Vol 71 Nr 3 2006mdashJOURNAL OF FOOD SCIENCE S243Published on Web 3272006

copy 2006 Institute of Food TechnologistsFurther reproduction without permission is prohibited

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utritiv

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Rheological Properties of Tomato-based Products after Thermaland High-pressure TreatmentIIIIISABELSABELSABELSABELSABEL VVVVVERLENTERLENTERLENTERLENTERLENT M M M M MARARARARARCCCCC H H H H HENDRICKXENDRICKXENDRICKXENDRICKXENDRICKX P P P P PIERPIERPIERPIERPIERPAAAAAOLOLOLOLOLOOOOO R R R R ROOOOOVEREVEREVEREVEREVERE P P P P PAAAAAULULULULULAAAAA M M M M MOLDENAERSOLDENAERSOLDENAERSOLDENAERSOLDENAERS ANDANDANDANDAND A A A A ANNNNNNNNNN VVVVVANANANANAN L L L L LOEOEOEOEOEYYYYY

ABSTRAABSTRAABSTRAABSTRAABSTRACTCTCTCTCT D D D D Drrrrrastic losses in the rastic losses in the rastic losses in the rastic losses in the rastic losses in the rheological prheological prheological prheological prheological properoperoperoperoperties of tomato homogenate wties of tomato homogenate wties of tomato homogenate wties of tomato homogenate wties of tomato homogenate wererererere obsere obsere obsere obsere observvvvved when thered when thered when thered when thered when thermally trmally trmally trmally trmally treatedeatedeatedeatedeatedat atmospherat atmospherat atmospherat atmospherat atmospheric pric pric pric pric pressuressuressuressuressure and the highest loss was found at 60e and the highest loss was found at 60e and the highest loss was found at 60e and the highest loss was found at 60e and the highest loss was found at 60 degC degC degC degC degC These losses wThese losses wThese losses wThese losses wThese losses wererererere more more more more more pre pre pre pre pronounced with incronounced with incronounced with incronounced with incronounced with increas-eas-eas-eas-eas-ing pring pring pring pring pressuressuressuressuressures up to 300 MPes up to 300 MPes up to 300 MPes up to 300 MPes up to 300 MPa after which the negativa after which the negativa after which the negativa after which the negativa after which the negative change in re change in re change in re change in re change in rheological prheological prheological prheological prheological properoperoperoperoperties of tomato homogenate de-ties of tomato homogenate de-ties of tomato homogenate de-ties of tomato homogenate de-ties of tomato homogenate de-crcrcrcrcreased Aeased Aeased Aeased Aeased At tempert tempert tempert tempert temperaturaturaturaturatures up to 60es up to 60es up to 60es up to 60es up to 60 degC combined with 500 MPdegC combined with 500 MPdegC combined with 500 MPdegC combined with 500 MPdegC combined with 500 MPa fora fora fora fora formation of a tomato gel strmation of a tomato gel strmation of a tomato gel strmation of a tomato gel strmation of a tomato gel structuructuructuructuructure occurre occurre occurre occurre occurred and aned and aned and aned and aned and animprimprimprimprimprooooovvvvvement in the rement in the rement in the rement in the rement in the rheological prheological prheological prheological prheological properoperoperoperoperties of tomato homogenate was obserties of tomato homogenate was obserties of tomato homogenate was obserties of tomato homogenate was obserties of tomato homogenate was observvvvved Hed Hed Hed Hed Hooooowwwwwevevevevevererererer at higher temper at higher temper at higher temper at higher temper at higher temperaturaturaturaturaturesesesesesand 500 MPand 500 MPand 500 MPand 500 MPand 500 MPa the ra the ra the ra the ra the rheological prheological prheological prheological prheological properoperoperoperoperties of the tomato prties of the tomato prties of the tomato prties of the tomato prties of the tomato product woduct woduct woduct woduct wererererere unaltere unaltere unaltere unaltere unaltered and no gel was fored and no gel was fored and no gel was fored and no gel was fored and no gel was formedmedmedmedmed

KKKKKeyworeyworeyworeyworeywords tomato homogenateds tomato homogenateds tomato homogenateds tomato homogenateds tomato homogenate r r r r rheological prheological prheological prheological prheological properoperoperoperopertiestiestiestiesties ther ther ther ther thermal and high-prmal and high-prmal and high-prmal and high-prmal and high-pressuressuressuressuressure pre pre pre pre processingocessingocessingocessingocessing

Introduction

The consumer demand for high-quality processed products withfresh-like characteristics has increased remarkably in the past few

years Preferences shift toward fresh healthy and rich-flavored ready-to-eat foods with enhanced shelf life Tomato is one of the most impor-tant fruit products It is mainly marketed as processed products thatis pastes concentrates ketchup salsa and so on Besides microbialsafety important quality aspects of such tomato products are colortaste and viscosity (Gould 1992) Viscosity changes during processingof tomato fruits are closely related to changes in pectin a cell wallpolysaccharide owing to the action of pectinmethylesterase (PME) andpolygalacturonase (PG) Hence both enzyme systems should be con-trolled during processing to obtain the desired viscosity

