Scalping ofFlavors in

Packaged FoodsM.G. Sajilata, K. Savitha, R.S. Singhal, and V.R. Kanetkar

ABSTRACT: Food packaging, although an integral part of the food chain, has a major drawback in that, often, thepackaging material interacts with the flavor constituents of the food, causing either a selective or an extensive lossof desirable food flavors or absorption of undesirable off-flavors from the packaging material, thereby resulting inan eventual loss of quality of the packaged food item. The process is called “scalping” and is of great concern to thefood industry, which is always looking out for new avenues in “packaging solutions” for its final product qualityneeds. The review highlights the various attributes of the scalping process, explores approaches to the reduction ofthe manifested undesirable effects, and covers other relevant aspects.

IntroductionFood packaging in recent years has experienced rapid ad-

vances and continuous growth due to consumer desire and de-mand for conveniently packaged products. This has promoted atrend in packaging materials that has evolved from simple foodwraps to sophisticated containers. Besides providing an adequateshelf life and product quality it is desirous that the packagingmaterial also participates in the overall flavor management ofthe packaged food, flavor being an underlying factor in the con-sumer acceptability of all food products. However, sorption offood aromas, particularly by plastic packaging materials, is usu-ally perceived as a major factor contributing to the quality alter-ation of most foods during storage. This causes changes in boththe intensity and characteristics of the food flavors owing to theirabsorption by the packaging material, a phenomenon commonlyreferred to as “scalping” (Roland and Hotchkiss 1991).

Flavor scalping is a term used in the packaging industry todescribe the loss of quality of a packaged item due to either itsvolatile flavors being absorbed by the package or the food absorb-ing undesirable flavors from the packaging material. The inabilityto prevent scalping is a sore point for many packagers, a classicexample being the absorption of various plastic flavors when softdrinks are stored in plastic bottles for an extended period of time.Tropicana discovered that its gable-topped orange juice cartonsscalped flavor from the juice by absorbing important chemicals.Polyethylene (PE), the most common plastic used in commercialproducts, including beverages, is a top scalper.

Commercial plastics for food packaging include simple ho-mopolymers or copolymers such as low-density polyethy-lene (LDPE), medium-density polyethylene (MDPE), high-density

MS 20060142 Submitted 3/1/2006, Accepted 9/18/2006. Authors are with FoodEngineering and Technology Dept., Inst. of Chemical Technology, Univ. ofMumbai, Matunga, Mumbai-400 019, India. Direct inquiries to author Singhal(E-mail: rekha@udct.org).

polyethylene (HDPE), polypropylene (PP), polyethylene tereph-thalate (PET), aluminum sheet (Surlyn), ethylene vinyl alcoholcopolymer (EVOH), and others (Gremli 1996). Figure 1 showsthe molecular structures of the polymers used as packaging ma-terials (Willige 2002). Among the plastic materials, polyethylenesand polypropylenes (homo- and copolymers) are mostly used inpackage design preferred for contact with the product both inmonolayer and laminated or coextruded structures because oftheir good chemical resistance and inertness to most foods, goodbarrier properties to water, and thermosealability. However, theirpolyolefinic nature gives them high lipophilicity enabling reten-tion of large amounts of nonpolar compounds such as most ofthe aroma compounds, thereby causing an aroma imbalance inthe packaged food. Trace concentrations of residual solvents,monomers, plasticizers, inhibitors, or mold-release agents con-ceivably could migrate to the food product and, also, flavor in-gredients can travel to the package. Moreover, flavor scalpingfrom the product does not require the product to be in direct con-tact with the polymer because the aroma compounds are volatileenough to transfer from the headspace to the package. Thus, al-though the polymeric films may be adequate moisture or gas bar-riers for a product, these benefits can be offset by flavor scalping.Studies in scalping will therefore help determine the type of plas-tic films best suited for a particular food product (Risch 1991).The frequent use of PE on the inner surface of the food-contactpolymer containers has made it the principal polymer in mostscalping studies.

Interaction between Food Flavor and Packaging MaterialThe first and foremost function of a food package is to pro-

tect the product and preserve its inherent quality (Nielsen andJagerstad 1994). Over the past decade, the use of plastic pack-ages has increasingly replaced metal and glass for food and bev-erage packaging, the advantages being numerous—lower costs,lighter in weight, less apt to shatter, transparent, flexible, andconvenient to the consumer. In spite of all these advantages,

there are some properties of plastics that limit their use in foodpackaging (Salame 1989). Food packaging interactions, definedas an interplay between food, packaging, and the environment,can produce an adverse effect on the food and/or the package(Hotchkiss 1997). Interactions between the plastic package andthe flavor constituents can cause adsorption and absorption of fla-vor volatiles by the packaging material, permeation of the flavorvolatiles through the plastic material, food- and flavor-inducedchanges in the physical properties of the plastic polymer, as wellas interactions of low-molecular-weight compounds in the plasticsuch as solvent and plastifiers with the food flavor or the food it-self, thereby resulting in an overall imbalance in the flavor profileof the food (Gremli 1996).

Figure 2 shows the possible interactions between foodstuff,polymer film, and the environment and their adverse conse-quences (Nielsen and Jagerstad 1994). Flavor losses that resultfrom interaction with the polymer package can be either losses

Figure 1 --- Molecular structures of polymers

Figure 2 --- Possible interactions between foodstuff, poly-mer film, and the environment and their adverse conse-quences (Nielsen and Jagerstad 1994)

occurring by permeation or migration through the package orthose from sorption by the container (Strandburg and others1991).

Modeling Studies in Flavor ScalpingThere are several models, more aptly theoretical considera-

tions, to predict the losses due to flavor scalping. Flavor lossesthat result from interaction with the polymer package can beeither losses occurring by permeation or migration through thepackage or those from sorption by the container (Strandburg andothers 1991).

The permeation process can be described as a multistep event.First, collision of the penetrant molecule with the polymer is fol-lowed by sorption into the polymer. Next, migration through thepolymer matrix occurs and, finally, desorption of the permeantfrom the polymer completes the process. For a polymer with thinfilm geometry, Fick’s first law can be written as

�Mx

�t L= P A�px

where �Mx /�t is the transport rate of the material x through a filmof area A, with a thickness of L, and under a chemical potentialcreated by pressure difference across the film of �px . P is thepermeability coefficient and is a steady-state parameter.

P = DS

The permeability consists of 2 parameters, the diffusion coeffi-cient (D) and the solubility coefficient (S). The diffusion coeffi-cient is a kinetic parameter. It is a measure of how fast trans-port events will occur. It reflects the ease with which a penetrantmolecule moves within a polymer host. The diffusion coefficientis determined from the following equation:

D = L 2

7.2t1/2

where L is the film thickness and t 1/2 is the time required to reachone-half the steady-state mass transport rate. Once the permeabil-ity coefficient and the diffusion coefficient have been obtained,the solubility coefficient can be calculated.

The solubility coefficient is a thermodynamic parameter. It is ameasure of the concentration of penetrant molecules that will bein position to migrate through the polymer.

Cx = Spx

where Cx is the concentration in the polymer and px is the partialpressure of x in the vapor phase.

The sorption of the flavor ingredient from solution onto thepolymer involves the process of partitioning and diffusion. Themechanism of sorption consists of both adsorption onto the solidsurface followed by dissolution into the polymer and diffusionaway from the surface under the driving force of a concentrationgradient until equilibrium is established. The partitioning step in-volves adsorption onto the polymer surface and mixing into thepolymer matrix, which depends on the relative forces of attrac-tion between the solution and the polymer for the solute. Theseforces are governed by the thermodynamics of the system and areaffected by structure and polarity. These factors can be consid-ered on the basis of cohesive energy density (CED) forces or itssquare root, the solubility parameter of the Hildebrand equation.

The Hildebrand approximation for the heat of mixing is

�Hm = v1(δ1 − δ2)�22

where �Hm is the partial molar heat of mixing, v1 is the partialmolar volume of the penetrant, �2 is the volume fraction of thepolymer in the mixture, and δ1 and δ2 are the square roots of theCED of the penetrant and the polymer. The differences in the CEDvalues of the polymer-penetrant combination in a given systemwill affect the size of heat of mixing and a permeant with a largerCED will have larger heat of mixing and less negative �G thanthe one with a smaller CED. The diffusion step depends on themobility of the solute, which can depend on its size and interac-tion with the polymer to plasticize and relax its chain segments(Halek and Luttman 1991).

Organic vapors often solvate host polymers that eventually ex-hibit increased permeability to oxygen and other gases suggestingthat oxygen permeability might provide information on volatilescalping (Salame and Steingiser 1977). In one study, limoneneabsorption by LPDE increased oxygen permeability, which wasproportional to the limonene concentration (Sadler and Braddock1990, 1991). As oxygen permeability is directly proportional topenetrant concentration, Mt /Mα will equal Pt /Pα, where Mt andPt are, respectively, the absorbed volatile weight and oxygen per-meability of the polymer at time t, and Mα and Pα are the cor-responding volatile weight and polymer oxygen permeability att = α. If results are expressed as a dimensionless ratio, then oxy-gen permeability can be used to solve mass transfer equations.Crank (1967) presented an applicable equation for membraneabsorption and desorption (Sadler and Braddock 1990), where

(Mt/Mα)desorption =∑

n→0 to α

{8 exp{ − D(2n + 1)2π2t}}/(2n + 1)2π2}4τ 2

and the absorption equation is (Mt /Mα)absorption = 1–(Mt /Mα)desorption, where D is the diffusion coefficient, and τ

is 12 the membrane thickness when both surfaces are exposed to

the penetrant. For single-surface penetrant exposure, τ is the fullthickness of the membrane.

Migration testing using food simulants is the normal procedurefor checking compliance of a food packaging material againstspecific migration limits (SMLs). However, as Feigenbaum andothers (2002) point out, this is not practical for several reasons.Industries that put packaged foods or materials intended for foodcontact on the market do not know the identity of the potentialmigrants; sometimes, those who manufacture and sell these rawmaterials do not know the processing conditions or the final ap-plication; and even if the identity of the migrants were knownit may be difficult to analyze them. Therefore, the use of math-ematical modeling to predict migration, which can reduce theamount of tests to be undertaken, has been recently introducedinto legislation. Practical examples for the application of this newconcept are described in the Practical Guide (European Commis-sion 2003).

Mass transfer from a plastic into food consists essentially of ki-netic (diffusion in the polymer and foodstuff) and thermodynamic(equilibrium partitioning between the packaging and food) fac-tors, besides the geometric dimensions of a given food/packagingsystem. The most important variables that control migration of thesubstances are contact time t and temperature T . Numerous mi-gration tests have shown that the majority of measurements couldbe represented as being approximately proportional to the squareroot of time t and the initial concentration of the migrant in thepolymer, c P ,0. With the assumption that material transport obeysthe law of diffusion, the following equation can be applied to de-

scribe approximately the migration of a substance from a polymerinto a food (simulant) for situations where mF, t /mF, e < 0.5:

mF,t = 2kcP ,0(DP t/π )1/2

DP is the actual diffusion coefficient of the migrant in the polymerwith a homogeneous distribution of the migrant at an initial con-centration, cP ,0. The constant k is the measure of the influenceof factors that lie outside the polymer/migrant system and has,by definition, a value of k = 1 in the absence of such influences(such as for contact with ethanol and oil) as long as no poly-mer swelling takes place. A practical application of this equationis that for cases with k = 1, the actual diffusion coefficients ofmigrants in the polymer can be determined from the kinetic mea-surement of mF , t from systems with known/measured values forcP ,0. Provided that the migration potential in the polymer, that is,the initial amount of migrant dissolved in the polymer, mP ,0, isknown, then

mF,e = mP ,0

1 + kP ,F (VP /VF )

where

kP ,F = cP,e

cF,e

The partition coefficient kP , F is defined as the concentration ratioat equilibrium of a migrant in the polymer, cP ,e, divided by thatat equilibrium in the foodstuff or food stimulant, cF ,e. The valuesfor kP , F range over several orders of magnitude, depending onthe polarity of the polymer involved, the food simulant, and thenature of the migrant. The kP , F value for limonene, a nonpolarsubstance in the LDPE/water system at 23 ◦C, was found to behigher than 5000. This makes it retained in the polymer, whereasa much more polar compound such as cis-hexenol shows a muchlower value of 0.33, causing considerable transfer into water.

