhydrogen peroxide must not be greater than 05 ppm(Reuter 1988) An improperly designed system may resultin more incidence of non-compliance with regulationspertaining to residues Prolonged contact between liquidhydrogen peroxide and the packaging material alsoincreased the dif culty of removing residues

Trans IChemE Vol 81 Part C March 2003

62 ANSARI and DATTA

Hydrogen peroxide is frequently used to sterilize thepackaging material as it decomposes to a product whichwill neither taint the food nor render it toxic It is reportedthat the traces of residual of hydrogen peroxide left in foodis less than 025ppm and normally this amount does nothave any harmful effect when consumed (Hedrick 1973Smith and Brown 1980) Residual hydrogen peroxide andthat trapped in the package headspace at the time of sealinghas an adverse effect on product stability particularly onascorbic acid degradation in fruit juices (Toledo 1986)When the concentration of hydrogen peroxide at the time ofpackaging exceeded 01 mg=mliexcl1 a marked increase inascorbic acid degradation in bottled orange juice andorange juice concentrates was observed by Toledo (19751986) Aseptic packaging of foods containing ascorbic acidin a system that uses hydrogen peroxide for sterilization ofmaterial is not desirable unless some means is used toremove hydrogen peroxide vapours from the package headspace and packaging material is completely free fromhydrogen peroxide residues

There are three commercial methods for applyinghydrogen peroxide to packaging materials dippingspraying and rinsing

Dipping the packaging material (ie plastic laminateswith cardboard lms of thermoformable plastics and lami-nates) are taken from a reel and dipped into a bath ofaqueous 30ndash33 hydrogen peroxide (Reuter 1988) Countreductions on the order of 4ndash5 logs are claimed Wettingagents are added to ensure uniform wetting of the surfacesExcess solution is removed by squeeze rolls or air jets afterremoval of the material from the bath which leaves a thin lm of solution that is then dried by the application of hotair To increase the ef cacy especially in the case of dusty orslightly soiled material prior treatment of the material withrotating brushes sterile compressed air jets or ultrasoundapplied to the bath may be added

Spraying this is the method (Reuter 1988) for preformedcontainers in which the hydrogen peroxide is sprayed in theform of small dispersed droplets into the container Sprayingdoes not result in a cohesive lm due to the hydrophobiccharacteristics of plastics rather only 30ndash40 of the innersurfaces of the container are covered and thereby it does notensure good sterility of lm A conventional spray givesdrops of over 30 mm diameters on the surface and only30ndash40 of the surface area is covered An ultrasonic systemcan be used to give particle sizes of only 3 mm diameterwhich will give an average surface cover of about 60The droplet size distribution between 2 and 80 mm and thedistribution of droplets on the inner surfaces of the variousgeometric shapes of the containers is problematical Theef cacy is dependent upon the volume of solution sprayedhowever the larger the volume the longer the drying timeThe drying must be carried out with hot sterile air Improvedaerosol sprayers can limit the droplet size to 2ndash4 mm andincrease the coverage to 60 while permitting reducedvolume of peroxide solution to be applied hence thedrying time This method is being replaced by the use ofa mixture of hot air and vapourized peroxide Sterilizationby hydrogen peroxide vapour would be a cost-effectivealternative as the least amount of hydrogen peroxide isused Furthermore the amount of hydrogen peroxideadsorbed on the treated surface from the vapour phase willbe several orders of magnitude smaller than a liquid

lm Therefore ushing the vapour-treated surface withlow-temperature sterile air free of hydrogen peroxidevapours can effectively eliminate residues

Low levels of hydrogen peroxide which remain in thevapour phase in air at near ambient temperature has beenshown to have sporicidal activity suitable for in-line ster-ilization of aseptic packaging materials (Wang and Toledo1986) Air saturated with hydrogen peroxide vapour carryrelatively low concentration of hydrogen peroxide(76 mg liexcl1 at 70macrC) but can induce 6 decimal reductionsof spores of Bacillus subtilis var niger in 12 min Incomparison the same organism requires 12 min to achieveone decimal reduction in hot air alone at 150macrC Since theamount of hydrogen peroxide vapourized from solution isless than that from water the solution is enriched duringprolonged operation and a system must be devised to addmake-up water to maintain the desired concentration

Rinsing prefabricated intricately shaped containers forwhich the spraying process is unsuitablecan be rinsed with anaqueous 30ndash33 H2O2 solution For ambient temperaturesterilization this can be combined with peracetic acid Thecontainers are drained allowed to drip dry and then com-pletely dried by hot sterile air Glass containers metal cansand blow-moulded plastic bottles are treated in this manner