Nowadays 2 conventional thermal processes namely ldquocold breakrdquoand ldquohot breakrdquo procedures are applied in the industrial productionof tomato based products In a ldquocold breakrdquo process the chopped to-matoes are pumped into a heat exchanger and preheated to a tem-perature of approximately 655 degC whereby the pectolytic enzymesPG and PME present in the tomatoes retain a sufficient part of theiractivity and consequently are able to degrade the cell wall pectinduring subsequent processing Hence quality defects such as a de-creased viscosity and syneresis occur although such products appearto keep their natural tomato color and fresh flavor To overcome prob-lems of viscosity loss and syneresis a ldquohot breakrdquo process can beapplied In such a thermal process the chopped tomatoes are imme-diately preheated to a temperature between 77 degC and 93 degC In thiscase the pectolytic enzymes are inactivated resulting in a more vis-cous tomato product that does not separate upon standing Howeverquality losses in terms of flavor color and nutritional value arecaused due to this severe thermal treatment (Gould 1992 Saacutenchezand others 2002 Tiziani and Vodovotz 2005)

Therefore there is a growing interest in food processing and pres-ervation technologies that do not make use of heat or at least thatreduce the heat input of conventional technologies by reducingtreatment time andor temperature (Mertens and Knorr 1992) Oneof the technologies that is a possible alternative for the conventionalthermal processes is high-pressure processing Besides the possibil-ity to inactivate vegetative microorganisms (Mertens 1992 Knorr1993) it is also known that high-pressure processing may display ei-ther enhancement or reduction in enzyme activity (Hendrickx andothers 1998) In this way to improve or preserve the viscosity of toma-to-based products high-pressure processing may be used to selec-tively inactivate PG while maintaining PME activity (Crelier and oth-ers 2001 Fachin and others 2002 2003 2004) An additionaladvantage of using high pressure is the improved preservation of thenutritional and sensorial quality of processed products (Knorr 1993)

The influence of thermal processing on the rheological propertiesof tomato-based products has already been partly described in theliterature but only for temperatures higher than 65 degC (Xu and others1986 Saacutenchez and others 2002 Valencia and others 2003) Howeverliterature on the effect of high-pressure processing on the rheolog-ical properties of such products is very scarce (Porretta and others1995 Krebbers and others 2003) So the aim of the present study isto investigate the changes in rheological properties of tomato homo-genate after processing in a broad temperature-pressure domain

Materials and Methods

A batch of light red tomatoes (Lycopersicon esculentum var Flan-dria Prince Tradiro) was purchased at commercial maturity Ster-

ilized cold (65 degC) and hot (95 degC) break tomato puree both concen-trated to about 125deg Brix by vacuum heating (62 degC 02 bar) were pro-duced by La Stazione Sperimentale per lrsquoIndustria delle ConserveAlimentari (SSICA Parma Italy) All chemicals were of analytical grade

PPPPPrrrrreparepareparepareparation of tomato homogenateation of tomato homogenateation of tomato homogenateation of tomato homogenateation of tomato homogenateTomatoes (~350 g) equilibrated at room temperature were

washed and chopped into large pieces and their seeds were removedby hand The tomato pieces were then homogenized twice in a blend-er (Buumlchi Mixer B-400 Flawil Switzerland 9000 trmin) for 5 s Halfof the homogenized sample was kept as control The other part of

JFS S Sensory and Nutritive Qualities of Food

MS 20050510 Submitted 82305 Revised 101305 Accepted 122505 Au-thors Verlent Hendrickx and Van Loey are with Centre for Food and Micro-bial Technology Faculty of Bioscience Engineering Katholieke Univ LeuvenKasteelpark Arenberg 22 B-3001 Leuven Belgium Author Rovere is with LaStazione Sperimentale per lrsquoIndustria delle Conserve Alimentari (SSICA)Parma Italy Author Moldenaers is with Dept of Chemical Engineering Fac-ulty of Engineering Katholieke Univ Leuven Leuven Belgium Direct in-quiries to author Hendrickx (E-mail MarcHendrickxbiwkuleuvenbe)

S Sensory amp Nutritive Qualities of Food

S244 JOURNAL OF FOOD SCIENCEmdashVol 71 Nr 3 2006 URLs and E-mail addresses are active links at wwwiftorg

Rheological properties of tomato product

the sample was packed and thermal or pressure treated as de-scribed subsequently The total time needed to prepare the packedhomogenized sample and consequently to start the thermal or pres-sure treatment was standardized at 15 min