In recent years, the possibility of modeling with easily availablehardware and software opens a large field of complex applica-tions. MIGRATEST Lite (1997, 1999), COATING TEST (1999), andMIGRATEST (2000) are specialized softwares offered for migra-tion modeling (Brandsch and others 2000).

Sorption of Food Flavors by the Packaging MaterialsSorption of food flavors in polymers involves the process of

both partitioning and diffusion. Moreover, although the intensityof aroma of a packaged foodstuff depends on the vapor pres-sure (influenced by the other food components), interaction ofthe volatile organic moieties with other food constituents, andaroma barrier characteristics of the package (Mahoney and oth-ers 1988), the nature of the aroma is also imperative in deter-mining the extent of sorption. The extent of flavor absorption isinfluenced by the properties of the polymer, the flavor molecules,and also external conditions. The chemical composition, chainstiffness, morphology, polarity, and crystallinity of the polymer in-fluence flavor absorption as much as the chemical composition,concentration, and polarity of the flavor compounds, as well asthe presence of other chemical constituents. External factors suchas duration of storage, relative humidity, temperature, and thepresence of other food components can also affect the solubilityof aroma compounds in a polymer (Nielsen and others 1992a;Leufven and Hermansson 1994).

Some of the major factors influencing the scalping process aredescribed in detail below.

Flavor Characteristics

ConcentrationThe higher the concentration of the sorbed material, the higher

the rate of transport into the polymer structure (Brody 2002). Thepresence of copermeants can affect the rate of transport of themolecules through the polymer usually by increasing the rate.

Carbon chain length, boiling point, and polarityThe carbon chain length is closely related to the boiling point

of a molecule, and several researchers have indicated a relation-ship between solubility and boiling points of sorbates. Thus thecarbon chain length of volatiles affects their solubility in polymerfilms. In a sorption study, permeation and diffusion of volatilecompounds into PE or ethylene vinyl copolymer films were eval-uated using volatile compounds such as alkanes, aliphatic ethylethers, aldehydes, and alcohols containing 4 to 10 carbon atoms.Sorption and solubility coefficient were found to increase with anincrease in the carbon chain length of the volatile compounds. Astudy on the sorption behavior of flavor compounds by PE linerof laminated pouches (PET/Al/PE) containing 43 volatile com-pounds showed the distribution ratio of the flavor compoundsto increase with carbon chain length of the flavor compoundsfrom 0.01 to 0.1 for alcohols, 0.03 to 1.23 for aldehydes, 0.02 to5.77 for aliphatic esters, and 0.11 to 11.6 for benzoates (Shimodaand others 1988). The distribution ratio, defined as the ratio ofamount sorbed into PE film and the adhesive layer to the amountremaining in the solution with respect to each flavor component,was used as the basis of comparison. In each series, the distri-bution ratios increased about 3-fold for each methylene group,but in the compounds composed of 11 or more carbon atoms,the increments were either less or, in the case of aldehydes, neg-ative. The sorption of esters, ketones, and aldehydes by PP hasalso been shown to increase as the number of carbon atoms in thecompounds increased. Moreover, compounds with 8 or more car-bon atoms were demonstrated to be sorbed from yogurt drinks byHDPE with shorter molecules remaining in the product (Linssenand others 1991). In the same study, it was observed that highlybranched molecules were sorbed to a greater extent than linearmolecules.

Esters and aldehydes are reportedly absorbed much more thanthe alcohols in LDPE. The absorption of aldehydes has been cor-related to their structure, the shorter chain aldehyde, decanal, be-ing absorbed to a lesser extent than the C12 aldehyde, and dode-canal. The chain length of the lipophilic portion of the moleculeappears to affect its absorption, the unsaturated aldehydes (forexample, perillaldehyde and geranial) being absorbed to a lesserextent than the saturated aldehydes (Charara and others 1992).The length of the carbon chain might account for the differencesbetween the esters; the longer the chain, the lesser the polarityand ease of absorption of the compounds by the nonpolar poly-olefins.

According to Shimoda and others (1988), the effect of func-tional groups on the distribution ratios of flavor compounds be-tween a film and a solution was greater in the liquid phasebecause of closer interactions, whereas the effect of molecularweight or boiling point was much larger than the functional groupin the vapor phase. Strandburg and others (1991) obtained a linearrelationship between the logarithm of the solubility coefficient invinylidene chloride copolymer and the boiling point of linear es-ters, alkanes, and ketones. Similar results are reported for the sol-ubility of alkyl esters in polyvinyl alcohol (Landois and Hotchkiss1988). The higher solubility of linalool compared to limonene inPE is also shown to be influenced by its boiling point. Linalool isa more linear, less bulky molecule than limonene, which wouldfacilitate its ability to move into the polymer matrix. In addition,

Figure 3 --- Structure of linalool

its higher boiling point (198 ◦C for linalool, 175 ◦C for limonene)is indicative of its ability to condense and remain within the ma-trix (Roland and Hotchkiss 1991). Boiling point has been corre-lated to higher solubility by several workers. Also, linalool, usu-ally present at 1/10 the concentration of limonene, may have agreater potential in affecting aroma due to sorption by a PE pack-age than limonene. The higher solubility of linalool coupled withits higher sensory impact increases the effect of linalool sorption.Figure 3 and 4 differentiates linalool and limonene with respectto their structures.

Solute polarity is one of the predominant controlling factorsinfluencing sorption. Flavors are absorbed more easily in a poly-meric film of similar polarity. The sorption of a number of cit-rus flavors by LDPE is almost instantaneous, the partitioningdepending on the polarity of the compounds. Comparing car-vone (C10H14O) and limonene (C10H16), both similar terpenesbut with different polarity, it has been shown that the less po-lar limonene is not only sorbed at a faster rate but also dif-fuses more rapidly, probably due to lesser cohesive forces (Halekand Luttman 1991). In the sorption of citrus oil constituents bypolyolefins, terpenes showed the highest affinity for the poly-mers, followed by sesquiterpenes (C15); larger amounts of es-ters and aldehydes were sorbed than alcohols, due to polarity(Charara and others 1992). For the same reasons, saturated alde-hydes were sorbed to a greater extent than those with doublebonds.

Orange juice aromas have been demonstrated to be sorbed todifferent extents, starting with hydrocarbon compounds, whichshowed the highest affinity to LDPE, followed by ketones, esters,aldehydes, and finally alcohols (Linssen and others 1991; Nielsenand others 1992a; Linssen and Roozen 1994). Factors that affectabsorption include molecular size of the aroma compounds andpolarity and solubility properties of both the polymer and thearoma compounds. Table 1 shows the solubility parameters andhydrogen-bonding characters of aroma compounds and the poly-mers used in the study by Nielsen and others (1992b). In general,the smaller the difference between δ values of the polymer andthe flavor compound, the greater the solubility of the flavor intothe polymer. Alcohols have much larger solubility parameter val-ues than LDPE, LLDPE, and PP, which might explain why they

Figure 4 --- Structure of limonene

are less absorbed compared to esters and aldehydes while PEThas a solubility parameter closer to that of the alcohols. Also, po-larity and hydrogen-bonding character play an important role inthe prediction of solubility. The reason why the alcohols are notabsorbed to a greater extent into PET might be due to their stronghydrogen-bonding character, which PET lacks. Esters and alde-hydes have solubility parameters very close to LDPE, LLDPE, andPP, which partly explains their large partition coefficient in thesepolymers. The differences between the esters might be accountedfor by the length of the carbon chain—the longer the chain, theless polar the compounds, facilitating easier absorption by thenonpolar polyolefins (Nielsen and others 1992b).

Polymer Characteristics

Surface areaThe degree of sorption is directly related to the available surface

area in contact with the food (Gremli 1996).

PolarityDifferent plastic materials have different polarity, hence they

differ in their affinity toward the flavor compounds. Flavors areabsorbed more easily in a polymeric film of similar polarity (Gallo

Table 1 --- Solubility parameters (δ values)

Compound/ δ Hydrogen-bondingpolymer values (H) character

Ethyl butyrate 8.5 ModerateButyl acetate 8.5 ModerateIsopentyl acetate 7.8 ModerateEthyl-2-methyl butyrate 8.3 ModerateButyl propanoate 8.8 ModerateHexyl acetate 8.6 ModerateHexanal 8.3 ModerateTrans-2-hexenol 8.5 ModerateIsopentyl alcohol 10 StrongHexanol 10.7 StrongLow-density polyethylene 8 PoorLinear low-density polyethylene 8 PoorPolypropylene 9.2 PoorPolyamide 11.1 StrongPolyester 10.7 Moderate

Table 2 --- Approximate partition coefficient values for various plastic/food package systems

Substance (i) Polymer (P) Food phase (L) KP /L

Nonpolar (n-alkane) Nonpolar (PO) Very polar (water) 1 × 105–1 × 106 (40000)Nonpolar to middle polarity (d-limonene) Nonpolar (PE) Very polar (water) 1200Nonpolar to middle polarity (d-limonene) Nonpolar (PO) Polar (fruit juices) 400–600Middle polarity (ketone) Middle polarity (PVC) Polar (ethanol) 10–20Middle polarity (ester) Nonpolar (PO) Very polar (water) 10–40Nonpolar (large n-alkanes) Nonpolar (PO) Polar (ethanol) 5Nonpolar (n-alkane) Nonpolar (PO) Nonpolar (n-alkane) 1All polarities All polymers Oil 0.1–10Polar (alcohol) Very polar (PA) Very polar 1Polar to middle polarity (alcohol and hydrocarbon) Polar (PET) Polar 0.1 --- 3Polar (alcohol) Nonpolar (PO) Very polar (water) 0.5Polar (alcohol) Middle polarity (PVC) Polar (ethanol) 0.1 (0.07)Polar (alcohol and middle polarity) Very polar, polar Polar >0.1

(PA) (propanol) 0.05–0.1(PET) (methanol) 0.001–0.05

Very polar (acid) Nonpolar (PO) Very polar (water) 0.001 (0.001)

and others 1999). Polyolefins are highly lipophilic and are incom-patible for packaging products with nonpolar substances (such asfats, oils, aromas) since they can be absorbed and retained by thepackage (Hernandez and others 2001). The polyesters, however,are more polar than the polyolefins and hence show less affinityfor the nonpolar components. Table 2 shows the approximate up-per and lower limits for partition coefficients one may normallyencounter in plastic/food systems based on the polarities of thesolutes, plastics, and foods (Baner 2000a). It also shows the ap-proximate ranges of partition coefficient values for various solutesbetween typical food-contact plastics and liquid phases.

LLDPE and oriented polypropylene (OPP) (both nonpolar)have been shown to easily absorb limonene and myrcene and,to smaller extents, decanal, hexyl acetate, nonanone, carvone,linalool, octanol, and hexanal (Willige and others 2002a). Theflavor molecules carvone and limonene have similar structure butlimonene is a nonpolar terpene, while carvone is an oxygenatedpolar terpene. Hence, limonene is absorbed in larger quantitythan carvone.

Glass transition temperatureSorption rates to a certain extent depend on the glass transition

temperature (Tg ) of the polymer that determines the flexibility ofthe polymer molecules. Below Tg , the polymer molecules are stiff(glassy state) and the chance of a flavor molecule finding a suffi-ciently large void is limited. Above Tg , the polymer molecules arehighly flexible (rubbery state), which makes this chance higher.Table 3 shows the glass transition temperatures of polymers com-monly used for food packaging.

Rubbery polymers such as the polyolefins PE and PP, whichhave a Tg below ambient temperature, have a high diffusioncoefficient for flavors with steady-state permeation being estab-lished quickly in such structures (Giacin and Hernandez 1997).Stiff-chained polymers that have a high glass transition temper-ature generally have low permeability, unless they also have ahigh free volume (Miller and Krochta 1997). Polymers such asthe polyesters PET, PC (polycarbonate), and PEN (polyethylenenaphthalate) have a Tg above ambient temperature. At room tem-perature, these glassy polymers have very stiff chains and a verylow diffusion coefficient for flavor molecules at a low concen-tration. Glassy polymers such as polyvinylidene chloride have alow diffusion coefficient for aroma molecules at a low concen-tration and hence display high flavor-barrier properties (Brody

2002). PEN has excellent performance characteristics in compar-ison to PET for carbonated beverages, allowing hot-fill, rewash,and reuse, due to its high Tg (VanLune and others 1997).

