oxygen and aroma barrier

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A ReviewOxygen and aroma barrierproperties of edible films:

A reviewK.S. Miller and J.M. Krochta

Interest in maintaining food qual ity while reducing packagingwaste has encouraged the exploration of the oxygen andaroma transport properties of edible films . This review articleintroduces the theoretical basis for oxygen and aroma barrierproperty determination and presents a brief historical per-spective of the development of barrier polymers. The effectsof structure and composition on mass transport in ediblefilms are examined and compared with those of the morethoroughly investigated synthetic polymers. A survey of ediblefilm oxygen and aroma barrier research is presented; areas re-quiring additional investigation are suggested, for applicationsas well as basic research. The potential of edible films andcoatings to provide excellent aroma retention and superioroxygen barrier properties makes this quite a promising area ofresearch for both food and packaging scientists .

Food quality is easily diminished by the deleterioustransport of aroma compoundsand oxygen. Food is re-quired to satisfy the biological need for a source of nu-trition; however, it is the flavor and aroma of a food thatprovide the impetus for its consumption. In fact, a largesegment of commercial manufacturing deals with theproduction of packaging that extends the shelf life offood by controlling oxygen and aroma transport. Afoods characteristic flavor and aroma are the result of acomplex construct of hundredsof individual constituentcompounds interacting to produce a recognizable tasteand aroma. Thus, if one or more flavor constituents arealtered or diminished, food quality may be reduced. Areduction in food quality may result from the oxidationof aroma componentsdue to the ingressof oxygen, or itmay be the result of the loss of specific aroma com-pounds to the packaging material or environment.Therefore, it is critical to identify both the oxygen andaroma mass ransfer properties of food packaging.K.S. Miller (formerly of the Department of Biological and AgriculturalEngineering, University of California, Davis, CA 95616, USA) is now atFrito-Lay, Inc., 7701 Legacy Drive, Plano, TX 75024, USA. J.M. Krochta(corresponding author] is at the Departments of Biological and AgriculturalEngineering and Food Science & Technology, University of California, Davis,CA 95616, USA (fax: +l-916-752-4759; e-mail: [email protected]).

228 Copyright 01997. Elsevw Science Ltd All rights resewed 0924.2244/97~$17.00PI,: 50924.2244(97~01O51-0

Origin and definition of edible polymer filmsFoods such as fruit and nuts have natural built-inpackaging protection in the form of skins and shells.These natural barriers regulate the transport of oxygen,carbon dioxide and moisture and also reduce flavor andaroma oss. However, processed oods dominate todaysdiet; and no such natural barriers exist for processedfoods.Humankinds instinct to cover food (perhaps stem-ming from a desire to hide this precious commodity)may have inadvertently led to the implementation offood packaging. The very first package probably con-sisted of leaves, animal skin or the shell of a nut orgourd. Around SOOOBC,he different types of packag-ing materials hat were available included sacks,basketsand bagsmade from plant or animal material, as well asprimitive pottery and ceramic vessels.By -15OOBC, hollow glass objects had begun to ap-pear, but it was not until -AD~OO that the woven,pressedsheets hat eventually became known as paperappearedr. Lard or wax was used to enrobe fruit andother food items in 16th-century England2. The firstplastic, a cellulose-basedpolymer, was introduced in1856; then in 1907, phenol formaldehyde plastic(Bakelite) was discovered. From then on, a series ofdiscoveries and inventions led to todays multitude ofprimarily synthetic polymer packaging materials.Polymer scientists have produced a variety of syn-thetic polymers and polymer laminates hat are excellentbarriers to both oxygen and aroma compounds. How-ever, despite the availability of these synthetic barriers,the food industry is now considering natural packagingbiopolymers such as edible and biodegradable polysac-charide or protein films. Although these biopolymersshare their origins with the early, all-natural packagingmaterials, they have many of the same properties andare as convenient as the synthetic polymers that theyaugment. Environmental and economic reasonsas wellas product development and consumer trends havepushed food and packaging scientists along this cyclicpath.Edible polymer films may be formed as either foodcoatings or stand-alone ilm wraps and pouches. Thesebiopolymer films have potential for use with food asoxygen and/or aroma barriers2. This reduces the re-quirements of the synthetic polymer to the provisionof a barrier to moisture loss and protection of the foodfrom external contamination. Thus, the amount of syn-thetic packaging is reduced and recyclability is in-creasedbecause he need for synthetic laminates, oftenused o improve oxygen and aroma barrier properties, isdiminished.Regardlessof whether it is a synthetic polymer orbiopolymer, a polymers mass transport properties areinfluenced by similar factors; these nclude compositionand structure, which directly affect a films performanceas a barrier to quality loss. For these reasons,environ-mental and processingconditions that affect the compo-sition and structure of polymer films are of great interestto both food and polymer scientists.