Ethylene oxideImplementation of ethylene oxide sterilization has made

possible the use of sterile low-cost disposable thermoplas-tic devices for industrial application It has been used to pre-sterilize paperboard cartons (Hedrick 1973) and plasticpackaging materials (Alguire 1973) However no commer-cial system uses ethylene oxide as a sterilizing agent inaseptic packaging of low acid foods for ambient temperaturestorage and distribution The vapour form is ammable andexplosive (Merck 1989) and hence employed as a mixturewith an inert gas such as dichlodi uoromethane Alguire(1973) reported that 100 ethylene oxide has better micro-bicidal properties than an ethylene oxide inert gas mixtureAn important consideration in bulk sterilization of pre-formed packaging materials is ethylene oxidersquos ability topermeate and contact all surfaces that need to be sterilizedThe use of lower concentration of ethylene oxide is saferthan use of 100 ethylene oxide from standpoint ofpotential for injury to personnel The D-value for Clostri-dium botulinum 62 A require a minimum of 72 min for a12D process thus making it impractical to use ethyleneoxide for in-line sterilization of aseptic packaging materialsin high-speed packaging lines Ethylene oxide is effectiveagainst bacteria moulds yeasts as well as viruses The mostprobable activity is alkylation of nucleic acids and othercellular components (Phillips 1977) It is a particularlyeffective sterilizing agent because it rapidly penetratespackaging material The ethylene oxide concentrationhumidity and temperature in uence the rate of sterilizationA clean object can be sterilized if treated for 5ndash8 h at 38macrCor 3ndash4 h at 54macrC when the relative humidity is maintained at40ndash50 and the EtO concentration at 700 mg liexcl1 Extensiveaeration of the sterilized materials is necessary to removeresidual EtO because it is so toxic However one disadvan-tage of it is its comparatively slow rate of action againstmicroorganisms Spores of Bacillus subtilis var niger ATCC9372 are used as indicators in dry heat and ethylene oxidesterilization (Anonymous 1995 1999)

Trans IChemE Vol 81 Part C March 2003

STERILIZATION METHODS FOR PACKAGING MATERIALS 63

Other sterilization methodsThere are many other chemicals such as peracetic acid

beta propiolactone alcohol chlorine and its oxide andozone etc that have been suggested as having potentialfor use in sterilizing aseptic packaging materials Peraceticacid is a peroxide of acetic acid and on decomposition itforms acetic acid and water The low pH and oxidizingproperties make it an excellent sporicidal agent It has all theadvantages of hydrogen peroxide and is unaffected bycatalase and peroxidases Peracetic acid is a liquid sterilantwhich is particularly effective against spores of aerobic andanaerobic bacteria and is effective at lower temperaturesthan hydrogen peroxide In practice it is used in a solutionwhich also contains hydrogen peroxide The solutioncontaining peracetic acid and hydrogen peroxide is effectiveagainst resistant bacterial spores even at 20macrC for example a1 solution will eliminate 10iexcl7ndash10iexcl8 of most resistantspore stains in 5 min at 20macrC and the most resistant strainsin 60 min The maximum usable temperature is 40macrC whenthe sterilization times are about 5 times shorter In spite ofits sporicidal properties peracetic acid is not an approvedsterilant for use on aseptic packaging materials Its vapour isvery pungent and irritating Therefore a system that utilizesthis compound must be airtight to prevent environmentalrelease Unlike hydrogen peroxide vapours for which atolerance is allowed inside sealed packages no such toler-ance is allowed for peracetic acid and vapours in the head-space can cause disagreeable vinegar like off- avour insome food products In spite of these problems thiscompound deserves attention as a possible sterilant foraseptic packaging materials because of its effectiveness asa sporicidal agent The presence of small amounts of aceticacid may not be considered off- avours At ambienttemperature and halogen concentration of 100 ppm expo-sure times for effective sterilization would be on the order ofhours Under these conditions it would be impractical to usethese compounds as in-line sterilants in aseptic packagingOne major problem with the use of strong chlorine prepara-tions for sterilization is that available chlorine reacts withorganic material Reaction between chlorine and the materialbeing sterilized may produce off- avours

Beta propiolactone is bactericidal sporicidal fungicidaland virucidal It lacks the penetrating power of the ethyleneoxide but it is considerably more active against microorgan-isms Only 2ndash5 mg of beta propiolactone per litre is requiredfor sterilization compared with 400ndash800 mg of ethyleneoxide per litre However its poor penetration of materialsand its cancer-inducing properties have restricted its use as apractical sterilizing agent

Ethanol is the only industrially signi cant alcohol used asbactericide and fungicide but not for sporicide Theirmechanism of action appears to involve protein denaturationand membrane lipids dissolution Ethanol liquid and vapourhave been suggested (Doyen 1973 Mita 1985) Howeverethanol is effective only against vegetative cells and notagainst fungal conidia or bacterial spores therefore its use inpackaging of foods is limited to extension of shelf lifeof packaged foods which are normally stored underrefrigeration The two most popular alcohol germicides areethanol and isopropanol usually used in about 70ndash80concentration