Polymer crystallinityMorphological properties that influence permeability and dif-

fusivity include structural regularity or chain symmetry leading to3-dimensional order or crystallinity (Brody 2002). The more or-dered the polymer molecular structure, the lower the rate of sorp-tion of the food aroma constituents. Crystalline regions can re-duce sorption or act as impermeable barriers for diffusion throughthe polymer. They can act as crosslinks to prevent swelling ofthe polymer. The crystalline regions can also restrain polymersegmental mobility affecting the solute permeability to the amor-phous regions. Thus, the degree of sorption for a given solute isoften related to the degree of the amorphous polymer content(Charara and others 1992; Letinski and Halek 1992). PP is an ex-ample of a crystalline polymer that can exist in various degreesof crystallinity.

Absorption of limonene and myrcene by PC is found to bemuch higher than by PET and PEN, although the Tg of PC ismuch higher than the Tg of PET and PEN. This is attributed to thelack of crystalline regions in PC, which is a totally amorphouspolymer (Nielsen 1994).

Surface polymer hydrolysisSurface hydrolysis of ethylene-vinyl acetate copolymer can

alter sorption behavior. While the sorption of hydrocarbons,ethyl octanoate, and decanal into hydrolyzed EVA film is de-pressed, the sorption of alcohols is promoted. This is mainly at-tributed to solubility of the individual compounds (available from:www.foodproductdesign.com).

Polymer densityThe amount of aroma compounds sorbed in polymers has been

shown to decrease with an increase in polymer density. The sorp-tion rate for limonene decreased from 75% to 63% and that ofcarvone decreased from 17% to 10% when the PP density in-creased from 0.8830 g/cm3 to 0.9213 g/cm3, respectively (Gremli1996).

Free volume of the polymerThe free volume of a polymer is the molecular “void” volume

that is trapped in the solid state (Salame 1989). The permeatingmolecule finds an easy path in these voids. Generally, a polymerwith poor symmetry in the structure or bulky side chains willhave high free volume and hence high permeability. A polymer’smolecular configuration is altered through solvation by organicpenetrants (Takeuchi and Okamur 1976).

Use of recycled plasticsIn an effort to reduce wastes from food packaging materials,

refillable PET bottles for carbonated drinks have been adopted in

Table 3 --- Common applications in food packaging applica-tions and their glass transition temperatures (Tg) (Helm-roth and others 2002)

Polymer Tg (◦C)

LDPE –20HDPE –20PP 5PS 90–100PET 67PVC 80PC 149

many countries. A major problem associated with refilling is thesorption of flavors from foodstuffs by the container causing off-flavors in the products filled into the same container. The extentof sorption is dependent on temperature, the polymer type, thecarbon chain length, and the type of functional groups of the sor-bate. Washing with sodium hydroxide solutions removes less thanhalf the sorbed amount of terpenes from plastic bottles (Gremli1996). A study to evaluate the efficacy of washing of PET bottlesintended for reuse with sodium hydroxide solution in order toremove limonene, linalool, and linalyl acetate showed limoneneto be significantly sorbed onto the walls of the bottles even afterwashing (Safa 1999).

Recycling of old polymers as new bilayer bottles may be ofvalue if a virgin polymer layer is placed between the recycledpolymer and the food. The virgin polymer layer plays the roleof a functional barrier. In the case of solid foods, the processof contamination by the recycled polymer is controlled by ra-dial diffusion through the bottle and the food. An increase in thevolume of the bottle is associated with a decrease in the areaof the bottle–food interface per unit volume of food (Rosca andVergnaud 1998).

External Factors

pHpH influences the sorption of aroma compounds into the pack-

aging material. pH has been shown to affect sorption by a factorof 40 between pH 3 and 5 for 2-hexanal into PE (Leufven andHermansson 1994). Lowering the pH to 3 increased the sorptionof alcohols into LDPE (Nielsen and others 1992a). Sorption oflimonene is shown to be 1.3 times more in LDPE at 22 ◦C andat pH 5.2 than at pH 2.6 (Blain 1994). Nonetheless, a study onlimonene in orange juice showed pH to have no significant effecton scalping (Nielsen and others 1992a).

Leufven and Hermansson (1994) studied the interaction ofaroma compounds such as trans-2-hexanal, 2-heptanone, 6-methyl-5-hepten-2-one, 6-methyl-5-hepten-2-ol, and limonenefrom tomato juice with polymers such as PET, PE, and EVOH as afunction of pH. The amount of extractable aroma compounds inthe different polymers at all pH values decreased in the followingorder: EVOH > PE > PET. Between these polymers, the sorp-tion of the aromas was less influenced by pH in PET than in theother 2 polymers, where PE contained larger amounts of the com-pounds at pH 4 than at pH 7. For EVOH, there was an increasein sorption of d-limonene with increasing pH. Table 4 shows theamount of added aroma compounds sorbed in the polymer films.It is hypothesized that pH has a direct relationship with polarity,solubility, and structure of molecules and can be influenced bythem.

Food compositionFood constituents play an important role in influencing the

sorption of flavors. Not only the type of plastic used is ofimportance for the uptake of aroma compounds, but also the pos-sible interactions between the flavor and food components. Fla-vor components may be dissolved, adsorbed, bound, entrapped,encapsulated, or retarded in their diffusion through the food ma-trix by food components. The relative importance of each of thesemechanisms varies with the properties of the flavor chemical(functional groups, molecular size, shape, volatility, and so on)and the physical and chemical properties of the components inthe food. Proteins, carbohydrates, and lipids interact with flavors,changing the concentration of free flavor in solution and, con-sequently, affecting their absorption. Absorption of flavor com-pounds by linear low-density polyethylene (LLDPE) was studied

Table 4 --- Amount of added aroma compound (μg cm−3) extracted from polymer films stored in spiked tomato juice

Days of Trans-2- 6-methyl-5 6-methyl-5-Polymer pH storage hexenol 2-heptanone hepten-2-one hepten-2-ol Limonene

PE 4 1 11 20 44 36 6057 20 92 298 132 848

7 1 22 77 12 55 2067 21 21 30 45 436

PET 4 1 11 11 8 4 197 22 31 24.4 14 58

7 1 10 8 7 5 177 14 28 21 11 50

EVOH 4 1 28 50 50 155 737 21 28 29 89 52

7 1 28 68 81 248 1227 45 43 43 94 81

in model systems representing differences in composition of thefood matrix. The extent of flavor absorption by LLDPE was influ-enced by food components in the order oil or fat > polysaccha-rides and proteins > disaccharides (Willige and others 2000a).

Due to the lipophilic character of many flavor compounds,food products with high oil/fat content tend to lose less flavor byabsorption into LLDPE packaging than food products containingno or a small quantity of oil (Willige and others 2000b). The pres-ence of a relative small amount of oil (50 g/L) has been shownto decrease absorption substantially. Also, proteins, particularly,β-lactoglobulin and casein, are able to bind flavors, resulting insuppression of absorption of the flavor compounds. Polysaccha-rides such as pectin and carboxymethylcellulose increase viscos-ity and consequently decrease absorption. Disaccharides such aslactose and saccharose increase absorption, probably by a “salt-ing out” effect of less nonpolar flavor compounds.

Selective flavor sorption by packaging materials constitutes amajor problem in beverages packaged in PE that has very highaffinity for nonpolar compounds. Since most beverages are wa-ter based, the compounds more miscible in PE are absorbed.It is hypothesized that by lowering the solution polarity or cre-ating nonpolar regions (emulsified oil droplets) the equilibriumpartition coefficient could be reduced thereby increasing flavorretention. A study to determine whether oil or sucrose in solutioncould reduce flavor sorption by PE showed that the sorption andpartition coefficient of various compounds into PE were greatlyinfluenced by the presence of oil in solution, but not sucrose. Forcompounds most preferentially absorbed by PE, addition of oilgreatly reduced flavor loss, the reduction magnitude being verysignificant with only 0.1% oil. It is postulated that flavors incorpo-rated into an oleoresin could be used to make beverages resistantto flavor scalping (available from: www.foodproductdesign.com).

Durr and others (1981) reported a 50% loss of limonene fromorange juice to LDPE during 2 wk of storage. The sorption oflimonene by LDPE from model solutions was much greater thanthat from orange juice attributed to the presence of other con-stituents in the juice, which increased the solubility of limonenein the aqueous phase (Manheim and others 1987). This suggeststhat the tendency of a beverage to lose flavor might be overcomeby the addition of components to the beverage that would reducethe transfer of the individual compounds to the polymer.

Relative humidity (RH)The presence of water vapor often accelerates the diffusion

of gases and vapors in polymers with an affinity for water. Thewater diffuses into the film and acts like a plasticizer, opening thepolymer structure to molecular transport. The solubility of ethyl

propionate in polyvinyl alcohol has been shown to reduce with anincrease in RH (Landois and Hotchkiss 1988). This is attributed tothe competition between ethyl propionate and water moleculesfor available sorption sites.

Recently, the use of EVOH materials providing a high barrierto organic compounds is gaining much attention as a means toreduce food aroma scalping. However, although EVOH is an ex-cellent barrier to food aroma when dry, a property that even im-proves at low RH, the solubility and diffusivity of the compoundsincrease dramatically with humidity at medium to high water ac-tivity. An increase in moisture content of moisture-sensitive plas-tics such as polyamides or EVOH is reported to increase the rateof permeation of aromatic compounds (Brody 2002). However,even at 100% RH, EVOH outperforms LDPE as a barrier to organicvapors (Carballo and others 2005).

Storage conditionsGenerally, permeability, diffusion, and solubility coefficients

follow a van’t Hoff-Arrhenius relationship, resulting in increasedpermeation with an increase in temperature (Brody 2002). Sincethe effect is accentuated at ambient temperatures compared torefrigerated temperatures, the adverse effects have appeared morefrequently in foods contained in retort pouches, aseptic packages,and hot-filled flexible packages.

The increased flavor absorption at higher temperatures is dueto increased mobility of the flavor molecules: changes in the poly-mer configuration such as swelling or decrease of crystallinity andchanges in the volatile solubility in the aqueous phase (Gremli1996). In 1 study, the total amount of flavor absorbed increasedconsiderably with temperature from 4 ◦C to 40 ◦C, the absorp-tion by polyolefins such as LLDPE and OPP being several timeshigher than by the polyesters such as PC, PET, and PEN, indicat-ing polyesters to be more preferable over polyolefins as packagingmaterials (Willige and others 2002b).

Storage temperature has been demonstrated to affect the degreeof sorption of aromas by PE. Sorption of limonene and myrcenefrom commercial orange drink stored for 12 wk in PET bottleswas found to be 3.40 and 9.90 μg of limonene/g PET and 0.11and 0.33 μg of myrcene/g PET at 4 ◦C and 25 ◦C, respectively.The higher values at 25 ◦C could be due to the greater mobility ofthe molecules or more swelling of the polymer generating morespace for incorporation of the dissolved molecules (Nielsen andothers 1992a; Gremli 1996). Similarly, the solubility coefficientfor aldehyde vapors in linear LDPE was maximal at 25 ◦C, withsorption being lower at both 5 ◦C and 75 ◦C (Leufven and Stollman1992). The sorption of aldehyde vapors by polyvinyl chloride(PVC) also decreased as the temperature decreased. VanLune andothers (1997) suggested that the crystallinity of PET decreased

CRFSFS: Comprehensive Reviews in Food Science and Food Safety

while free volume increased at higher temperatures resulting inmolecules’ easier absorption.

Tawfik and others (1998) demonstrated that PET stored for15 d at 37 ◦C in a model solution containing 320 ppm limoneneabsorbed 7 times more limonene than when stored at 5 ◦C, but4 times more after 45 d. It is concluded that the diffusion pro-cess is temperature dependent as evidenced by the slower rateat a lower temperature. Nonetheless, absorption of decanal andmyrcene by PET film was found to decrease on prolonged storagenotably due to desorption from the polymer to the model solu-tion on degradation (Willige and others 2002b). With increasingstorage time and temperature, degradation of aldehydes such asoctanol, decanal, and citral was also observed in orange juice(Durr and others 1981; Moshanos and Shaw 1989a).

Examples of Specific Flavor Systems with PackagingMaterials

Citrus flavorsCitrus flavors, which contribute to the flavor of orange juice,

have been the major subject of study with respect to flavor scalp-ing for quite some time. Citrus components from cold-pressedand terpeneless oils have been shown to be absorbed into vari-ous polymers such as LDPE, HDPE, PP, and surlyn used in asepticpackaging (Charara and others 1992). Citrus essential oils consistof terpenes, sesquiterpenes, oxygenated compounds, and non-volatile components while terpeneless oil consists of a higherconcentration of oxygenated compounds and a very low con-centration of terpenes.