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Box 1. Polymer film ma ss transport propertiesThe diffusion coefficient describes the movement of permeant mol- When S is independent of the sorbed permeant concentrationecules through a polymer, and thus represents a kinetic property of and vapor pressure (i.e. at sufficiently low permeant concen-the polymer-permeant system. Figure 1 shows the activated diffu- trations), then the relationship between c and p become s linear andsion process used to describe permeant movement in a polymer. S is referred to as the Henrys law solubility coefficient. This re-

Activated diffusion is described as the opening of a void sp ace lationship is often used to calculate the solubility coefficient fromamong a series of segments of a polymer chain due to oscilla tions sorption isotherms, which are plots of the permeant concentrationof the segments (an active s tate), followed by translational motion in the headspace above a polymer versus the concentration of theof the permeant within the void space before the segmen ts return permeant within the polymer.to their normal state3. DiBenedetto pointed out that both the ac- The permeability coefficient incorporates both kinetic andtive and normal states are long-lived, as compared with the trans- thermodynamic properties of the polymer-permeant system, andlational rate of the permeant. thus provides a gross mass transport property. The permeability

Ficks first law in one dimension defines the diffusion coefficient: coefficient, P, is most commonly related to D and S as:,=-D$ (1) P=DS (3)

when both D and S are independent of concentration.where / is the diffusive mass transfer rate of permeant per unit area, Permeability is defined at steady state with D and S constant byc is the concentration of permeant, x is the length and D is the dif- integrating Eqn 1 and combining it with Eqns 2 and 3 to obtain:fusion coefficient.

The solubility coefficient describesthe dissolutio n of a permeant (dM/dt) ;;$dy Lin a polymer, and thus represents a thermodynamic property of the P= (4)polymer-permeant system. The solubility coefficient may be de- ASPfined by an adaptation of the Nernst distribution function as: where M is the quantity of permeant (which can be expressed as

c=sp (2) either mass or volume), t is time, L is the polymer film thickness,A is the cross-sectional area of the polymer, Ap is the partial press-where p is the vapor pressure of the permeant and S is the solubility ure difference across the polymer, and P is the permeability coeffi-coefficient. The solubility coefficient is a function of temperature cient. The term (dM/dt) is the slope of the transmission curve andand may be a function of the vapor pressure (or concentration of is required to be at steady state for the permeability coefficientdissolved permeant). calculation.

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This article first reviews the parameters hat are used essential or activated diffusion. Factors affecting a poly-to characterize mass ransport in polymer films, includ- mers structure have a direct effect on segmentalmobilitying the relationship between polymer structure and and, therefore, influence its mass transport properties.those mass ransport parameters.The compositions and Several polymer properties influence permeability:structures of edible films are then compared with those chemical structure, method of polymer preparation, poly-of synthetic polymers, and current researchon oxygen mer processing conditions, free volume, crystallinity,and aroma transport in edible polymers is summarized. polarity, tacticity, ciosslinking and grafting, orientation,Finally, potential applications for edible oxygen and presence of additives, and use of polymer blends4.aroma barrier films are examined, and corresponding Researchers ave shown that an increase n crystallinity,basic and applied researchneedsare identified. density, orientation, molecular weight or crosslinkingresults n decreased olymer permeability5x6.Structural influences on polymer mass ransport A barrier polymer inhibits permeantprogress, herebyproperties presenting a greater barrier to mass transport than theA films mass ransportpropertiesareoften described y permeant would otherwise meet in the absenceof thethree common coefficients: diffusion (the rate of move- polymer. The necessary haracteristics f a barrier polymerment of a permeantmolecule hrough the tangledpolymer include: a degreeof polarity, high chain stiffness, nertnessmatrix, basedon, for example, the sizeof the permeantmoleculeand the struc- Permeant moleculeture of the polymer matrix), solubility(the partitioning behavior of a permeant I of polymer