Thermal treatment of tomato homogenateThermal treatment of tomato homogenateThermal treatment of tomato homogenateThermal treatment of tomato homogenateThermal treatment of tomato homogenateHalf of the prepared tomato homogenate (~150 g) was packed in

double polyethylene plastic bags (250 360 mm thickness 50 micronMedisch Labo Service Menen Belgium) and vacuum-sealed (MultivacA30016 Wolfertschwenden Germany) up to 35 mbar The sample wasspread over the whole internal area of the plastic bag resulting in amaximal average thickness of the filled bags of only 2 mm to avoid orreduce temperature gradients in the sample The sample was placedin a temperature-controlled water bath during 15 min and afterwardimmediately cooled in ice water for 2 min After equilibration at roomtemperature for 2 min the consistency of the sample was measuredby means of a Bostwick consistometer (see ldquoBostwick consistometerrdquosection) During the thermal treatment of the sample the consistencyof the control was also measured

CCCCCombined prombined prombined prombined prombined pressuressuressuressuressure-tempere-tempere-tempere-tempere-temperaturaturaturaturatureeeeetrtrtrtrtreatment of tomato homogenateeatment of tomato homogenateeatment of tomato homogenateeatment of tomato homogenateeatment of tomato homogenate

The combined pressure-temperature experiments were performedin a pilot-scale single vessel high-pressure equipment (SO5-7422-0warm isostatic press Engineered Pressure Systems Int Temse Bel-gium) with a volume of 590 mL (5-cm dia 30-cm length) The appa-ratus allows pressurization up to 600 MPa in combination with tem-peratures from ndash30 degC to 100 degC The high-pressure pumping systemuses an electrically driven high-pressure intensifier with a displace-ment of 83 mLmin The pressure medium is a propylene glycol-based fluid (60 Dowcal N The Dow Chemical Co Horgen Switzer-land) A cryostat allows heating or cooling of the system from theoutside of the vessel An overshoot of pressure as compared with thepreset pressure is always observed due to technical limitations of theequipment which makes it difficult to control the exact pressure ofthe high-pressure process Consequently the conditions of the high-pressure process could slightly deviate from the desired conditions

Half of the prepared tomato homogenate was poured in a flexiblepolyethylene plastic flask (LDPE ~100 mL Medisch Labo Service)further packed in double polyethylene plastic bags (120 170 mmMedisch Labo Service) and vacuum-sealed (Multivac A30016 Wolf-ertschwenden Germany) up to 35 mbar The sample was placed in thevessel already equilibrated at a preset temperature Pressurization wasdone automatically up to the preset pressure and after 15 min of treat-ment time the pressure was manually released and the sample wasimmediately cooled in ice water for 2 min After equilibration at roomtemperature for 2 min the consistency of the sample was measuredby means of a Bostwick consistometer (see the ldquoBostwick consistom-eterrdquo section) During the pressure treatment of the sample the con-sistency of the control was also measured

Rheological measurRheological measurRheological measurRheological measurRheological measurementsementsementsementsementsBBBBBostwick consistometerostwick consistometerostwick consistometerostwick consistometerostwick consistometer The Bostwick consistometer is used to

determine the consistency of the tomato homogenate by determininghow far the homogenate flows under its own weight along a centimeterscaled level surface in 30 s The farthest point of flow on the scale at theend of this time period was recorded as the index of consistency (cm)for the tomato homogenate To determine the alteration (loss or im-provement) in consistency (cm) between thermal or pressure treated(sample) and untreated tomato homogenate (control) the index ofconsistency of the control was subtracted from the index of consistencyof the sample Note that after each measurement the consistometerwas washed with water and dried completely before using again

Drying is necessary as a moist surface will decrease the friction coef-ficient of the instrument and will result in false readings

Immediately after the consistency measurement of sample orcontrol the homogenate was packed in double polyethylene plasticbags (250 360 mm thickness 50 micron) vacuum-sealed up to 35mbar and treated at 90 degC for 15 min to inactivate the enzymeswhich are possibly still present in the tomato homogenate Aftercooling in ice water the homogenate was stored at 4 degC until deter-mination of its rheological properties by means of a rotational rhe-ometer (see the ldquoRotational rheometerrdquo section)

The reproducibility of the consistency measurements was testedfor 2 selected (T P) conditions namely at 40 degC and atmosphericpressure (01 MPa) and at 40 degC and elevated pressure (300 MPa) Atleast 3 replications were performed for each condition (T P) Theaverage loss in consistency was found to be 655 plusmn 045 cm at 01 MPaand 1210 plusmn 110 cm at 300 MPa yielding a standard deviation of lessthan 10 for both (T P) conditions