Table 5 shows the percent absorption of flavors from cold-pressed and terpeneless orange oil extracted from 4 polymersusing cyclohexane. LDPE has the highest absorption value be-cause of its large amorphous area and low crystallinity, the ab-sorption and diffusion taking place in the amorphous area of thepolymer. Terpene hydrocarbons being lipophilic tend to diffuseinto the amorphous regions of the polymers. HDPE and PP ab-sorbed constituents to a lesser extent probably due to their highercrystalline structure. About 70% of limonene was lost from cold-pressed orange oil in contact with LDPE for 4 d, while only a 30%loss was observed from orange oil in contact with HDPE and PP.Surlyn was shown to absorb flavor constituents moderately.

Table 5 --- Percent absorption of components from cold-pressed and terpeneless orange oil (Charara and others1992)

% absorption

Component LDPE HDPE PP Surlyn

Cold pressedLimonene 68 30 28 53Pyrene 58 21 20 49Myrcene 66 28 22 78TerpenelessLimonene 54 22 36 39Copaene 40 ND ND 17Caryophyllene 34 4 9 19Octylacetate 35 ND 13 21Undecanal 30 ND 12 14Dodecanal 22 7 9 16Decanal 18 6 6 11Perillaldehyde 10 ND ND 7Geranial 1 1 ND 1Linalool 3 <1 1 2

The most extensively studied aroma compound with respectto its sorption by polymers is limonene. Limonene is an unsatu-rated terpene hydrocarbon present in citrus flavors. It is a highlynonpolar substance and has shown high affinity for many poly-meric packaging materials. It is a highly nonpolar substance withhigh affinity for many polymeric packaging materials. A decreasein limonene content in stored orange juice is attributed to itslipophilic nature and, hence, the ease of its diffusion into the poly-mer (Moshanos and Shaw 1989b). The absorption of caryophyl-lene and copeane was lower than that of limonene while thatof octyl acetate and sesquiterpenes was similar. For limonenein model solutions, the affinity onto the polymer is in this or-der: LDPE > polyamide > polystyrene. Also, an increase in thethickness of the polymer film and, consequently, the mass of thepolymer available as sorbent increase the sorption of limonene(Ikegami and others 1991; Pieper and others 1992). The sorp-tion of limonene by LDPE has been demonstrated to be bipha-sic, showing both adsorption and absorption (Halek and Luttman1991). A study on the flavor absorption by LDPE, PC, and PETfrom model solutions containing 7 flavor compounds showed va-lencene to be almost completely absorbed by LDPE, followed toa lesser extent by decanal, hexyl acetate, octanol, and nonanone(Willige van and others 2003). Fewer and less of the flavor com-pounds were absorbed from the model solution by PC and PET.However, limonene was readily absorbed from the orange juiceby LDPE, while myrcene, valencene, pinene, and decanal wereabsorbed in smaller quantities. Berlinet and others (2005) havereported the evolution of aroma compounds from orange juicemade from concentrate and stored in glass, standard monolayerpolyethylene terephthalate (PET 1), multiplayer PET (PET 2), andplasma-treated PET (internal carbon coating) (PET 3) to be due tothe high acidity of the matrix, implying acid-catalyzed reactions.PET packaging materials and their corresponding oxygen perme-ability were found to have no correlation with the loss of aromacompounds.

The quality of juices asceptically packed in laminated car-tons has been the subject of extensive research during the lastdecades (Marshall and others 1985; Moshanos and Shaw 1989a,1989b). In another study, 2 polyester/polyolefin adhesive lami-nates coated with a PVDC-PVC copolymer were surface-exposedto lemon juice and a hot sauce product as well as to food sim-ulant systems at 45 ◦C (Schroeder and others 1990). Sorption offlavor volatiles resulted in delamination of both laminates fol-lowing exposure to the food products, d-limonene being iden-tified as the primary component sorbed by the laminates. Theshelf life of aseptically filled orange and grape fruit juices hasalso been shown to be significantly shorter in laminated cartonsthan in glass jars, one of the attributes being the absorption ofd-limonene by the PE-contacting surface (Manheim and others1987). Orange juice packaged in “juice board,” paperboard sand-wiched between 2 layers of LDPE has been shown to have a slightturpentine-like off-flavor owing to the absorption of limonene intothe paperboard (available from: www.foodproductdesign.com).The retention of aldehydes, ethyl butanoate, and other flavorcompounds in orange juice is reported to be significantly higher inPET than in HDPE and LDPE (Ayhan and others 2001). Commer-cially, most aseptically filled juices are packed in LDPE-laminatedcarton packs such as Tetra Brik and Combibloc. Food industriesoften correct the absorptive effect by adding excess flavors to thefood for keeping taste and flavor acceptable for consumers untilthe end of the product’s shelf life.

While flavor absorption may be high enough to affect thesensory quality of a packaged food, only a few authors haveconducted sensory tests to go along with the analytical re-sults (Kwapong and Hotchkiss 1987; Manheim and others 1987;Sharma and others 1990). Durr and others (1981) showed that

the absorption of d-limonene up to 40% did not affect the sen-sory quality of orange juice during 3 mo of storage at 20 ◦C.They suggested that d-limonene contributed scarcely to the flavorof orange juice. Moreover, they considered limonene absorptioneven as an advantage, since limonene is known as a precursorto off-flavor compounds. They also reported the storage temper-ature to be the main quality parameter for the shelf life of orangejuice.

Apple aroma compoundsEvaluation of 5 polymer packaging films with respect to the

sorption of 10 apple aroma compounds from its aqueous so-lution showed major differences in the amount of aroma com-pounds absorbed by the polymers (Nielsen and others 1992b).LDPE, LLDPE, and PP absorbed larger quantities of aroma com-pounds than did the polar polymers. PP absorbed larger amountsof aroma compounds than the 2 polyethylenes, which may bea consequence of differences in crystallinity and morphology ofthe polymers. LDPE is reported to possess long-chain branch-ing while LLDPE exhibits short-chain branching, but similar den-sity and equal heat of fusion and degree of crystallinity. Kon-czal and others (1992) reported larger amounts of apple aro-mas to be sorbed by LDPE than by EVOH with high ethylenecontent.

Migration from Packaging MaterialsMigration is an important safety aspect to be considered when

designing food packaging materials. Plastic additives, frequentlyused to improve polymer properties, and residual monomers oroligomers are not chemically bound to the polymer moleculesand can, therefore, move freely within the polymer matrix. Conse-quently, at the interface between the packaging material and foodthey can dissolve in the food product and thus adversely affectthe flavor and acceptability of the food (Hernandez and Gavara1999). Some of the additives that are more likely to migrate arethe plasticizers used in a range of plastics, but more particularlyin PVC films where they can migrate in PVC food-contact sit-uations (available from: www.foodscience.afisc.csiro.au). Whilepaper and board materials may transmit taint or odor to a food,plastics have a much greater potential to do this. The presenceof monomers can contribute to significant off-flavors, styrenemonomers at 0.5% in polystyrene (PS) being responsible for thecharacteristic plastic odor of many packaged foods. Fats andoils in food products, however, can help mask off-flavors frommonomers and odorous residues from plastic processing. Never-theless, as consumers shift toward lower fat products, off-flavorscontributing directly via packaging materials may become moreof a problem. A number of components of ink may cause unpleas-ant flavors in food if the manufacture of the packaging material isnot carefully controlled. Solvents associated with packaging inksand with resins used in bonding the various layers of laminatedpackaging films can be absorbed by the product to contributeoff-flavors (available from: www.foodprodcutdesign.com).

The nature and strength of off-odors migrating to the fooddepend on a number of factors, including the chemical natureof the migrant (its volatility and polarity), the lipophilicity ofthe food matrix, the physical structure of the food (solid com-pared with liquid), time/temperature conditions during storage,and the aroma activity (strength) of the migrant (available from:www.foodproductdesign.com). Migration of off-flavors from thepackaging material could be a result of plasticizers, inks, anddyes used for graphics or from resins used in the bonding of thepackage.

Some factors that could affect migration or be a source of unde-sirable flavors in the final packaged product are described briefly.

Molecular weight of the migrantMigration is a term used to describe the process of mass transfer

from a food packaging material to its contents. If one reduces themigration process to the diffusion in and from the plastics, then themigratability of plastic constituents corresponds predominantlyto the volatility or molecular weight of these organic substancesand to the basic diffusivity of a polymeric type (Franz 2000). For agiven plastic, this means that the mobility of a migrant decreaseswith increasing molecular weight.

Solubility of the migrant in the food matrixThermodynamic properties such as polarity and solubility in-

fluence the migration rate due to interactions between polymer,migrant, and food stimulant (Helmroth and others 2002). For in-stance, if a migrant has poor solubility in the food simulant, itwill rather remain in the polymer than migrate into the food sim-ulant. This is often the case with nonpolar additives in nonpolarpolymers such as LDPE, PP, and PS in contact with more polarfood simulants such as water or 3% acetic acid. Higher migrationrates are, therefore, often found for fatty simulants such as oliveoil, 95% ethanol, or isooctane than for aqueous food simulants(Till and others 1987; Riquet and Feigenbaum 1997; Linssen andothers 1998). If the food simulant itself has a high affinity for thepolymer, then it may be absorbed by the polymer.

Most of the plastic constituents have a lipophilic rather thanhydrophilic character and, therefore, migration increases stronglywith increasing fat content and especially so where the fat or oilrepresents the outer phase of the food matrix (Franz and others2000). The increase in migration is not necessarily due to an in-crease in the substance’s diffusion coefficient due to interactionsbetween the fat and the plastic as is often assumed (Baner andothers 1992). The amount of substances migrating from plasticsinto foodstuffs with high fat content is higher than in foodstuffswith water content owing to the higher solubility of the migratingorganic compounds in fat compared to water. In fact, the higherthe fat content of the product, the higher the taste threshold con-centration. For styrene, Bruck and Hammerschmidt (1977) devel-oped an equation that relates the fat content of the food productto the taste threshold concentration as threshold (mg/L) = 0.0025(80 × % fat in food + % water in food). Conversely, water andproducts with high water content (juices, skim milk) have lowertaste thresholds usually on the order of 50 ppm.

Storage temperatureAt lower temperatures, including refrigeration, migration of

plastic components to foods may occur, the migration beingmuch slower than at higher temperatures (available at: http:/www.foodscience.afisc.csiro.au).

An understanding of the interactions between food ingredientsand polymeric packaging materials requires knowledge of severalfactors. It is worthwhile to examine the relative balance of thevarious factors and their overall effect on migration.

Resins as a source of off-flavorsResins used in bonding of paper layers can be a culprit in

the migration of off-flavors in packaged foods. Quaker Oats Co.discovered a piney/spruce off-flavor in packaged ready-to-eatcereals (available from: www.foodproductdesign.com) that con-tributed an uncharacteristic flavor note to the cereal. When thepackaging material used for the outer boxes was examined, it didnot possess a piney odor, but the inner glassine liner containeda strong piney odor. The liner was composed of 2 sheets of Kraftpaper laminated with a resin in microcrystalline wax and over-waxed on both sides with paraffin wax.

Odors from inks and dyesResidual solvents from inks and dyes used for packaging

graphics can cause off-flavor problems in foods (available from:www.foodprodcutdesign.com). A nondairy coffee creamer manu-facturer noticed shipping boxes of foil-wrapped nondairy creamerto have a severe musty odor. Results showed that all of the newproduction samples contained the odor problem and had a peakidentified as 1,3,5-trimethylbenzene (mesitylene). The trimethyl-benzene peak had a strong musty odor identical to the odor ofcontaminated boxes. Further checking revealed that the packag-ing supplier had used a new type of ink containing high levels of1,3,5-trimethylbenzene for graphics on the foil packets.

Inks and model ink components incorporated deliberately intocarton boards at low temperature and during microwave heatinghave shown benzophenone to migrate to the packaged food evenfrom PE-coated boards, attributed mainly to the permeability ofPE to low-molecular-weight substances (Johns and others 2000).Migration of benzophenone, benzylbutyl phthalate, butyl ben-zoate, chlorodecane, and dimethyl phthalate was also detectedafter storing the food at –20 ◦C for 1 wk in the impregnated carton-board. It was concluded that for inks used to print food-contactmaterials, the content of low-molecular-weight volatiles shouldbe controlled to lower the migration levels.