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Segments I Imolecule between the surface of the chains I II Ipolymer and the surroundingheadspace) Iandpermeability (the rate of transportof 4a permeantmolecule through a polymer l= x-- gg%J-;I I ggasa resultof the combinedeffects of dif- I Reference I Ifusion and solubility). These are for- I ----- pos ition I Imally defined in Box 1.Figure 1 depicts the activated diffu- Normal Activated Normal state aftersion processand clearly shows he im- state state one diffusional jumpportanceof polymer structure or perme- F;n ,ant transport. The ability of a segment @ of the polymer chain to relax and shift The activation process for diffusion. Adapted from Molecular Properties o f Amorphous High Polymers. II.its structure, allowing the permeant An Interpretation of Gaseous Diffusion Through Polymers in). Polym. SC;.: Part Al, Copyright 0 1963,access o newly formed void spaces, s A.T. DiBennedetto, and reproduced with permission from John Wiley & Sons, Inc.Trends in Food Science & Technology July 1997 [V ol. 81 229


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to permeants, high chain-to-chain packing, some inter- them open to engage in hydrogen bonding even when themolecular crosslinking and a high glass transition tem- cohesive energy density is relatively high. In the case ofperature. The effects of the previously ment ioned poly- a polymer with a simple carbon repeat ing unit, a hydro-mer properties on mass transport have been defined gen substituent results in an oxygen permeability coeffi-primarily in terms of oxygen and moisture transport. The cient that is 117500 times greater than that of the samediversity of aroma compounds has impeded the thorough backbone with a hydroxyl group substituent. One wouldinvestigation of their myriad polymer-permeant interac- also expect polymers with a higher cohesive energy den-tions and of the associated effects on aroma permeabili ty. sity to be better barriers to nonpolar aroma compounds.Chemical structures Free volumeKnowledge of the effects of differing chemical struc-tures on a polymers mass transport properties is im-portant for todays packaging industry. The types ofsubstituent groups present in a polymer can have atremendous effect on the variability of the permeabilitycoefficient by influencing two main factors: how tightlythe polymer chains are bound together and how muchfree volume exists between the chain9.

Cohesive energy densityCohesive energy density is a measure of the polarity ofa polymer and of the energy binding the polymer chains to-gether. In general, the higher a polymers cohesive energydensity, the more difficu lt it is for the polymer chains toopen and allow a permeant to pass (highly polar permeantssuch as water being an exception to this rule). An empiri -cal correlating parameter, dubbed the Permachor value, canbe used to predict gas permeation, if free volume and co-hesive energy density are known8. The effects of varioussubstituent groups on polymer permeabil ity are shown inTable 1. As increasingly polar substituents are added to thesame carbon backbone (thus increasing the cohesive en-ergy density), oxygen permeabil ity decreases by five ordersof magnitude. However, water, being highly polar, doesnot rely on the polymer chains to open and can force