Chlorine and its oxide and ozone are used mainly forwater disinfection Ozone can be used as a sterilizing agent

because of its high oxidizing capability Death of almost allmicroorganisms usually occurs within 30 min Ozone wasrecommended recently as an alternative to chlorine (Kim1998) and hydrogen peroxide (Khadre and Yousef 2001)They found that hydrogen peroxide at 10000-fold higherconcentration was less effective than ozone against Bacillusspores Antimicrobial power of ozone increases as tempera-ture decreases below ambient (Herbold et al 1989)Comparatively low concentration is needed to eliminatelarge population of spores at ambient temperature in shorttime period and this makes ozone best suited for anindustrial setting

CONCLUSIONS

Sterilization of packaging materials in-line of processingposes certain dif culties Based on the review presented it isevident that hydrogen peroxide sterilization followed by hotair appears to have the most potential for use as in-linesterilants for packaging materials This combination is time-tested and no adverse report seems to have put any doubtover the methodrsquos acceptability Sterilization of packagingmaterials using hydrogen peroxide and followed by ultra-violet irradiation also has been accepted for industrialapplication Dry heat saturated steam and superheatedsteam can be effective sterilants but the degree of heatdamages many packaging materials so they nd limitedapplication Infrared rays cannot be applied as there is atemperature rise of the packaging material due to infraredapplication and this results in softening of the plasticsIonizing rays are not accepted as they have harmful effecton personnel Light pulses have not been adequately studiedso far as their effect on food material is concerned Ethyleneoxide requires a very long time which precludes its use forin-line applications Peracetic acid produces an off- avourin food if residual deposit is enclosed in the container Betapropiolactone lacks the penetrating power of ethylene oxideand ethanol is not effective against spores

REFERENCES

Alguire DE 1973 Ethylene oxide gas sterilization of packaging materialsFood Technol 27(9) 64

Anonymous 1994 The Pure Bright Process Pure Pulse ProprietaryInformation

Anonymous 1995 Biological indicators for dry heat sterilization controland ethylene oxide sterilization control in United States Pharmaco-peia=National Formulary (US Pharmacopeial Convention RockvilleMD) pp 200 202

Anonymous AE 1999 Biological indicators of sterilization in BritishPharmacopeia Vol II (British Pharmacopeia Commission London) ppA281ndashA283

Bachmann R 1975 Sterilization by intense ultraviolet radiation BrownBoveri Rev 62 206ndash209

Bayliss CE and Waites WM 1979a The synergistic killing of spores ofBacillius subtilis by hydrogen peroxide and ultraviolet irradiation FEMSMicrobiol Lett 5 331ndash333

Bayliss CE and Waites WM 1979b The combine effect of hydrogenperoxide and ultraviolet irradiation on bacterial spores J Appl Bacteriol47(2) 263ndash269

Bayliss CE and Waites WM 1982 Effect of simultaneous high intensityultraviolet irradiation and hydrogen peroxide on bacterial spores J FoodTechnol 17 467ndash470

Bernard D 1983 Microbiological considerations of testing asepticprocessing and packaging systems in Processing of National FoodProcessors Association Conference Capitalizing on Aseptic (FPIWashington DC) pp 13ndash14

Trans IChemE Vol 81 Part C March 2003

64 ANSARI and DATTA

Hydrogen peroxide is frequently used to sterilize thepackaging material as it decomposes to a product whichwill neither taint the food nor render it toxic It is reportedthat the traces of residual of hydrogen peroxide left in foodis less than 025ppm and normally this amount does nothave any harmful effect when consumed (Hedrick 1973Smith and Brown 1980) Residual hydrogen peroxide andthat trapped in the package headspace at the time of sealinghas an adverse effect on product stability particularly onascorbic acid degradation in fruit juices (Toledo 1986)When the concentration of hydrogen peroxide at the time ofpackaging exceeded 01 mg=mliexcl1 a marked increase inascorbic acid degradation in bottled orange juice andorange juice concentrates was observed by Toledo (19751986) Aseptic packaging of foods containing ascorbic acidin a system that uses hydrogen peroxide for sterilization ofmaterial is not desirable unless some means is used toremove hydrogen peroxide vapours from the package headspace and packaging material is completely free fromhydrogen peroxide residues

There are three commercial methods for applyinghydrogen peroxide to packaging materials dippingspraying and rinsing

Dipping the packaging material (ie plastic laminateswith cardboard lms of thermoformable plastics and lami-nates) are taken from a reel and dipped into a bath ofaqueous 30ndash33 hydrogen peroxide (Reuter 1988) Countreductions on the order of 4ndash5 logs are claimed Wettingagents are added to ensure uniform wetting of the surfacesExcess solution is removed by squeeze rolls or air jets afterremoval of the material from the bath which leaves a thin lm of solution that is then dried by the application of hotair To increase the ef cacy especially in the case of dusty orslightly soiled material prior treatment of the material withrotating brushes sterile compressed air jets or ultrasoundapplied to the bath may be added