Off-odors from polyamide/ionomer laminatesAn off-odor with a smell of cat urine is reported to have oc-

curred in cooked ham products packed in polyamide/ionomerlaminate films (Piringer and Ruter 2000). A gas chromatograph(GC) study of such products was able to identify the presence ofdiacetone alcohol (DAA), which was used in the printing ink ofthe film that subsequently migrated into the laminate. Dehydra-tion reaction of DAA by the ethylene ionomer resulted in theformation of 4-methyl-4-mercaptopentane-2-one off-odor. It isproposed that when printing ionomer films or laminates contain-ing ionomer in which foods with sulfur-containing proteins arepacked, the absence of mesityloxide and its chemical precursorssuch as acetone and diacetone alcohol must be guaranteed.

Styrene taint in dairy productsAn example of a product that has had styrene taint problems

over the years has been dairy products such as coffee cream-ers and condensed milk packed in thermoformed PS single-serveportion-pack containers, the sensory threshold being on the orderof 0.1 mg/kg (ppm) of styrene in the product (Baner 2000b). TheBritish Ministry of Agriculture Foods and Fisheries (MAFF 1994)carried out a trade survey of 22 coffee creamer portion packs andfound styrene monomer levels in the product ranging from 23to 223 μg/kg (ppb) with an average of 134 μg/kg. Of the com-monly PS-containing portion-pack materials, mono-material PSwas found to have the greatest migration followed by PS/PE; andthe material with the lowest migration was PS/EVOH/PE. Surpris-ingly, even the product packed in PS/EVOH/PE barrier materialcontained styrene at a perceptibly significant level at the end ofthe shelf life despite the EVOH barrier layer between the PS layerand the product. The possible explanation for styrene in the prod-uct comes from the fact that styrene from the PS layer transfers tothe inner PE layer during shipment and storage. This takes onlya few days given the relatively high diffusion coefficient of PSand PE. However, lower temperatures and shorter shelf life canreduce the amount of styrene transferred to the product.

Measurement of styrene monomer migration from PS cups intomilk samples containing 0.5%, 3.5%, or 10% fat, as well as 4 wa-ter/ethanol mixtures containing 0%, 15%, 50%, or 100% ethanol,indicated that water, the simulant prescribed for milk under Eu-ropean Commission legislation (Directive 85/572/EEC), does notexhibit the required physicochemical properties for an adequate

simulation under practical migration conditions (O’Neill andTuohy 1994). Styrene migration was found to depend stronglyupon the fat content of the milk and on the ethanol concentrationin the simulant. Pure water gave migration values considerablylower than all of the 3 samples of milk and 50% ethanol wasshown to correlate approximately with 3.5% fat milk. Styrenemigration from PS cups into different food systems such as wa-ter, milk (0.5%, 1.55%, or 3.6% fat), cold beverages (apple juice,orange juice, carbonated water, cola, beer, and chocolate drink),hot beverages (tea, coffee, chocolate, and soup with 0%, 0.5%,1%, 2%, or 3.6% fat), take-out foods (yogurt, jelly, pudding, andice cream), aqueous food simulants (3% acetic acid, 15%, 50%,or 100% ethanol), and olive oil was also found to be fat depen-dent, the maximum migration being for the fatty foods (Tawfik andHuyghebaert 1998). Studies of styrene migration from PS foam ar-ticles have shown the amount of styrene migrating into food oil tobe proportional to the square root of the time of exposure (Licklyand others 1995).

Cork taint in winesOne of the most critical problems in the enological industry

is associated with cork taint. Significant research has been car-ried out with respect to cork taint, particularly in wines. The maincompounds responsible for this off-flavor are some chloroanisolessuch as 2,4,6-trichloroanisole (TCA), 2,3,4,6-tetrachloroanisole(TECA), and pentachloroanisole (PCA). Tests have confirmed thepresence of 2,4,6-TCA and 2,3,4,6-tetrachlorophenol with ex-tremely low-flavor thresholds, the dominant odor being close tothat of mold (or moldy cork), leaving a sensation of dryness inthe mouth very similar to the feeling of a plastic film covering thetaste buds (Cantagrel and Vidal 1990). Thus, use of compoundscontaining chlorine in bad conditions in the steps in the manu-facture of cork (or in the wine-making process) runs the risk ofimparting an off-flavor.

Off-odors in premiumsA common promotional tool is the insertion of premiums in-

side food packages. The volatiles in the inserts, if not properlycontrolled, can impart an undesirable flavor to packaged foods.Odor testing of unwrapped premiums intended to be inserted inpackaged dry mix beverages demonstrated a very strong “solvent-like,” objectionable odor (Apostolopoulos 1998). The source ofthe residual chemicals was found to be solvents used with eitherthe resin or the paint employed in the manufacturing and paintingof the premiums. Overwrap films are recommended to preventdirect contact and contamination of the food products.

Influence of Food Processing on Scalping of Flavors

SorptionHigh-pressure food processing. Single-layer and multilayer

plastic structures can be used in combination with high-pressureprocessing (HPP) without concern that the process will enhancesorption. In 1 study, sorption behavior and flavor scalping po-tential of selected packaging films in contact with food simu-lant liquids (FSL) (ethanol and acetic acid solutions) were evalu-ated after high-pressure processing (Caner and others 2004). HPPtreatment (800 MPa, 10 min, 60 ◦C) showed d-limonene con-centration in the films and food simulants to be insignificantlyaffected by HPP, except for the metallized PET/EVA/LLDPE incomparison to the control pouches, which were exposed to at-mospheric pressure at 60 ◦C for 10 min in an electric oven.

Irradiation. Recently, irradiation has been explored as a viablemeans in the shelf life extension of many foods. Apple cider pack-aged in 3 plastic polymers, PS, LDPE, or nylon-6 (N6), and irra-diated at 2 kGy was compared with unirradiated cider packaged

in glass containers (Crook and Boylston 2004). The extent of fla-vor absorption by the packaging materials was influenced by thepolarity of the polymer and the flavor compound, with plasticpolymer having a greater affinity for compounds with similar po-larity. Cider irradiated and stored in PS containers was shown tohave a lower rate of loss than untreated cider or irradiated ciderpackaged in LDPE for volatile flavor compounds characteristic ofthe apple or fruity taste and aroma.

MigrationMicrowave treatment. In the microwave oven, plastic contain-

ers and wraps are often warmed to a point that plasticizers arereleased from the plastic into the food (Bishop and Dye 1982).Numerous studies indicate that some plasticizers can cause un-desirable side effects in animal tissue, and that the amount ofmigration could be as high as 23% of the total weight. In 1 study,migration of plasticizers such as dioctyladipate (DOA) from plas-ticized PVC and acetyltributylcitrate (ATBC) from polyvinylidenechloride (PVDC/PVC) (SaranTM) films into ground meat heated for4 min was found to be 84 mg/kg (14.7 mg/dm2) and 95.1 mg/kg(2.5 mg/dm2), respectively (Badeka and Kontominas 1998). Suchmigrations could result in deterioration of the final food quality,posing off-flavor and/or safety problems. The migration of com-pounds from a polymeric material into a food-contacting phasewould depend on several factors such as the nature and thick-ness of the packaging material, the nature of the food in contact,initial concentration of additives in the polymer, compatibility ofthe additive/polymer system, temperature, time of contact, andothers.

One development in food packaging that has received muchattention lately is the microwave susceptor, a package devel-oped to meet consumer demands for convenience and qual-ity in microwaveable food products. A susceptor is typicallya piece of polyester film that has been metallized with alu-minum laminated either to paperboard or between 2 layers ofpaper. Food/package interactions that result from the use ofmicrowave susceptors and dual ovenable trays have been ex-tensively studied by the U.S. Food and Drug Administration(FDA) ’s Indirect Additives Laboratory. Susceptors are typicallyconstructed of plastic such as PET, adhesives, and paper, andtemperatures in excess of 212 ◦F are common for such pack-aging. At elevated temperatures, both volatile and nonvolatilechemicals from the package can migrate into foods (Hollifield1991). Generally, the more paper in the susceptor construction,the greater the amount and number of volatile substances re-leased on heating. The number of substances released at lev-els greater than 0.5 μg/inch2 of susceptor surface from suscep-tors containing acrylic-based or vinyl-acetate-based adhesivesincludes acetone, methyl vinyl ketone, pentanal, toluene, hex-anal, furfural, heptanal, benzaldehyde, nonanal, furan, isobu-tanol, acetic acid, butanol, octanal, styrene, octyl acetate,5-hydroxymethylfurfural, and crotonaldehyde (Hollifield 1991).

Ionizing radiation. Irradiation for the control and reduction ofmicroorganisms and insects and the extension of product shelf lifehas become a potential preservation technique in many countries.Several researchers have studied migration into food simulants,describing it to be a consequence of irradiation. Lox and others(1991) found an increase in migration as a function of dose upto 10 kGy and a decrease at higher doses from extruded PVCirradiated with 3 to 25 kGy γ -rays. With accelerated electrons, themigration increased steadily with the absorbed dose. However, nosignificant migration was observed with LDPE and PP at absorbeddoses of up to 25 kGy with either electron irradiation or a cobalt-60 γ -source (Rojas and Pascat 1990).

The gas permeability of LDPE, HDPE, PET, and PVC pack-aging materials is unaffected by irradiation doses of up to

8 kGy (Buchalla and others 1993). However, irradiation of thesepolymers can result in the release of hydrogen, CO2, CO, andmethane gases and the formation of volatile oxidation prod-ucts, including peroxides, alcohols, aldehydes, ketones, and car-boxylic acids. Flexible food packaging materials (LDPE, EVAc,PET/P/EVOH/PE) irradiated with low (5 kGy, corresponding tocold sterilization), intermediate (20 kGy), and high (100 kGy)doses produced these volatile compounds in amounts that in-creased with increasing irradiation dose (Riganakos and others1998). The extent of radiation-induced changes depends on manyfactors such as the type of polymer, processing exposure, andirradiation conditions (Buchalla and others 1993). Odor inten-sity values for various polymers have been reported by Tripp(1959). More intense odors on irradiation were observed in LDPEthan in HDPE, developing stronger off-odors than PS and vari-ous polyamides and polyesters. The intensity of off-odors of ir-radiated PE was found to increase with oxygen concentrationin the atmosphere with a good correlation existing between theamount of products formed by irradiation and the intensity ofoff-odor (Azuma and others 1984). The odor-producing volatilesfrom LDPE films irradiated with a dose of 20 kGy from a 2.5 MeVelectron beam were identified to be mainly aliphatic hydrocar-bons, aldehydes, ketones, and carboxylic acids.

Migration studies with 9 different polymers irradiated at 60 kGyshowed increased migration into distilled water for polyamide-6,polyamide-11, polyvinyl chloride-vinyl acetate, and poly (vinyli-dene chloride-vinyl chloride) after γ -irradiation (Killoran 1972).The extracted substances were identified as low-molecular-weight materials of the polymers and, in the case of PVC, as anester-type plasticizer. Transfer of off-odors from 2 laminates (withLDPE and HDPE as inner layers) into water was evident at the low-est dose applied (10 kGy) and increased with the absorbed doseto reach a maximum at 150 kGy; however, during storage, the off-odors gradually disappeared. Nevertheless, polyester-aluminum-HDPE did not produce any off-odors up to 500 kGy.

Retorted foods. Migration from multilayer laminated filmpouches intended for retort foods was investigated using HPLCanalysis, a fluorescence detector, and measurements of residue onevaporation, consumption of potassium permanganate, and totalorganic C (Uematsu and others 2005). HPLC analysis revealedthat the levels of migrants in oil and water heated in the pouches(121 ◦C, 30 min) were 10 times those in the corresponding officialsimulants, n-heptane (25 ◦C, 60 min), and water (95 ◦C, 30 min).Bisphenol A diglycidyl ether and related compounds were foundin oil and water heated in the pouches, as well as in the officialsimulants. Results indicate that these compounds were present inthe adhesive between the laminated films and migrated throughthe food-contact film of the package.