RRRRRotational rotational rotational rotational rotational rheometerheometerheometerheometerheometer To obtain rheological parameters suchas viscosity () and viscoelastic properties (storage [Grsquo] and loss[Grdquo] moduli) steady-shear and oscillatory-shear experiments werecarried out by means of the rotational Physica Modular CompactRheometer (MCR) 300 (Anton Paar GmbH Graz Austria) using a 6-bladed vane geometry The primary reason for using the vane ge-ometry instead of the cylinder for shearing the tomato homogenatewas to eliminate serious wall-slip effects An additional advantageof using the vane is the minimum amount of disturbance of thethixotropic homogenate when adding the vane into the tomatosample (Barnes and Nguyen 2001) Temperature was controlled bymeans of Peltier elements and kept at 200 degC

The tomato homogenate was poured into the cup and the vanewas lowered into the sample First steady-shear experiments de-scribing the viscosity of the tomato homogenate were carried outusing shear rates from 10s to 01s The time duration for eachmeasuring point was 20 s and a total of 14 points were obtained foreach sample Second a 5-min rest period (shear rate was 0s) wasintroduced because tomato-based products are thixotropic Hencethe network disturbed in the steady-shear experiments could re-store again Third oscillatory-shear experiments measuring theviscoelastic properties of the tomato homogenate were done atangular frequencies between 100 and 01 rads using a constantstrain of 01 Finally amplitude tests (oscillation) were carried outin which the angular frequency is kept constant at 10 rads whilevarying the strain between 001 and 10 This final test is requiredbecause the strain amplitude used in the oscillatory-shear experi-ments should be situated in the linear viscoelastic region that iswhere the storage modulus Grsquo is nearly invariant with strain

The reproducibility of the data obtained with the rheometer was alsotested for 2 selected (T P) conditions namely at 40 degC and atmosphericpressure (01 MPa) and at 40 degC and elevated pressure (300 MPa) At least3 replications were performed for each condition (T P) A deviation ofless than 5 with regard to both the average viscosity and the averagemoduli was found for both (T P) conditions Hence measurementsperformed with the rotational rheometer were reproducible

CCCCConsistency of theronsistency of theronsistency of theronsistency of theronsistency of thermal-trmal-trmal-trmal-trmal-treatedeatedeatedeatedeatedcoldhot brcoldhot brcoldhot brcoldhot brcoldhot break tomato pureak tomato pureak tomato pureak tomato pureak tomato pureeeeeeeeee

Originally the aim was to use hot andor cold break tomato pu-rees as real tomato-based products from which PME and PG wereinactivated First Tomato PG and PME were extracted using a meth-od modified from that of Pressey (1986) which is described by Verlentand others (2004) Further different amounts of these purified tomatoPME (1 PME unit is defined as the amount of enzyme that produc-es 1 mol of acid per min at pH 7 and 22 degC [Verlent and others

Vol 71 Nr 3 2006mdashJOURNAL OF FOOD SCIENCE S245

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Rheological properties of tomato product

2004]) (30 to 200 units100 L) and PG (1 PG unit is defined as theamount of enzyme that produces 1 mol of reducing groups permin at pH 44 and 35 degC [Verlent and others 2004]) (10 to 150 units100 L) enzymes were added to several samples of 150 g of eitherthe hot or the cold break tomato purees The enzyme-puree mix-tures were then subjected to thermal treatment at temperatures of30 degC 40 degC 50 degC or 60 degC for 1 h performed as described for to-mato homogenate (see ldquoThermal treatment of tomato homoge-naterdquo section) followed by a consistency measurement with theBostwick consistometer (see ldquoBostwick consistometerrdquo section)However at all enzyme concentrations tested these treatmentshad no effect on the consistency of the tomato puree that is no dif-ference was observed between the index of consistency of the sam-ple and the index of consistency of the respective control or un-treated hotcold break tomato puree Besides the fact thatinterfering compounds can be present in real food systems anotherpossible explanation may be that free enzymes cannot easily ac-cess pectin in contrast to endogenous enzymes Hence to counterthis problem another real tomato-based product was used namelythe previously described tomato homogenate in which the en-zymes are endogenous and active

CCCCConsistency of high-pronsistency of high-pronsistency of high-pronsistency of high-pronsistency of high-pressuressuressuressuressure-tre-tre-tre-tre-treated tomato pieceseated tomato pieceseated tomato pieceseated tomato pieceseated tomato pieces Tomatoes(~350 g) equilibrated at room temperature were washed andchopped and their seeds were removed by hand Half of the pieces waskept as control and homogenized (Buumlchi Mixer B-400 Flawil Switzer-land 9000 trmin) twice for 5 s after which the consistency was mea-sured with the Bostwick consistometer (see the ldquoBostwick consistom-eterrdquo section) The other portion of tomato pieces was packed in doublepolyethylene plastic bags (60 220 mm thickness 50 microns Me-disch Labo Service) and vacuum-sealed (Multivac A30016 Wolfertsch-wenden Germany) up to 35 mbar After a pressure treatment with thepilot-scale single-vessel high-pressure equipment (SO5-7422-0 warmisostatic press Engineered Pressure Systems Int) the sample wasimmediately cooled in ice water After equilibration at room temper-ature for 2 min the sample was homogenized after which the consis-tency of the sample was also measured with the Bostwick consistom-eter (see the ldquoBostwick consistometerrdquo section)