Spraying this is the method (Reuter 1988) for preformedcontainers in which the hydrogen peroxide is sprayed in theform of small dispersed droplets into the container Sprayingdoes not result in a cohesive lm due to the hydrophobiccharacteristics of plastics rather only 30ndash40 of the innersurfaces of the container are covered and thereby it does notensure good sterility of lm A conventional spray givesdrops of over 30 mm diameters on the surface and only30ndash40 of the surface area is covered An ultrasonic systemcan be used to give particle sizes of only 3 mm diameterwhich will give an average surface cover of about 60The droplet size distribution between 2 and 80 mm and thedistribution of droplets on the inner surfaces of the variousgeometric shapes of the containers is problematical Theef cacy is dependent upon the volume of solution sprayedhowever the larger the volume the longer the drying timeThe drying must be carried out with hot sterile air Improvedaerosol sprayers can limit the droplet size to 2ndash4 mm andincrease the coverage to 60 while permitting reducedvolume of peroxide solution to be applied hence thedrying time This method is being replaced by the use ofa mixture of hot air and vapourized peroxide Sterilizationby hydrogen peroxide vapour would be a cost-effectivealternative as the least amount of hydrogen peroxide isused Furthermore the amount of hydrogen peroxideadsorbed on the treated surface from the vapour phase willbe several orders of magnitude smaller than a liquid

lm Therefore ushing the vapour-treated surface withlow-temperature sterile air free of hydrogen peroxidevapours can effectively eliminate residues

Low levels of hydrogen peroxide which remain in thevapour phase in air at near ambient temperature has beenshown to have sporicidal activity suitable for in-line ster-ilization of aseptic packaging materials (Wang and Toledo1986) Air saturated with hydrogen peroxide vapour carryrelatively low concentration of hydrogen peroxide(76 mg liexcl1 at 70macrC) but can induce 6 decimal reductionsof spores of Bacillus subtilis var niger in 12 min Incomparison the same organism requires 12 min to achieveone decimal reduction in hot air alone at 150macrC Since theamount of hydrogen peroxide vapourized from solution isless than that from water the solution is enriched duringprolonged operation and a system must be devised to addmake-up water to maintain the desired concentration

Rinsing prefabricated intricately shaped containers forwhich the spraying process is unsuitablecan be rinsed with anaqueous 30ndash33 H2O2 solution For ambient temperaturesterilization this can be combined with peracetic acid Thecontainers are drained allowed to drip dry and then com-pletely dried by hot sterile air Glass containers metal cansand blow-moulded plastic bottles are treated in this manner

Ethylene oxideImplementation of ethylene oxide sterilization has made

possible the use of sterile low-cost disposable thermoplas-tic devices for industrial application It has been used to pre-sterilize paperboard cartons (Hedrick 1973) and plasticpackaging materials (Alguire 1973) However no commer-cial system uses ethylene oxide as a sterilizing agent inaseptic packaging of low acid foods for ambient temperaturestorage and distribution The vapour form is ammable andexplosive (Merck 1989) and hence employed as a mixturewith an inert gas such as dichlodi uoromethane Alguire(1973) reported that 100 ethylene oxide has better micro-bicidal properties than an ethylene oxide inert gas mixtureAn important consideration in bulk sterilization of pre-formed packaging materials is ethylene oxidersquos ability topermeate and contact all surfaces that need to be sterilizedThe use of lower concentration of ethylene oxide is saferthan use of 100 ethylene oxide from standpoint ofpotential for injury to personnel The D-value for Clostri-dium botulinum 62 A require a minimum of 72 min for a12D process thus making it impractical to use ethyleneoxide for in-line sterilization of aseptic packaging materialsin high-speed packaging lines Ethylene oxide is effectiveagainst bacteria moulds yeasts as well as viruses The mostprobable activity is alkylation of nucleic acids and othercellular components (Phillips 1977) It is a particularlyeffective sterilizing agent because it rapidly penetratespackaging material The ethylene oxide concentrationhumidity and temperature in uence the rate of sterilizationA clean object can be sterilized if treated for 5ndash8 h at 38macrCor 3ndash4 h at 54macrC when the relative humidity is maintained at40ndash50 and the EtO concentration at 700 mg liexcl1 Extensiveaeration of the sterilized materials is necessary to removeresidual EtO because it is so toxic However one disadvan-tage of it is its comparatively slow rate of action againstmicroorganisms Spores of Bacillus subtilis var niger ATCC9372 are used as indicators in dry heat and ethylene oxidesterilization (Anonymous 1995 1999)