Analytical Techniques

Sorption of food flavorsIn the quantitative analysis of flavor sorption by packaging

materials, solvent extraction of flavor compounds is generallyfollowed by determination of the extract concentration by GC(Imai and others 1990). Increasing the extract concentration bydistillation or vacuum evaporation is usually used to increasethe sensitivity of measuring low-magnitude sorption (Kwapongand Hotchkiss 1987). Solvent extraction and multiple headspaceextraction using a purge and trap injector (MHE-PTI) can alsobe used to study sorption. While the solvent extraction step ishighly tedious and time consuming, the MHE-PTI is more con-venient, rapid, fully automated, and suitable for volatile aromacompounds.

The techniques currently employed in the evaluation of flavorsorption include GC-MS (mass spectroscopy) analysis using static

headspace sampling, dynamic headspace sampling, and solid-phase microextraction (SPME) (available from: www.foodprodu-ctdesign.com, Paik 1992); solvent extraction coupled withGC (Shimoda and others 1988; Imai and others 1990;Nielsen and others 1992a); electronic nose (available from:www.foodproductdesign.com), supercritical fluid extraction (SFE)coupled with GC (Nielsen and others 1991; Johansson and oth-ers 1993); and vacuum microgravimetry (Roland and Hotchkiss1991).

A novel method developed by Nielsen and others (1991) usingsupercritical carbon dioxide extraction directly coupled to GCmight prove essential in the continuous investigation of absorbedfood components in plastic packaging materials. It is based on theadvantageous properties that carbon dioxide has at a pressure andtemperature above the critical point when the carbon dioxide isin the supercritical stage. Leufven and Hermansson (1994) usedSFE with carbon dioxide to extract aromas from film samples anddeposit them in a chemical adsorbent for thermal desorption ontoa GC-FID (flame ionization detection) capillary column. Nielsenand Jagerstad (1994) also used SFE to strip the polymer off thesorbed volatiles, which was then deposited directly onto a GC-FID capillary column for elution. These SFE techniques are limitedto aromas that are soluble in the given supercritical fluid (Nielsenand others 1991).

In a study on the interaction of orange juice with inner pack-aging material of an aseptic product, the volatile components ab-sorbed by the inner packaging material were extracted by etherand analyzed by GC-MS (Jeng and others 1991). The empty pack,after rinsing with deionized water and air-drying, was filled withethyl ether for 24 h at room temperature and the ethyl ether ex-tract so obtained was concentrated with a vigreux column. Ethylcinnamate was added as an internal standard. The volatile com-ponents so extracted from the inner packaging material were sub-jected to GC analysis on a Varian 3400 gas chromatograph usinga fused silica column with a stationary phase equivalent to Car-bowax 20 M. In another study, using headspace chromatography,attempts were made to “fingerprint” key flavor compounds sorbedby aseptic packages containing juice products (Arora and others1991). The package that contained juice showed extra peaks inthe chromatogram compared to the control container, which didnot contain any juice. The extra peaks represented the uptake offlavor compounds by the package.

In a study on the sorption of 5 classes of flavor compounds(aldehydes, methyl ketones, methyl esters, alkylpyrazines, andsulfur compounds) representing a wide range of food productssuch as dairy, fruit, vegetable, and meat products by PP at roomtemperature, the PP was cut into small discs and mounted onto asilanized stainless steel wire alternating with silanized glass beads(Arora and others 1991). The stainless steel wire with PP discs andglass beads were inserted into a silanized screw tight glass bottle.Each of the glass bottles containing a 2% alcoholic buffer of simu-lated milk ultrafiltrate (SMUF) and a flavor compound was placedon a shaker for uniform dispersal of the flavor compound for 24 hat room temperature (24 ◦C). Two sets of controls (buffer solutioncontaining only the flavor compound and buffer solution contain-ing the flavor compound, wire, and glass beads) were placed onthe shaker along with the bottles containing the PP samples. After24 h, the flavor compound in the buffer solution was extractedwith methylene chloride and analyzed by GC. The GC analysiswas performed using a Supelcowax 10 fused silica capillary col-umn. The buffer solution in the controls was also extracted inthe same manner as the buffer solution containing the PP discs.The quantity of flavor compound extracted was determined usingpeak areas. Calibration curves were generated using peak areasfrom known concentrations of flavor compounds.

In the use of refillable bottles such as PET and PC, there is

a possibility that the bottles returning to the loop could havebeen used for the storage of detergents, pesticides, and otherchemicals, posing a threat of severe contamination problems(Demertzis and Franz 1998). A quick test to predict the sorptionproperties of refillable bottles involves measuring the uptake ofmodel contaminants onto bottle strips and subsequent remigra-tion of the chemicals in food simulants. The model contaminantsinclude alcohol-type compounds such as ethylene glycol, phe-nol, n-hexanol, 2-phenylethanol, menthol, and 1,2-decanediol;ester/ketone-type compounds such as ethyl acetate, cyclohex-anone, iso-amyl acetate, benzopheneone, linalyl acetate, andmethyl stearate; hydrocarbon-type compounds such as toluene,n-heptane, p-xylene, limonene, phenyl cyclohexane, and phenyldecane; and chlorinated compounds such as chlorobenzene and1,1,1-trichloroethane. The GC analytical methodology is usuallyapplied in the detection of the contaminants.

In the headspace solid phase microextraction (HS-SPME),volatiles are captured by SPME based on the theory of equilibriumpartitioning of the analytes between the solid phase of SPME, liq-uid or solid sample matrix, and headspace above the matrix andanalyzed by GC (Zhang and Pawliszyn 1993). Traditional meth-ods for the extraction and concentration of volatile compoundsfor analysis by GC such as liquid-liquid extraction, solid phase ex-traction, SFE, static headspace sampling, or dynamic headspace(purge-and-trap) methods have one or more drawbacks such ashigh cost, multistep preparation, low sensitivity, prolonged ex-traction time, artifact formation, or solvent contamination (Brag-gins and others 1999). Compared to these methods, the SPMEtechnique is a simple, rapid, and economical method requiringno solvents (Yang and Peppard, 1994). Nevertheless, SPME anal-ysis is reported to be affected by factors such as the nature of theSPME solid phase, adsorption time, adsorption temperature, saltaddition, stirring condition, and sample size (Steenson and oth-ers 2002; Lee and others 2003; Rodriguez-Bencomo and others2003).

The oxygen electrode (OE) method provides a rapid and accu-rate test for gauging the absorption of volatile compounds. In astudy to measure scalping of limonene by LDPE, the LDPE testfilm was mounted on a YSI 5331 OE. The probe was then placedin the headspace of a flask that contained the selected volatileat the test temperature (Sadler and Braddock 1991). Absorption-induced changes in the readings were monitored on a strip chartrecorder. When stable permeation readings indicated maximumabsorption, the probe was withdrawn and placed in an air streamequilibrated to the test temperature. As the air stream forceddesorption of limonene from the probe membrane, a strip chartrecorder was used to monitor permeability loss in response to des-orption. Desorption was monitored until the permeation readingwas within 5% of its reading prior to volatile treatment. Limoneneabsorption produced an increase in oxygen permeability, whichappeared to be proportional to limonene concentration. Time-course changes in the readings were used to calculate diffusioncoefficients of volatiles in the polymer. Diffusion coefficients werefound to be proportional to the volatile’s solubility in the polymer.

A vacuum-gravimetric method to quantify the sorption ofaroma compounds by food-contact polymers developed byRoland and Hotchkiss (1991) consisted of a microgravimetricbalance contained in a vacuum chamber. Polymer samples wereplaced on the balance in a sealed chamber and outgassed to 10−4

torr. Small amounts of aroma compounds (d-limonene or linalool)were admitted to the evacuated chamber and the mass increase inthe polymer resulting from compound sorption and the pressurewithin the vessel were recorded by means of a computer. Thismethod is proposed to be a useful predictor of individual flavor–polymer interactions at flavor concentrations and amounts thatare usually encountered in foods.

In a method based on weight change in order to study the ab-sorption of citrus flavor volatiles by LDPE, tarred LDPE sheetswere stored in a closed desiccator over a 50-mL pool of theselected volatile (Sadler and Braddock 1991). After 1 wk, thesamples were removed, weighed, and desorbed in a forced-airstream. Weight loss was recorded every 15 min until the poly-mer was within 5% of its weight prior to volatile treatment. Thesolubility of the volatile in the polymer was calculated as:

(A − B)/B × 100

where B and A represented the polymer weights before and aftervolatile absorption.

Cava and others (2005) analyzed the diffusion behavior oflimonene in LDPE as a function of sample thickness, perme-ant concentration, and the outer medium by transmission Fouriertransform infrared (FT-IR) spectroscopy. A surprising reduction indiffusion (D) and permeability (P) coefficients with reducing filmthickness was observed, most likely attributed to the morpholog-ical differences arising during cast film extrusion. Moreover, thelimonene desorption kinetics were found to slow down consid-erably when desorption of the volatile was carried out in watercompared to desorption in air. Finally, the sorption kinetics oflimonene in LDPE were found to be much slower when the poly-mer was put in contact with pressed orange juice (similar to a realjuice packaging case) than when it was put in contact with thepure volatile. A remarkable finding arising from this work is thatthe diffusion coefficient of limonene in LDPE can vary by up to 2orders of magnitude depending on the testing conditions, mainlylimonene concentration but also polymer morphology, and, con-sequently, these observations may well account for the extensivevariability reported for this permeant in the existing literature.

Migrants from packaging materialsWith regard to migration, the control of transfer from plastic

packaging materials into foods is based on the measurement ofthe substance in the food or simulant. The direct migration mea-surement consists of measuring either directly in foodstuffs ormore commonly mimicking as closely as possible a given foodpackaging application using food simulants. The semi-direct mi-gration test applies more severe test conditions by using volatilesolvents with strong interactions toward the plastic to enhance themigration rate from the plastic. Thus, the extraction test is basedon accelerated mass transport mechanisms where the diffusioncoefficients of migrants are increased by several orders of magni-tude compared to the original migration test. As a rule, extractiontests are designed such that they make use of the following prin-ciple: polar polymer + polar migrant + polar solvent = worstcase = nonpolar polymer + nonpolar migrant + nonpolar sol-vent (Franz and others 2000).

When an off-odor problem occurs and is suspected to be pack-aging related, it is suggested that the packaging supplier be ap-proached to find out the nonproprietary aspects of the compo-nents used during manufacture. The next step is to obtain ananalytical profile of the product, typically done using dynamicheadspace GC-MS, while simultaneously conducting taste andodor evaluation. To confirm that the off-flavor problem is indeedpackaging related, a control should be run where the productis stored using a relatively inert barrier such as glass. Finally,one needs to look for volatiles that correlate with the defect andfind their threshold levels. If off-flavor increases over time, thenit would be best to look for alternative packaging materials orpackaging materials with different barrier compositions.

In general, the first stage in the determination of the migrantsin polymers is their separation from the matrix. When the mi-grant is a volatile compound such as styrene, the headspace GC

technique is very suitable for the analysis; it has been used inthe EU project Specific Migration by heating the PS dissolvedin dimethylacetamide at 90 ◦C for 120 min. If the migrant is anonvolatile compound it is essential to use a liquid extractionstep. The solvent used should both dissolve the target compoundand also swell or dissolve the polymer matrix. When the polymeris dissolved in a solvent, the polymer is then usually reprecip-itated by addition of a solvent in which the polymer is insolu-ble.

Usually, extraction procedures are carried out with hand shak-ing (Garcia and others 2006). Nevertheless, sometimes ultrason-ics (Marque and others 1998) or maceration (Monteiro and others1998) could be employed to improve the process. Gramshaw andVandenburg (1995) used dynamic headspace to extract styrenefrom thermoset polyester passing through the U-tube nitrogen at25 mL/min and heating the oven at 200 ◦C. To analyze styrenedimers and trimers, Soxhlet extraction with dichloromethane wasused, followed by size exclusion chromatography to clean up thedichloromethane extracts.