Results and Discussion

Rheological prRheological prRheological prRheological prRheological properoperoperoperoperties of tomatoties of tomatoties of tomatoties of tomatoties of tomatohomogenate after therhomogenate after therhomogenate after therhomogenate after therhomogenate after thermal trmal trmal trmal trmal treatmenteatmenteatmenteatmenteatment

The effect of temperature on the alteration in the consistency theviscosity and the viscoelasticity of tomato homogenate was studiedfor temperatures ranging from 30 degC to 90 degC

At all temperatures tested loss in consistency of the tomato homo-genate after thermal treatment was observed that is the index ofconsistency of the sample was always higher than the index of consis-tency of the respective control yielding positive values (Figure 1 andFigure 2) The highest loss in consistency was found at 60 degC (Figure 1)

Generally loss in consistency is due to pectin depolymerization bythe action of PG on pectin However PME also contributes indirect-ly to loss in consistency as PME creates a good substrate for PG Ver-lent and others (2005 ldquoEffect of temperature and pressure on the com-bined action of purified tomato pectinmethylesterase andpolygalacturonase in presence of pectinrdquo submitted to Enzyme andMicrobial Technology) investigated the effect of temperature and pres-sure on the combined action of purified tomato PME and PG in thepresence of pectin at pH 44 and found an optimal temperature forPME activity and PG activity around 60 degC and 50 degC respectivelyexplaining the drastic and even highest loss (60 degC) in consistency oftomato homogenate treated at these temperatures The fact that onlyPG contributes to loss in consistency was investigated in literature

(Porretta and Poli 1997 Errington and others 1998) by using transgen-ic tomato fruits which had reduced amounts of PG activity Theyfound no loss in consistency of both cold (655 degC) and hot (77 degC to93 degC) break tomato puree as PG activity was inhibited Errington andothers (1998) also investigated the alteration in consistency whenusing transgenic tomatoes with reduced amounts of PME activity Theyfound no significant differences between the consistency of the controland the transgenic tomato sample indicating that PME alone cannotcause loss in consistency and consequently that no good substrate canbe created for PG because of the reduced PME activity

Figure 3 represents steady-shear experiments describing theviscosity (Pas) of tomato homogenate in the function of the shearrate (s) after treatment for 15 min at various temperatures Thefigure shows how the viscosity values decrease with increasingshear rate at all given temperatures indicating that tomato homo-genate displays shear-thinning flow behavior (this behavior is alsocalled ldquopseudoplasticrdquo) This specific non-Newtonian behavior isalso described in the available literature (Fito and others 1983 Ver-cet and others 2002 Tiziani and Vodovotz 2005) The viscosity curve

Figure 1mdashLoss in consistency of tomato homogenate afterthermal treatment at 01 MPa for 15 min

Figure 2mdashComparison of the index of consistency of untreatedtomato homogenate or control (a) with tomato homogenateor sample treated at 40 degC and 01 MPa for 15 min (b)

a)

b)

S Sensory amp Nutritive Qualities of Food

S246 JOURNAL OF FOOD SCIENCEmdashVol 71 Nr 3 2006 URLs and E-mail addresses are active links at wwwiftorg

Rheological properties of tomato product

of 60 degC is positioned lowest with regard to the viscosity curves of theother given temperatures pointing out that the viscosity of tomatohomogenate decreases most when treated at 60 degC These viscosityobservations are comparable with the consistency experiments

The viscoelastic behavior of tomato homogenate determined withan oscillatory-shear experiment using angular frequencies between 100and 01 rads and a constant strain of 01 after treatment for 15 minat 50 degC is represented in Figure 4 Similar graphs were obtained for allother temperatures tested The storage modulus Grsquo represents the elasticbehavior of the sample and the loss modulus Grdquo represents the viscousbehavior The curve functions of both parameters together describe theviscoelastic behavior The figure shows that tomato homogenate is vis-coelastic in which the elastic portion (Grsquo) dominates over the viscousportion (Grdquo) concluding that tomato homogenate behaves as a weak gelThis observation is consistent with available literature (Saacutenchez andothers 2002 Valencia and others 2003 Tiziani and Vodovotz 2005) Forboth sample and control the storage modulus (Grsquo) reaches a plateauwith decreasing angular frequency (rads) indicating that a network inthe tomato homogenate is formed Under the experimental conditionsexplored here no relationship was observed between the temperaturetreatments and the viscoelastic properties

Figure 5 illustrates the results of a typical strain sweep experimentusing a constant angular frequency of 10 rads and strains between