Trans IChemE Vol 81 Part C March 2003

STERILIZATION METHODS FOR PACKAGING MATERIALS 63

Other sterilization methodsThere are many other chemicals such as peracetic acid

beta propiolactone alcohol chlorine and its oxide andozone etc that have been suggested as having potentialfor use in sterilizing aseptic packaging materials Peraceticacid is a peroxide of acetic acid and on decomposition itforms acetic acid and water The low pH and oxidizingproperties make it an excellent sporicidal agent It has all theadvantages of hydrogen peroxide and is unaffected bycatalase and peroxidases Peracetic acid is a liquid sterilantwhich is particularly effective against spores of aerobic andanaerobic bacteria and is effective at lower temperaturesthan hydrogen peroxide In practice it is used in a solutionwhich also contains hydrogen peroxide The solutioncontaining peracetic acid and hydrogen peroxide is effectiveagainst resistant bacterial spores even at 20macrC for example a1 solution will eliminate 10iexcl7ndash10iexcl8 of most resistantspore stains in 5 min at 20macrC and the most resistant strainsin 60 min The maximum usable temperature is 40macrC whenthe sterilization times are about 5 times shorter In spite ofits sporicidal properties peracetic acid is not an approvedsterilant for use on aseptic packaging materials Its vapour isvery pungent and irritating Therefore a system that utilizesthis compound must be airtight to prevent environmentalrelease Unlike hydrogen peroxide vapours for which atolerance is allowed inside sealed packages no such toler-ance is allowed for peracetic acid and vapours in the head-space can cause disagreeable vinegar like off- avour insome food products In spite of these problems thiscompound deserves attention as a possible sterilant foraseptic packaging materials because of its effectiveness asa sporicidal agent The presence of small amounts of aceticacid may not be considered off- avours At ambienttemperature and halogen concentration of 100 ppm expo-sure times for effective sterilization would be on the order ofhours Under these conditions it would be impractical to usethese compounds as in-line sterilants in aseptic packagingOne major problem with the use of strong chlorine prepara-tions for sterilization is that available chlorine reacts withorganic material Reaction between chlorine and the materialbeing sterilized may produce off- avours

Beta propiolactone is bactericidal sporicidal fungicidaland virucidal It lacks the penetrating power of the ethyleneoxide but it is considerably more active against microorgan-isms Only 2ndash5 mg of beta propiolactone per litre is requiredfor sterilization compared with 400ndash800 mg of ethyleneoxide per litre However its poor penetration of materialsand its cancer-inducing properties have restricted its use as apractical sterilizing agent

Ethanol is the only industrially signi cant alcohol used asbactericide and fungicide but not for sporicide Theirmechanism of action appears to involve protein denaturationand membrane lipids dissolution Ethanol liquid and vapourhave been suggested (Doyen 1973 Mita 1985) Howeverethanol is effective only against vegetative cells and notagainst fungal conidia or bacterial spores therefore its use inpackaging of foods is limited to extension of shelf lifeof packaged foods which are normally stored underrefrigeration The two most popular alcohol germicides areethanol and isopropanol usually used in about 70ndash80concentration

Chlorine and its oxide and ozone are used mainly forwater disinfection Ozone can be used as a sterilizing agent

because of its high oxidizing capability Death of almost allmicroorganisms usually occurs within 30 min Ozone wasrecommended recently as an alternative to chlorine (Kim1998) and hydrogen peroxide (Khadre and Yousef 2001)They found that hydrogen peroxide at 10000-fold higherconcentration was less effective than ozone against Bacillusspores Antimicrobial power of ozone increases as tempera-ture decreases below ambient (Herbold et al 1989)Comparatively low concentration is needed to eliminatelarge population of spores at ambient temperature in shorttime period and this makes ozone best suited for anindustrial setting

CONCLUSIONS

Sterilization of packaging materials in-line of processingposes certain dif culties Based on the review presented it isevident that hydrogen peroxide sterilization followed by hotair appears to have the most potential for use as in-linesterilants for packaging materials This combination is time-tested and no adverse report seems to have put any doubtover the methodrsquos acceptability Sterilization of packagingmaterials using hydrogen peroxide and followed by ultra-violet irradiation also has been accepted for industrialapplication Dry heat saturated steam and superheatedsteam can be effective sterilants but the degree of heatdamages many packaging materials so they nd limitedapplication Infrared rays cannot be applied as there is atemperature rise of the packaging material due to infraredapplication and this results in softening of the plasticsIonizing rays are not accepted as they have harmful effecton personnel Light pulses have not been adequately studiedso far as their effect on food material is concerned Ethyleneoxide requires a very long time which precludes its use forin-line applications Peracetic acid produces an off- avourin food if residual deposit is enclosed in the container Betapropiolactone lacks the penetrating power of ethylene oxideand ethanol is not effective against spores

REFERENCES

Alguire DE 1973 Ethylene oxide gas sterilization of packaging materialsFood Technol 27(9) 64