Chromatographic separations in the gas and liquid phases arecurrently state of the art (Brandsch and others 2000). In the au-tomatic PTI coupled to a GC used for the dynamic MHE ofvolatile compounds from aqueous solutions as well as films,trapped volatiles after the purge time are thermally flash des-orbed and directly injected onto the GC column (Feigenbaumand others 1998). In 1 study, the source of an off-odor in pre-miums intended for use with dry mix beverages was evaluatedin accordance with the ASTM F 151-86 (ASTM 1986) methodusing headspace GC/mass spectrometry (Apostolopoulos 1998).The premiums were placed in Mason jars which were sealed withTeflonTM-lined lids, equipped with sampling ports. The Mason jarscontaining the premium samples were placed inside a mechani-cally convected oven and heated at 110 ◦C for 90 min to ensurevaporization of the premium residuals into the headspace of theMason jars. Using a preheated gas-tight syringe to avoid conden-sation of the volatiles, headspace aliquots of 1 mL were with-drawn from the Mason jars and injected into GC-MS, equippedwith a CP-Sil 8 CB chromatographic column operated at 20 ◦C for2 min and then increased at 10 ◦C/min to 250 ◦C. The compoundspresent in the injected aliquots were separated in GC-MS scanswith mass spectral identifications. GC-MS was also used in theidentification of volatiles produced during electron beam irradia-tion (Riganakos and others 1998). About 1.2 to 1.6 g of packagingmaterials was weighed into glass headspace vials, sealed, and ir-radiated. Helium gas at a flow rate of 30 mL/min was passed for20 min through the headspace vials maintained at 80 ◦C. Thevolatile compounds were collected on a Tenax GC plug filledinto the glass liner of an Optic PTV-Injector.

The headspace gas chromatographic (HSGC) method for deter-mining volatile substances emitted from high-temperature foodpackaging is used to screen microwave susceptors and fatty foodsimulants (McNeal 2000). Levels of butadiene in a food or foodsimulant are usually determined by HSGC with automated sam-ple injection and by FID (Franz and others 2000). HS-SPME cou-pled to GC-electron capture detection can be used for the directdetermination of chloroanisoles in wine samples (Urunuela andothers 2004).

Additional selective methods of considerable importance in-clude direct mass spectrometry using an electro-spray-ionization(ESI) or atmospheric pressure ionization (API) ion source, whichcan quickly provide data for several species migrating into a stim-ulant in 1 run. The ESI-MS is especially well suited for complexnitrogen-containing and phenolic structures; combinations ofHPLC with ESI-MS or AI-MS also provide a very powerful tool. Ananalytical procedure based on headspace GC and mass selectivedetection is usually used to determine volatiles whereas HPLC

with UV detection is quite suitable for determining potentialmigrants of high molecular weight such as PET oligomers andadhesive plasticizers arbitrarily classified as nonvolatiles (Holli-field 1991).

Total migration of components from packaging materials intofat-releasing foodstuffs is based on the determination of the weightof a sample of the packaging material before and after its storage ina test fat under standard conditions, a correction being applied forthe residual fat retained in/on the sample (Figge and others 1978).The latter may be determined by physical or chemical methods,the calculation being based in most cases on a single componentof the triglyceride mixture that has migrated into the sample. Withmost test fats, differential migration of the individual componentsof the fat into and incomplete extraction of the absorbed fat fromthe test polymer are serious sources of error. Fewer problems areassociated with the use of 14C-labeled HB 307 as the fat simulantsuitable for all types of plastics, including the highly crosslinkedand insoluble types.

A reference material for the determination of overall migrationfrom a plastic coextrudate into the fatty food simulant olive oilwas produced and certified in an interlaboratory study (Lund andothers 2000). The analyses were carried out according to the ENV1186 standard from the European Committee for Standardization(CEN) with exposure of the coextrudate to olive oil for 10 d at40 ◦C. After an initial preliminary interlaboratory study, 8 lab-oratories participated in the certification round, and 2 differentmethods were used to obtain single-sided exposure of the plas-tic to the oil. A reference value of 8.6 mg/dm2 to 1.4 mg/dm2

(half width of the 95% confidence interval) was obtained, whichis within the range relevant for the regulatory limit (10 mg/dm2),making this reference material suitable for laboratories measuringaccording to the EU overall migration limit.

A neutron activation method developed for the analysis ofHDPE, LDPE, PP, PET, and PS food-contact plastics provides fordetermination of over 50 elements at a concentration of be-low 1 mg/kg (Thompson and others 1996). This technique hasnow been extended to study migration from food-contact mate-rials into standard food simulants (olive oil, acetic acid, ethanol,and water). Samples of plastic are irradiated in a thermal neu-tron flux to produce radionuclides of the elements present in theplastic. Over a period of time, the radionuclides of these ele-ments may travel from the plastic into the food simulants, andhence migration can be determined. Gamma ray spectrometryperformed on the simulants at the end of the test is used to quan-tify the migration, any activity being attributed to the migrationof radionuclides of elements in the plastic. Detection limits ofabout 0.002 mg/kg were achieved for antimony (Sb) in retail PETbottles.

It is imperative that every analytical method adopted for migra-tion studies be capable of reaching the required detection limit,typically 50 parts per billion or less (Buck and Bussey 2002). Toverify whether the methodology is capable of delivering the cor-rect results, FCN (Food Contact Notification) rules state that eachanalytical method be validated. Extracts of samples or controlsmust be spiked with the potential migrant at known levels andthen analyzed using the same method applied to the sample ex-tracts. Adequate recovery of the spiked material shows that themethod works as reported. While chemical analysis guides thesearch for off-flavor contributors in a food, identifying these alsorequires sensory evaluation tools.

LegislationA major concern in packaging is whether the packaging com-

ponents will migrate into the food product. In addition to con-cerns about the safety of a packaged food, packagers must also

address concerns about whether the packaging will affect ap-pearance, flavor, odor, and other factors influencing consumeracceptance. According to FDA regulations, any packaging mate-rial that comes in contact with food must receive FDA clearancebefore it can be used (Buck and Bussey 2002). While consumeracceptance issues will determine the success or failure of a givenproduct, unsafe packaging is a public health issue and will af-fect the future of the food and packaging manufacturers. Packagecomponent migration can result in food adulteration, which vi-olates government laws, and food safety professionals must beaware of regulations pertaining to this area. The major purposefor performing migration studies is to estimate exposure to foodpackaging components (Begley 2000). Exposure is calculated todetermine an estimate of daily intake (EDI). The EDI for a partic-ular migrant is compared with the acceptable daily intake (ADI)of the migrant, as determined by FDA toxicologists.

In addition to positive lists and SMLs, general provisions havebeen introduced under the European Community Directive con-trolling the suitability of materials and articles on the basisof overall migration limits. Also, European Council Directive82/711/EEC, as amended by European Commission Directive93/8/CEE (European Commission 1993) establishes in Article 3that “verification of compliance of migration into foodstuffs withthe migration limits shall be carried out under the most extremeconditions of time and temperature foreseeable in actual use.”Time and temperature are 2 factors that affect the migration of sub-stances from plastic materials, in addition to other factors such ascompression, stacking, or vacuum packing that should be takeninto account when carrying out migration tests under the worstforeseeable conditions. The transfer of substances is regulated by2002/72 with 2 different migration limits, that is, overall migra-tion limit and SML. The overall migration limit, the total quantityof substances released by the sample, is 60 mg/kg or 10 mg/dm2.Some substances are also regulated with SML, because toxico-logical data submitted to the Authority are limited or its impactin higher transfer level is considered to be a concern. SMLs arementioned in Annex II of 2002/72/EC.

The Plastic Materials and Articles in Contact with Food Regu-lations, 1998, set an overall migration limit for all food-contactplastics, the limit being 10 mg/dm2 of plastic surface area, ingeneral. However, a limit of 60 mg/kg of food applies specificallyin the case of containers or similar receptacles with a capac-ity between 0.5 L and 10 L, or which have a contact area thatcannot be determined, and for sealing devices such as caps, gas-kets, and stoppers (available from: www.food.gov.uk). The Aus-tralian Food Standards Code has set maximum migration levelsfor 3 specific monomers, vinyl chloride, acrylonitrile, and vinyli-dene chloride, because of their known potential toxicity (avail-able from: www.foodscience.afisc.csiro.au).

A packaging material with exceptional importance for foodsis PE. Depending on the source of this plastic, water in con-tact with it can produce characteristic flavor changes. The de-scription of the PE odor ranges from candle-like, stuffy, musty tosoapy, to rancid. For this reason, PE used for food applicationshas high-quality standard requirements. Another packaging ma-terial, for which toxicity and safety have been extensively studiedis styrene, which is reported to have no health consequence atthe levels commonly found in foods. The current European legis-lation sets no SMLs for styrene in food, which means its contentis controlled by the overall migration limit of 60 mg/kg in thefood. The overall migration limit is never reached because styrenecreates a strong astringent chemical plastic off-taste at levelsmuch lower than the 60 mg/kg. Recent surveys of styrene lev-els in foods by the MAFF in England have led to the conclusionthat there is no toxicological concern considering the levels (<1to 134 μg/kg) found in foods (MAFF 1994).

Approaches to the Reduction of the Scalping ProcessJudicious selection of polymers is the best way to reduce scalp-

ing by the use of crystalline polymers, polymer blends, or ediblefilms (Gallo and others 1999). Fluorination of polymer surfaceand inert coatings on the polymer surface is yet another approach.Some of the common measures to prevent the scalping process,thus maintaining the integrity of the packaged food items, aredescribed below.

Flavor encapsulationProtection mechanisms designed into commercial flavor de-

livery systems provide for release of the flavor at the appropriatetime. Advances in encapsulation technology allow encapsulationof hydrophobic or hydrophilic flavors, which are available in arange of particle size, shape, and solubility. Flavors can be en-capsulated to protect them from undesirable interactions with thecomponents and the package. The industry is developing micro-and macroencapsulation technologies utilizing various coatingssuch as polymers, waxes, fats, starch, hydrogenated vegetableoils, proteins, maltodextrins, mono- and diglycerides, gum ara-bic, alginates, gelatin, and cellulose compounds (available from:www.foodproductdesign.com)

Electron beam irradiationElectron beam irradiation of ethylene vinyl acetate copolymer

films is shown to markedly depress the sorption of flavor com-pounds such as hydrocarbons and low-polarity compounds (Mat-sui and others 1990). This effect is attributed to the radical reac-tions evolving changes in the structural characteristics such ascrosslinking and scission reactions (Lawton and others 1960).

Selective scalpingFew researchers have addressed how scalping, particularly se-

lective scalping, can improve the flavor profile of food systems.Polymer blends included with an agent that has an affinity for spe-cific compounds can be used to proactively remove deleterioussubstances in a packaging environment (Del Nobile and others2002).

The possibility to trap degradation products (produced bychemical reactions) with scavengers such as a zeolite additiveor antioxidants and hence prevent degradation products from mi-grating to the polymer film surface and further into food in contactwith the film was investigated by Anderson and Forsgren (2005).Adsorption of oxidative degradation products in a zeolite addi-tive or protection of LDPE by using antioxidants could preventoff-flavor in the packed product, such as water.

The migration of low-molecular-weight compounds formedduring polymerization, processing, and forming of packaging ma-terials could be a problem. For example, acetaldehyde is formedin the polyester PET by the thermal decomposition of the ethyleneglycol hydroxyl terminal group and the main chain of the poly-mer (Ikgarashi and others 1989). In their study, poly (m-xylideneadipamide) (nylon MXD6), D-sorbitol, and α-cyclodextrinaldehyde-scavenging agents were blended with poly (ethyleneterephthalate) and thermally pressed into films. The total amountof aldehydes sorbed by the films was 2 to 10 times higherfor films containing the aldehyde-scavenging agents than non-blended films. Aldehyde-scavenging films demonstrated selectivescalping, preferably for smaller-molecular-weight aldehydes.

Odor absorbent packaging can also be used in combinationwith base polymers such as PET, LDPE, and PP to eliminate off-odors produced during long-term storage (Perchonok 2003). Theodor-producing compounds targeted could be aldehydes and ke-tones associated with lipid oxidation and cooked odors. Ketonesare found in beverages and foods such as ultra-high tempera-ture (UHT) milk. The use of polymeric amines in packaging to

remove volatile food components may be an effective meansto remove unpleasant flavor notes since ketones and aldehy-des contain functional carbonyl groups that readily react withamines.

Choice of packaging materialThe choice of plasticizers must be made on the basis of the

film’s end use. A possible approach to reduce the potentialfor migration of plasticizers from PVC into foods is to replacemonomeric with polymeric plasticizer compounds of highermolecular weight with reduced tendency for migration due tolimited mobility in the polymer matrix, for example, a polyadipateester-type plasticizer used at 70% level for low-molecular-weightPVC or 50% level for a high-molecular-weight PVC (Demertzisand others 1990).