001 and 10 after treatment of the tomato homogenate for 15 minat 50 degC For all other temperatures tested (30 degC 40 degC 60 degC 70 degC80 degC and 90 degC) similar graphs were obtained Two regions can beobserved from the figure namely a linear viscoelastic region that iswhere the storage modulus (Grsquo) is nearly constant with strain and anonlinear region with decreasing values of Grsquo It can be seen that a01 strain amplitude is in the linear viscoelastic region Thereforethe 01 strain amplitude used in the oscillatory-shear experiments(Figure 4) is allowed to be used in these measurements

Rheological properties of tomato homogenateRheological properties of tomato homogenateRheological properties of tomato homogenateRheological properties of tomato homogenateRheological properties of tomato homogenateafter high-prafter high-prafter high-prafter high-prafter high-pressuressuressuressuressure tre tre tre tre treatmenteatmenteatmenteatmenteatment

The effect of combined pressure-temperature treatments on thealteration in the consistency the viscosity and the viscoelasticity oftomato homogenate was studied for temperatures ranging from 30 degCto 70 degC and pressures ranging from 100 to 500 MPa

For all conditions tested the highest loss in consistency of the toma-to homogenate after combined pressure-temperature treatment wasfound at 300 MPa at all temperatures tested (Figure 6) The combinedaction of purified tomato PME and PG on pectin during thermal andhigh-pressure processing was studied at pH 44 by Verlent and others

Figure 3mdashSteady-shear viscosity data of tomato homogenateobtained with the rheometer after treatment for 15 min at30 degC () 40 degC () 60 degC () 90 degC ()

Figure 4mdashViscoelastic behavior of tomato homogenatemeasured with the rheometer after treatment for 15 min at50 degC storage modulus (Grsquo) of sample () and control () andloss modulus (Grdquo) of sample () and control ()

Figure 5mdashAmplitude test (oscillation) performed with therheometer after treatment for 15 min at 50 degC to determinethe limit of the linear viscoelastic range storage modulus(Grsquo) of sample () and control () and loss modulus (Grdquo) ofsample () and control ()

Figure 6mdashLoss in consistency of tomato homogenate aftercombined pressure-temperature treatments for 15 min 30 degC() 40 degC () 50 degC () 60 degC () 70 degC ()

Vol 71 Nr 3 2006mdashJOURNAL OF FOOD SCIENCE S247

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URLs and E-mail addresses are active links at wwwiftorg

Rheological properties of tomato product

(2005 ldquoEffect of temperature and pressure on the combined action ofpurified tomato pectinmethylesterase and polygalacturonase in pres-ence of pectinrdquo submitted to Enzyme and Microbial Technology) andthey observed that tomato PME was very active in presence of tomatoPG at pressures up to 300 MPa PME creates a good substrate for PGwhich also has a sufficient high activity at 300 MPa These findingsmay explain the drastic loss in consistency of tomato homogenate treat-ed at 300 MPa Striking was that negative values were observed at tem-peratures up to 60 degC combined with 500 MPa indicating that theconsistency of the treated tomato homogenate was improved withregard to the control which is favorable (Figure 7)

However the disadvantage is that a jelly-like translucent structurewas formed and serious syneresis occurred which is unacceptablefor the consumer These 2 phenomena were not or only to a small ex-tent observed at lower pressure levels or at atmospheric pressure(Figure 8) Porretta and others (1995) and Krebbers and others(2003) also found that high-pressure-treated (gt500 MPa) tomato-based products resulted in an improved viscosity and that a jelly-likehomogenous structure was formed Porretta and others (1995) as-cribed this gel formation to protein-tissue coagulation and compact-ing At 70 degC combined with 500 MPa neither loss nor improvementin consistency of the tomato homogenate was observed So thisseems to be the best processing condition toward food quality butalso toward food safety as vegetative microorganisms are alreadyinactivated at 500 MPa and 25 degC and outgrowth of remaining bac-terial spores is prevented by the low pH (Mertens 1992 Porretta andothers 1995 Hendrickx and others 1998 Heinz and Knorr 2002) Sev-eral Salmonella and hepatitis outbreaks from tomato-based prod-ucts have already been reported However it was found in the liter-ature that toward food safety both Salmonella and hepatitis can beeasily inactivated by high-pressure processing even at pressureslower than 500 MPa combined with ambient temperature (Calci andothers 2005 Bayindirli and others 2006)