Anonymous 1994 The Pure Bright Process Pure Pulse ProprietaryInformation

Anonymous 1995 Biological indicators for dry heat sterilization controland ethylene oxide sterilization control in United States Pharmaco-peia=National Formulary (US Pharmacopeial Convention RockvilleMD) pp 200 202

Anonymous AE 1999 Biological indicators of sterilization in BritishPharmacopeia Vol II (British Pharmacopeia Commission London) ppA281ndashA283

Bachmann R 1975 Sterilization by intense ultraviolet radiation BrownBoveri Rev 62 206ndash209

Bayliss CE and Waites WM 1979a The synergistic killing of spores ofBacillius subtilis by hydrogen peroxide and ultraviolet irradiation FEMSMicrobiol Lett 5 331ndash333

Bayliss CE and Waites WM 1979b The combine effect of hydrogenperoxide and ultraviolet irradiation on bacterial spores J Appl Bacteriol47(2) 263ndash269

Bayliss CE and Waites WM 1982 Effect of simultaneous high intensityultraviolet irradiation and hydrogen peroxide on bacterial spores J FoodTechnol 17 467ndash470

Bernard D 1983 Microbiological considerations of testing asepticprocessing and packaging systems in Processing of National FoodProcessors Association Conference Capitalizing on Aseptic (FPIWashington DC) pp 13ndash14

Trans IChemE Vol 81 Part C March 2003

64 ANSARI and DATTA

Other sterilization methodsThere are many other chemicals such as peracetic acid

beta propiolactone alcohol chlorine and its oxide andozone etc that have been suggested as having potentialfor use in sterilizing aseptic packaging materials Peraceticacid is a peroxide of acetic acid and on decomposition itforms acetic acid and water The low pH and oxidizingproperties make it an excellent sporicidal agent It has all theadvantages of hydrogen peroxide and is unaffected bycatalase and peroxidases Peracetic acid is a liquid sterilantwhich is particularly effective against spores of aerobic andanaerobic bacteria and is effective at lower temperaturesthan hydrogen peroxide In practice it is used in a solutionwhich also contains hydrogen peroxide The solutioncontaining peracetic acid and hydrogen peroxide is effectiveagainst resistant bacterial spores even at 20macrC for example a1 solution will eliminate 10iexcl7ndash10iexcl8 of most resistantspore stains in 5 min at 20macrC and the most resistant strainsin 60 min The maximum usable temperature is 40macrC whenthe sterilization times are about 5 times shorter In spite ofits sporicidal properties peracetic acid is not an approvedsterilant for use on aseptic packaging materials Its vapour isvery pungent and irritating Therefore a system that utilizesthis compound must be airtight to prevent environmentalrelease Unlike hydrogen peroxide vapours for which atolerance is allowed inside sealed packages no such toler-ance is allowed for peracetic acid and vapours in the head-space can cause disagreeable vinegar like off- avour insome food products In spite of these problems thiscompound deserves attention as a possible sterilant foraseptic packaging materials because of its effectiveness asa sporicidal agent The presence of small amounts of aceticacid may not be considered off- avours At ambienttemperature and halogen concentration of 100 ppm expo-sure times for effective sterilization would be on the order ofhours Under these conditions it would be impractical to usethese compounds as in-line sterilants in aseptic packagingOne major problem with the use of strong chlorine prepara-tions for sterilization is that available chlorine reacts withorganic material Reaction between chlorine and the materialbeing sterilized may produce off- avours

Beta propiolactone is bactericidal sporicidal fungicidaland virucidal It lacks the penetrating power of the ethyleneoxide but it is considerably more active against microorgan-isms Only 2ndash5 mg of beta propiolactone per litre is requiredfor sterilization compared with 400ndash800 mg of ethyleneoxide per litre However its poor penetration of materialsand its cancer-inducing properties have restricted its use as apractical sterilizing agent

Ethanol is the only industrially signi cant alcohol used asbactericide and fungicide but not for sporicide Theirmechanism of action appears to involve protein denaturationand membrane lipids dissolution Ethanol liquid and vapourhave been suggested (Doyen 1973 Mita 1985) Howeverethanol is effective only against vegetative cells and notagainst fungal conidia or bacterial spores therefore its use inpackaging of foods is limited to extension of shelf lifeof packaged foods which are normally stored underrefrigeration The two most popular alcohol germicides areethanol and isopropanol usually used in about 70ndash80concentration

Chlorine and its oxide and ozone are used mainly forwater disinfection Ozone can be used as a sterilizing agent

because of its high oxidizing capability Death of almost allmicroorganisms usually occurs within 30 min Ozone wasrecommended recently as an alternative to chlorine (Kim1998) and hydrogen peroxide (Khadre and Yousef 2001)They found that hydrogen peroxide at 10000-fold higherconcentration was less effective than ozone against Bacillusspores Antimicrobial power of ozone increases as tempera-ture decreases below ambient (Herbold et al 1989)Comparatively low concentration is needed to eliminatelarge population of spores at ambient temperature in shorttime period and this makes ozone best suited for anindustrial setting