Vinylidene chloride polymers. Vinylidene chloride polymersare often used in connection with other structural polymers asa barrier to prevent the entry of oxygen into food containers andto limit flavor scalping during long periods of storage. Althoughthese polymers display outstanding characteristics for use in foodpackaging, they suffer from sensitivity to thermal treatments, un-dergoing heat-induced degradative dehydrochlorination at pro-cess temperatures of 150 ± 170 ◦C. To scavenge evolved hy-drogen chloride and thus prevent the formation of metal halidesby interaction with extruder walls, these materials are usuallyblended with a small amount of passive base prior to processing(Howell and Uhl 2000).

Biobased food packages resistant to scalping. In general, plas-tic materials are not inert and where direct contact between thepackaged product and the plastic container occurs, there can bemigration of substances into the product. The amount of any com-ponent that migrates into food depends on the original concentra-tion of the particular component in the polymer and its solubilityas well as the temperature, mechanical stresses, and contact time.Therefore, there is a considerable interest in replacing some orall of the synthetic plastics by biodegradable materials in manyapplications.

Polylactic acid (PLA) has finally arrived as an alternative toPET, PVC, and cellulosics in some high-clarity packaging roles.PLA, an aliphatic polyester featuring high-clarity, gloss, stiffness,and easy processing, is fabricated by polymerizing lactic acidmonomer (LA) produced by carbohydrate fermentation of corndextrose. Currently, PLA is being used as a food packaging poly-mer for short-shelf-life products with common applications suchas containers, drinking cups, sundae and salad cups, overwrapand lamination films, and blister packages (Ikada and Tsuji 2000).The novel resin is forging roles in thermoformed cups and con-tainers and also single-serve drink bottles. Synthesized from pro-cessed corn, a renewable plant feedstock, it biodegrades afteruse, if composted. PLA synthesis requires 30% to 50% less fos-sil fuel than polymers synthesized from hydrocarbons and thusreduces carbon dioxide emission. These “green” benefits couldprovide users with a marketing edge. Nonetheless, PLA faces hur-dles, including its high density (1.25 g/cc) relative to PP and PS.It also has high polarity, making it difficult to adhere without tie-layers to nonpolar PE and PP in multilayer structures. But thegreatest stumbling block is PLA’s cost, currently an average of$1.30/lb. Notably, PLA delivers a balance of properties that hasgenerated enthusiastic interest from some end users. A good ex-ample of PLA’s early success is a biaxially oriented PLA (BOPLA)film made by Mitsubishi Plastics in Japan. This BOPLA film is lam-inated to paperboard in a golf-ball package for Dunlop Japan.The reverse-printed PLA film becomes the clear viewing win-dow when a panel is die-cut out of the paperboard. BOPLA filmlaminates easily and justifies Dunlop’s “recyclable-compostable”claims.

PLA and PHB (polyhydroxy butyrate) bottles or cups could beused for packaging fresh unpasteurized orange juice (Haugaardand Festersen 2000). PLA cups have relatively low water vaporpermeability and high resistance to scalping compared to PE.Since PHB has a much lower oxygen transmission rate than PLA(Krochta and De Mulder-Johnston 1997) and PHB has high waterresistance (Hanggi 1995), coating of PLA with PHB is expected togive a useful biobased packaging material for beverages. Packag-ing materials based on 100% PHB are also expected to be usefulfor beverages.

One major concern for biobased packaging materials is thatthey are moisture-sensitive. To assure food safety, it is advisedthat these packages be tested under the worst condition of 100%RH for the full anticipated shelf life.

Recently, the application of the nanocomposite concept wasshown to be a promising option to improve mechanical andbarrier properties of packaging films (Avella and others 2005).Nanocomposite films were obtained by homogeneously dispers-ing functionalized layered silicates (clay minerals) in thermoplas-tic starch via polymer melt processing techniques. These filmswere made by using different starch matrices such as potato starchand a mixture of potato starch with biodegradable polyester. In thecase of starch/clay nanocomposites, a good intercalation of thepolymeric phase into clay interlayer galleries, together with goodmechanical parameters such as modulus and tensile strength, wasobtained. Moreover, the conformity of the starch/clay nanocom-posite films with actual regulations and European directives onbiodegradable materials assessed demonstrated that these mate-rials could be utilized in the food packaging sector owing to theirlow overall migration limit.

Packaging for wines and liquors. Closure types vary consider-ably in scalping properties and may cause considerable changesin flavor concentration in wines (Capone and others 2003). Clo-sures tested included screw caps, natural cork closures, a tech-nical cork closure, and 7 synthetic closures. Semillon wine sam-ples spiked with various aroma compounds, including pyrazines,oak-related phenols, ethyl esters of fatty acids, monoterpenes, orhydrocarbons, and sealed with the closures under test were storedfor 2 yr. No absorption of any of the flavor or aroma compoundson the screw caps was observed. Relatively nonpolar compoundswere absorbed by cork or synthetic closures. The more polar com-pounds were not absorbed by any of the closures. For the nonpo-lar compounds, absorption was greatest for the synthetic closures;technical cork closures performed similarly to the natural corks,but had a slightly higher absorption capacity.

For pale ale bottled in clear PET and PEN containers with plasticcaps, and a glass control bottle with a crown closure, sensory, andphysicochemical properties at the 14th wk, showed PEN bottlesto have provided the best protection, followed by glass and finallyPET (Goodrich 1997).

Packaging for aseptic foods. Aseptic foods are packaged in avariety of polymeric materials. The PE liner in brick type asep-tic containers scalps d-limonene, geranial, octanal, and decanalfrom juice products (Arora and others 1991). As a result, the pack-aged fruit juice lacks flavor notes characteristic of fresh juice.There are reports of 62% and 37% sorption of nonpolar and po-lar organics, respectively, by PP. PP is ranked second after PE inthe quantity of flavor compound sorbed. Juices packed in co-extruded PP containers, consisting of PP outer and inner layerssurrounding an internal EVOH layer, have shown a marked de-crease in d-limonene content compared to juices packed in foilcontainers due to sorption. A layer of Saran is suggested with PPand HDPE to reduce the sorption of d-limonene (Arora and others1991).

Commercial scalping-resistant packaging options. Some com-mercial packaging materials are specifically designed to circum-

vent the problems usually associated with flavor scalping.For juices. International Paper Co. (Memphis, Tenn., U.S.A.)

has introduced a new generation of “Super-Marketing” beveragepackaging for juice and milk products, which helps lock in vi-tamins and retain flavor longer. International Paper’s patentedBarrier-Pak (R) technology reduces flavor oil scalping by up to20%, preserving the fresh taste of juice products.

The Lisle, Ill., based EVAL Co. of America recommends theuse of EVOH in direct contact with juices to reduce flavor/aromamigration (available from: www.foodproductdesign.com). EVOHprovides an effective barrier even in the presence of moisture asflavor molecules are significantly larger than oxygen molecules.EVOH is not considered a recyclable material; however, 0.001inches of EVOH provides the same barrier protection as 4.5inches of HDPE (available from: www.foodproductdesign.com).It is advocated to use either PE/paperboard/PE/tie layer/EVOH/tielayer/PE/tie layer/EVOH in direct contact with the juice or aslightly simpler structure consisting of PE/PP/tie layer/EVOH. Thebarrier is an excellent way to protect juice from losing flavor andaroma compounds. Also, a relatively simple structure of HDPE/tielayer/EVOH/tie layer/EVA protects against flavor migration.

McLean Marketing has announced a new concept in pack-aging for Uncle Matt’s organic juices. The company is offeringits organic orange juice in PET bottles that are claimed to offermany advantages, which for consumers could translate into bet-ter juice quality and taste plus an “easy to recycle” container.The PET packaging produces better juice quality over the life ofthe product, providing a better oxygen barrier with less flavorscalping.

Polymer nanocomposites have been demonstrated to helpachieve packaged food a shelf life of 3 to 5 yr (Perchonok 2003).AegisTM OX polymerized nanocomposite, oxygen-scavengingbarrier nylon resin, specially formulated for high oxygen and car-bon dioxide barrier performance, even in high humidity, is com-mercially available for a host of co-injection molded PET bottleapplications, including bottles and orange juice containers. An-other grade, AegisTM NC, can be used as a coating or as the baseresin for cast or blown films and replace nylon 6 coatings in pa-perboard juice cartons. The new family of resins nearly doublesthe heat resistance of nylon 6 and has a potential for less flavorscalping.

For wines and liquor. A clear, high-barrier film that creates asuperior oxygen barrier to protect wine packaged in bag-in-boxwine casks is now available from Scholle Corporation (Chicago,Ill., U.S.A.), a supplier of wine bags worldwide (available from:www.bagnbox.com). This film replaces the traditional metallizedPET film that used to be standard in the wine industry. It essen-tially doubles the shelf life of the wine, resists puncturing andflex-cracking, and can withstand the rigorous transportation con-ditions required for export. DuraShield is now being used in wine-producing countries worldwide. Using the latest polymer tech-nology, it reduces flavor scalping. However, Flextank, a worldleader in using PE for wine aging, maintains that food-grade PEof any density will retain its porosity in the presence of wine formany years resisting clogging by wine components or scalpingflavor from the wine.

Cleartuf Power Polyester Resin (M&G Polymers USA, LLC,Tortona, Italy) is a PE terephthalate copolymer resin designedfor liquor packaging and custom-packaging applications. It is ahigh-molecular-weight polymer designed to provide highly desir-able container properties for the one-way packaging of tunnel-pasteurized beers (available from: www.mgpolymers.com). Theseinclude high clarity and sparkle, increased temperature and pres-sure stability, reduced flavor scalping, reduced carbon dioxideloss due to creep, and reduced carbon dioxide and oxygen perme-ability. Cleartuf Power Polyester Resin is designed with a special

catalyst and stabilizer system that offers property retention duringprocessing.

Miscellaneous options. Honeywell Specialty Films (Morris-town, N.J., U.S.A.) has announced the commercialization ofAclon, a polychlorotrifluoroethylene (PCTFE) resin for use in mul-tilayer containers. Aclon provides the moisture barrier and chem-ical resistance properties of PCTFE in resin form for the blow-molding of flexible and rigid plastic packaging. Incorporated asa contact layer in multilayer extrusions, Aclon has successfullydemonstrated the barrier properties needed to improve the shelflife and reduce flavor scalping of liquids.

Fabri-Kal (Kalamazoo, Mich., U.S.A.), a thermoformer of semi-rigid plastic packaging, has a patented approach for incorporatingdesirable aromas in thermoformed cups, bowls, tubs, and trays(Brody 2005). The aromas can be engineered for release by aninteger of trigger-time, opening of the container, microwaving,and so on. And the reverse, removal of undesirable aroma byactivated carbon, has been developed at the Univ. of Georgiawith funding by Soldier Systems (Natick, Mass., U.S.A.), Com-pelAroma, developed by ScentSational Technologies LLC, is an-other patented and proprietary encapsulated-aroma release tech-nology. The Jenkintown, Pa., based company incorporates spe-cially engineered, FDA-approved, food-grade flavors into mi-crowave trays and other food and beverage packages (Bertrand2005). The flavors, which are generally recognized as safe by theFlavor and Extract Manufacturers’ Assoc., are incorporated withinthe polymetric structure of thermoformed packaging at the timeof molding. These heat-stable flavor additives withstand the highheat histories that plastic packaging experiences during extru-sion and thermoforming and while cooking in the microwave.Although this technology is beneficial to most foods that are mi-crowaved, it benefits foods that lack flavor and aroma needed tosatisfy the consumers’ taste buds.

ConclusionsAs consumers look for better-tasting and ready-to-eat foods,

the food industry will continue to develop new packaging op-tions to meet consumer needs. With these proliferating optionscomes the challenge of living up to consumer expectations ofsafety and wholesomeness of the packaged food item. There isample evidence that sorption of flavor constituents and migra-tion of odors from the packaging material alter the overall fla-vor profile of a packaged product. Recent advances in molecularmodeling simulations serve as an attractive tool to verify and sub-sequently identify the most accurate mechanism for sorption andmigration. As new packaging materials are developed and newapplications are found for the existing materials, further researchmust be performed to confirm the safety of food packaging. Eval-uating the compatibility of the packaging material with the foodby determining the quantity of flavor compounds sorbed by thepackage and also the level of migration into the food will aidfood processors in judiciously selecting packaging materials thatwould help maintain flavor stability during the course of storageof the packaged food product. Also, flavorists, packaging special-ists, and analytical chemists must work closely together especiallywhen packaging films are involved in the development of a foodproduct.

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