The improved consistency and the gel formation at 500 MPa werefurther investigated First the treatment time of 15 min was short-ened However even after a treatment time of only 5 min at 40 degCand 500 MPa the same improvement in consistency was obtainedand the appearance of the pressure-treated tomato homogenate was

the same compared with a treatment time of 15 min Porretta andothers (1995) described that viscosity is strongly dependent on thepressure applied but independent of treatment time Second byaddition of the chelator EDTA (Sigma) it was investigated to what ex-tent the formation of the gel and the improved consistency at 40 degCand 500 MPa can be ascribed to the presence of calcium ions 5 gEDTA allowing calcium ion binding was added to 350 g chopped to-matoes from which the seeds were removed The whole was thenhomogenized and the obtained homogenate was pressure treatedat 40 degC and 500 MPa The index of consistency of the control wasmuch higher than usual indicating that in all probability the chelatorEDTA bounded present calcium ions resulting in a break down ofcurrent bonds between pectin and calcium ions After treatment at40 degC and 500 MPa the index of consistency was the same as com-pared with the control but no gel formation or syneresis occurredHence it is presumable that calcium ions play a crucial role in the gelformation and improvement of the consistency at 500 MPa due tocrosslinking of low-methoxyl pectin chains generated by PME withcalcium ions Very recently in the context of thermal processing An-thon and others (2005) observed an improved firmness of diced to-matoes during calcium treatment They found that an increasedPME activity leads to extensive pectin deesterification and increasedcalcium cross-linking of the pectins

In a final experiment tomato pieces were pressure-treated at 40 degCand 500 MPa for 15 min (see ldquoMaterials and Methodsrdquo section) afterwhich the consistency of the homogenized sample was measuredThe index of consistency of the sample was lower than that of thecontrol yielding again a negative Bostwick-value which is of coursefavorable Even though the tomato pieces appeared to be jelly-likebefore blending after homogenization no gel structure occurredanymore At 1st instance hardly no syneresis was observed but afterawhile separation of water was perceptible

Steady-shear experiments after treatment of the tomato homo-genate for 15 min at 30 degC combined with various pressures are rep-resented in Figure 9 Similar graphs were obtained for all other pres-sure-temperature combinations tested As at atmospheric pressurethe viscosity values decrease with increasing shear rate at all givenpressures at 30 degC pointing out that tomato homogenate displayspseudoplastic flow behavior The sample treated at 500 MPa hasthe highest viscosity and the sample treated at 300 MPa has thelowest viscosity These findings are comparable with the results ac-quired in the consistency experiments

An example of an oscillatory-shear experiment after treatment of

Figure 7mdashComparison of the index of consistency of untreatedtomato homogenate or control (a) with tomato homogenate orsample treated at 40 degC and 500 MPa for 15 min (b)

a)

b)

Figure 8mdashComparison of the appearance of processed tomatohomogenates at 40 degC for 15 min at 01 MPa (a) and at 500MPa (b)

S Sensory amp Nutritive Qualities of Food

S248 JOURNAL OF FOOD SCIENCEmdashVol 71 Nr 3 2006 URLs and E-mail addresses are active links at wwwiftorg

Rheological properties of tomato product

the tomato homogenate for 15 min at 30 degC and 500 MPa showingthe viscoelastic behavior of tomato homogenate is represented inFigure 10 which is similar to Figure 4 For all other pressure-temper-ature combinations tested similar graphs were obtained Howeveronly at pressures of 500 MPa the curves of the sample are positionedabove the curves of the control which is the opposite for all otherpressures tested lower than 500 MPa This observation is analogousto the negative Bostwick-values at 500 MPa in the consistency exper-iments Under the experimental conditions explored here no otherrelationships were observed between the temperature-pressuretreatments and the viscoelastic properties Apart from that the samefindings were observed as at atmospheric pressure

Conclusions

Pectin in tomato puree in which the pectin degrading enzymeswere inactivated cannot be degraded by freely added tomato PME

and PG Consequently to improve the rheological properties of to-mato-based products proper process conditions must be chosen toenhance and reduce the activity of the endogenous tomato PME andPG respectively Thermal treatments and high-pressure treatmentsup to 400 MPa of tomato homogenate lead to serious damages ofrheological properties On the contrary high-pressure treatments at500 MPa combined with temperatures up to 60 degC seemed to be very

efficient to improve the rheological properties of tomato homoge-nate However other quality defects such as syneresis and forma-tion of a jelly-like translucent structure occurred Nevertheless nogel structure was observed anymore after homogenization the prob-lem of syneresis still exists but to a lesser extent No change in rheo-logical properties or in appearance of the tomato homogenate treat-ed at 500 MPa and 70 degC was observed Hence the tomato productquality is preserved at this process condition but based on the liter-ature the microbial safety of the tomato product is also retainedThe conditions described previously need validation for other toma-to varieties that might have differences for example in pectin con-tent enzyme activity and pH

AcknowledgmentsThis research has been supported by the Flemish Government-IWT the Fund for Scientific Research Flanders and the ResearchCouncil of the Katholieke Universiteit Leuven

ReferencesAnthon GE Blot L Barrett DM 2005 Improved firmness in calcified diced toma-

toes by temperature activation of pectin methylesterase J Food Sci 70342ndash7Barnes HA Nguyen QD 2001 Rotating vane rheometrymdasha review J Non-Newtonian