CONCLUSIONS

Sterilization of packaging materials in-line of processingposes certain dif culties Based on the review presented it isevident that hydrogen peroxide sterilization followed by hotair appears to have the most potential for use as in-linesterilants for packaging materials This combination is time-tested and no adverse report seems to have put any doubtover the methodrsquos acceptability Sterilization of packagingmaterials using hydrogen peroxide and followed by ultra-violet irradiation also has been accepted for industrialapplication Dry heat saturated steam and superheatedsteam can be effective sterilants but the degree of heatdamages many packaging materials so they nd limitedapplication Infrared rays cannot be applied as there is atemperature rise of the packaging material due to infraredapplication and this results in softening of the plasticsIonizing rays are not accepted as they have harmful effecton personnel Light pulses have not been adequately studiedso far as their effect on food material is concerned Ethyleneoxide requires a very long time which precludes its use forin-line applications Peracetic acid produces an off- avourin food if residual deposit is enclosed in the container Betapropiolactone lacks the penetrating power of ethylene oxideand ethanol is not effective against spores

REFERENCES

Alguire DE 1973 Ethylene oxide gas sterilization of packaging materialsFood Technol 27(9) 64

Anonymous 1994 The Pure Bright Process Pure Pulse ProprietaryInformation

Anonymous 1995 Biological indicators for dry heat sterilization controland ethylene oxide sterilization control in United States Pharmaco-peia=National Formulary (US Pharmacopeial Convention RockvilleMD) pp 200 202

Anonymous AE 1999 Biological indicators of sterilization in BritishPharmacopeia Vol II (British Pharmacopeia Commission London) ppA281ndashA283

Bachmann R 1975 Sterilization by intense ultraviolet radiation BrownBoveri Rev 62 206ndash209

Bayliss CE and Waites WM 1979a The synergistic killing of spores ofBacillius subtilis by hydrogen peroxide and ultraviolet irradiation FEMSMicrobiol Lett 5 331ndash333

Bayliss CE and Waites WM 1979b The combine effect of hydrogenperoxide and ultraviolet irradiation on bacterial spores J Appl Bacteriol47(2) 263ndash269

Bayliss CE and Waites WM 1982 Effect of simultaneous high intensityultraviolet irradiation and hydrogen peroxide on bacterial spores J FoodTechnol 17 467ndash470

Bernard D 1983 Microbiological considerations of testing asepticprocessing and packaging systems in Processing of National FoodProcessors Association Conference Capitalizing on Aseptic (FPIWashington DC) pp 13ndash14

Trans IChemE Vol 81 Part C March 2003

64 ANSARI and DATTA

Blake DF and Stumbo CR 1970 Ethylene oxide resistance of micro-organisms important in spoiling of acid and high acid foods J Food Sci35(1) 26ndash29

Brandli G 1975 A new source of intense shortwave ultraviolet radiationBrown Boveri Rev 62 202ndash205

Burton H 1988 Ultra High Temperature Processing of Milk and MilkProducts (Elsevier Applied Science London)

Bushnell A Cooper JR Dunn J Leo F and May R 1998 Pulsed lightsterilization tunnels and sterile-pass-through Pharm Eng March=April48ndash58

Cerf O and Hermier J 1972 Diversite de la resistance des spores debacillus a leau oxygenee Le Lait 52 1ndash20

Cerny G 1977 Sterilization of packaging materials for aseptic packaging2 Report Investigation of the germicidal effects of UV-C rays Verpack-ungs-Rdsch Tech-nisch-wiss Beilage 28(10) 77ndash82

Collier CP and Townsend CT 1956 The resistance of bacterial spores tosuperheated steam Food Technol 10(10) 477

Denny CB and Mathys AW 1975 NCA test on dry heat as a means ofsterilization of containers lids and a closing unit for aseptic canningFinal Report on RF4614 December 1975 (National Canners AssociationResearch Foundation Washington DC)

Denny CB Brown CK and Yao MG 1974 NCA tests of prime pakaseptic canning machine using hydrogen peroxide for equipment andcontainer sterilization March 20ndash23 and 26ndash28 1973 Research Reportno 1-74 (National Canners Association Washington DC)

Doyen L 1973 Aseptic system sterilizes pouches with alcohol and UVFood Technol 27(9) 49

Dunn J Ott W and Clark W 1995 Pulsed light treatment of food andpackaging Food Technol 49(9) 95ndash98

Dunn JE Clark RW Asmus JF Pearlman JS Boyer K Pairchaud Fand Hofman G 1991 Methods and apparatus for preservation offoodstuffs US Patent no 5 034 235

Harmulv BG Snygg BG 1973 Radiation resistance of spores of Bsubtilis and B stearothermophilus at various water activities J ApplBacteriol 30 677

Hedrick TL 1973 Aseptic packaging in paperboard container FoodTechnol 27(9) 64

Herbold K Flehming B and Botzenhart K 1989 Comparison of ozoneinactivation in owing water or hepatitis A virus poliovirus1 andindicator organisms Appl Environ Microbiol 55 2949ndash2953

Ito KA 1973 Resistance of bacterial spores to hydrogen peroxide FoodTechnol 27(11) 58ndash66

Ito KA and Stevenson KE 1984 Sterilization of packaging materialsusing aseptic systems Food Technol 38(3) 60ndash62

Khadre MA and Yousef AE 2001 Sporicidal action of ozone andhydrogen peroxide a comparative case study Int J Food Microbiol 71131ndash138

Kim JG 1998 Ozone as an antimicrobial agent in minimally pro-cessed foods PhD Thesis The Ohio State University Columbus OH

Kodera T 1983 Method and apparatus for sterilizing food packages or thelike US Patent no 4 396 582

Larousse J and Brown BE 1997 Food Canning Technology (John Wileyand Sons Inc New York)

Maunder DT 1977 Possible use of ultraviolet sterilization of containersfor aseptic packaging Food Technol 31(4) 36ndash37

Mental R and Schmidt J 1973 Investigation on rhinovirus inactivation byhydrogen peroxide Acta Virol 17 351ndash354

Merck 1989 Merck Index 11th edition (Merck amp Co Rahway NJ)Mita K 1985 Japanese Patent no 60256457Nelson PE 1984 Outlook for aseptic bag-in-boxpackaging of products for

remanufacture Food Technol 38(3) 72ndash73

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Phillips CR 1977 Gaseous sterilization in Disfection Sterilization andPreservation 2nd edn Block SS (ed) (Lea and Febiger London)

Reuter H 1988 Aseptic Packaging of Food (Technomic Lancaster PA)Russell AD 1982 The Destruction of Bacterial Spores (Academic Press

San Francisco CA)ShaptonDA and ShaptonNF 1998 Principles and Practices for the Safe

Processing of Foods (Woodhead Cambridge)Smith QJ and Brown KL 1980 The resistance of dry spores of bacillus

subtilis var globigii (NCIB 8058) to solutions of hydrogen peroxide inrelation to aseptic packaging J Food Technol 15 169ndash179

Stannard CJ Abbiss JS and Wood JM 1983 Combined treatment withhydrogen peroxide and ultra-violet irradiation to reduce microbiologicalcontamination levels in pre-formed food packaging cartons J Food Prot46(12) 1060ndash1064

Stevenson KE and Shafer BD 1983 Bacterial spore resistance tohydrogen peroxide Food Technol 37(11) 111ndash114

Sturm W and Gilliand A 1974 Einsatz physilkalischer Sterilisationsver-fahren bel der Herstellung aseptischer Packungen Verpack-Rdschau25 298ndash302

Swartling P and Lindgren B 1968 The sterilizing effect against Bacillussubtilis spores of hydrogen peroxide at different temperatures andconcentration J Dairy Res 35 423ndash428

Toledo RT 1975 Chemical sterilants for aseptic packaging Food Technol29(5) 102ndash112

Toledo RT 1986 Post processing changes in aseptically packed beveragesJ Agric Food Chem 34(3) 405ndash408

Toledo RT Escher FE and Ayres JC 1973 Sporicidal properties ofhydrogen peroxide against food spoilage organisms Appl Microbiol 26592ndash597

Urbain WM 1978 Food irradiation Adv Food Res 24 155Von Bockelmann I 1974 SIK Rapport (Alnarp Sweden) no 359 p 86Von Bockelmann ILI and Von Bockelmann BAH 1972 The sporicidal

action of hydrogen peroxide a literature review Lebensm Wisensch-Technol 5 221ndash225

Wainess H 1982 Practical application of UHT Dairy Rec 83(2) 13583(3) 119

Wallhauser KH 1978 SterilizationDesinfektion Konservierung (ThiemeStuttgart)

Wang J and Toledo RT 1986 Sporicidal properties of mixtures ofhydrogen peroxide vapor and hot air Food Technol 40(2) 60ndash67

Yokoyama M 1990 Aseptic packaged foods in Food Packaging KadoyeT (ed) (Academic Press New York) pp 213ndash228

ACKNOWLEDGEMENT

Financial support from the Indian Council of Agricultural Research isgreatly acknowledged

ADDRESS

Correspondence concerning this paper should be addressed to ProfessorAK Datta Agricultural and Food Engineering Department Indian Instituteof Technology Kharagpur West Bengal-721302 IndiaE-mail akdagfeiikgpernetin

The manuscript was received 23 May 2002 and accepted for publicationafter revision 27 September 2002

Trans IChemE Vol 81 Part C March 2003

STERILIZATION METHODS FOR PACKAGING MATERIALS 65