Fluid Mech 981ndash14Bayindirli A Alpas H Bozo-glu F Hizal M 2006 Efficiency of high pressure treat-

ment on inactivation of pathogenic microorganisms and enzymes in appleorange apricot and sour cherry juices Food Control 1752ndash8

Calci KR Meade GK Tezloff RC Kingsley DH 2005 High-pressure inactivationof hepatitis A virus within oysters Appl Environ Microbiol 71339ndash43

Crelier S Robert MC Claude J Juillerat MA 2001 Tomato (Lycopersicon esculentum)pectin methylesterase and polygalacturonase behaviors regarding heat- and pres-sure-induced inactivation J Agric Food Chem 495566ndash75

Errington N Tucker GA Mitchell JR 1998 Effect of genetic down-regulation of polyg-alacturonase and pectin esterase activity on rheology and composition of tomatojuice J Sci Food Agric 76515ndash9

Fachin D Smout C Verlent I Ly Nguyen B Van Loey AM Hendrickx ME 2004 Inacti-vation kinetics of purified polygalacturonase by thermal and high pressure process-ing J Agric Food Chem 522697ndash703

Fachin D Van Loey AM Ly Nguyen B Verlent I Indrawati Hendrickx ME 2002 Com-parative study of the inactivation kinetics of pectinmethylesterase in tomato juiceand purified form Biotechnol Progr 18739ndash44

Fachin D Van Loey AM Ly Nguyen B Verlent I Indrawati Hendrickx ME 2003 Inac-tivation kinetics of polygalacturonase in tomato juice Innovative Food Sci-ence and Emerging Technologies (IFSET) 4135ndash42

Fito PJ Clemente G Sanz FJ 1983 Rheological behavior of tomato concentrate(hot break and cold break) J Food Eng 251ndash62

Gould WA 1992 Tomato production processing and technology 3 ed Timo-nium Md CTI Publications 536 p

Heinz V Knorr D 2002 Effects of high pressure on spores In Hendrickx MEG KnorrD editors Food engineering series Ultra high pressure treatments of foods NewYork Kluwer AcademicPlenum Publishers p 77ndash113

Hendrickx M Ludikhuyze L Van den Broeck I Weemaes C 1998 Effects of high pres-sure on enzymes related to food quality Trends Food Sci Technol 9197ndash203

Knorr D 1993 Effects of high-hydrostatic-pressure processes on food safety and qual-ity Food Technol 47156ndash61

Krebbers B Matser AM Hoogerwerf SW Moezelaar R Tomassen MMM van den BergRW 2003 Combined high-pressure and thermal treatments for processing of to-mato puree evaluation of microbial inactivation and quality parameters Innov FoodSci Emerg Technol 4377ndash85

Mertens B Knorr D 1992 Developments of nonthermal processes for food preserva-tion Food Technol 46124ndash33

Mertens B 1992 Under pressure Food Manuf 6723ndash4Porretta S Birzi A Ghizzoni C Vicini E 1995 Effects of ultra-high hydrostatic pres-

sure treatments on the quality of tomato juice Food Chem 5235ndash41Porretta S Poli G 1997 Tomato purEumle quality from transgenic processing tomatoes

Int J Food Sci Technol 32527ndash34Pressey R 1986 Extraction and assay of tomato polygalacturonases HortScience

21490ndash2Saacutenchez MC Valencia C Gallegos C Ciruelos A Latorre A 2002 Influence of pro-

cessing on the rheological properties of tomato paste J Sci Food Agric 82990ndash7Tiziani S Vodovotz Y 2005 Rheological effects of soy protein addition to tomato

juice Food Hydrocoll 1945ndash52Valencia C Saacutenchez MC Ciruelos A Latorre A Madiedo JM Gallegos C 2003 Non-

linear viscoelasticity modeling of tomato paste products Food Res Int 36911ndash9Vercet A Saacutenchez C Burgos J Montantildeeacutes L Buesa PL 2002 The effects of mano-

thermosonication on tomato pectic enzymes and tomato paste rheologicalproperties J Food Eng 53273ndash8

Verlent I Van Loey A Smout C Duvetter T Hendrickx ME 2004 Purified tomato po-lygalacturonase activity during thermal and high-pressure treatment Bio-technol Bioeng 8663ndash71

Xu SY Shoemaker CF Luh BS 1986 Effect of break temperature on rheologicalproperties and microstructure of tomato juices and pastes J Food Sci 51399ndash402 407

Figure 10mdashViscoelastic behavior of tomato homogenatemeasured with the rheometer after treatment for 15 min at30 degC and 500 MPa storage modulus (Grsquo) of sample () andcontrol () and loss modulus (Grdquo) of sample () and control ()

Figure 9mdashSteady-shear viscosity data of tomato homogenateobtained with the rheometer after treatment for 15 min at30 degC combined with 01 MPa () 100 MPa () 300 MPa ()500 MPa ()