Free volume is a measure of the degree of interstitia lspace between the molecules in a polymer. The diffusioncoefficient and the permeability coefficient both decreasewith a decrease in free volume for carbon dioxide, he-lium and methane in various polymers. Maeda and Paulpointed out that the addition of plasticizers to increase thefree volume resulted in lower glass transition tempera-tures, whereas the addition of anti-plasticizers to decreasethe free volume increased the glass transition temperature.Table 2 shows the dramatic effect of free volume on thepermeability of oxygen. As the fractional free volume de-creases from 0.204 for poly(4-methyl pentene-1) to 0.03for poly(viny1 alcohol) (PVOH; see Box 2 for al l polymerabbreviations), the oxygen permeability diminishes by sixorders of magnitude. Stiff-chained polymers that have ahigh glass transition temperature generally have low gaspermeability, unless they also have a high free volumea.These results suggest that nonpolar aroma compoundswould also have low permeabili ty coefficients in polymerswith a low free volume.CrystallinityCrystallinity is a measure of the degree of order of themolecules in a polymer. Polymer properties that affectcrystallinity include the structural regular ity of the poly-, mer chains; polymer chain mobility,

Table 1. The effects of cohesive energy density on permeability

Backbone: CCH,-CHjI

X Permeability at 25CbSubstituent CEDgroup C4 Polymer (cal/cm 3) Oxygen Water

-H Polyethylene 66 0.188 100--0 \ / Polystyrene 85 0.168 1100-OCOCH, Polybin yl acetate ) 88 0.023 8500-Cl Poly(vinyl chloride) 94 0.0036 250-CN Po lyacrylonitrile 180 0.000039 300-OH Poly(vinyl alcoh ol) 220 0.0000 016 (dry) d

Adapted from Ref. 8; reproduced with permission from Technomic Publishing Co., Inc.Units for permeability are cm pm/(m ? d.kPa), whereby a given volume of permeant (cm3) moves through aspecified cross-sectional area @polymer (m), which is of a given thickness (km) , in a certain time interval (d)with a defined pressure driving force (kPa) across that polymer thicknessUnannealed filmdPoly(vinyl alcohol) is soluble in wate rCED, Cohesive energy density

which allows variable conformation;the repeating presence of side chains,which engage in intermolecular bond-ing; and the absence of bulky sidechains, which interfere with the crystallattice formation. The mass transferof a gas or aroma in a semi-crystallinepolymer is primarily a function of theamorphous phase, because the crystal-line phase is usually assumed to beimpermeable. Table 3 illustrates the ef-fects of crystallinity on oxygen perme-ability. As the percent crystallinity of apolymer increases, the oxygen perme-abi lity decreases. The degree to whichoxygen permeability is affected is highlydependent on polymer structure. An in-crease in the crystallinity of polyethyl-ene from 43% to 74% results in a five-fold decrease in oxygen permeability,whereas an increase in the crystallin-ity of poly(ethylene terephthalate) from~10% to 45% yields a threefold de-crease in oxygen permeabil ity. The low

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diffusion coefficients for aroma compounds in glassypolymers suggest that the permeabili ty coefficients forpolymers with a high crystallinity would be correspond-ingly low.Orientation

Orientation refers to the alignment of the polymer chainsin the plane of the polymer backbone, and is a by-productof the processing operation. Sha and Harrison ment ionedseveral mechanisms for these orientation effects. Theyreported that the decrease in the fractional free volumeof the amorphous region with orientation correlated wellwith the decrease in permeability, solubility and diffu-sivity coefficients. However, others contend that the align-ment of the polymers crystallites increases the tortuosityof the permeants path, thus significantly reducing thepermeability only in the case of semi-crystalline poly-mer?. The minim al reduction in oxygen permeabilityfollowing 300% orientation of completely amorphouspolystyrene is cited in support of this observationx.TacticityTactic ity refers to the stereochemical arrangement of thesubstituted groups in relation to the plane of the polymerbackbone. Isotacticity occurs when al l of the substituentgroups lie on one side of the plane of the main chain. Ifsubstituent groups alternate on either side of the plane,the polymer is considered to be syndiotactic, and atactic ifthe substituent groups are randomly configured. Min andPaulI examined the influence of tacticity on the permeabil-ity of carbon dioxide, oxygen and nitrogen in poly(methy1methacrylate) (PMMA). It was concluded that permeabil-ity increased as the percentage of syndiotactic substitu-ents increased. Jasse et ~1.~ suggested hat these resultsmight be indicative of a more densely packed polymerstructure for isotactically substituted polymers.CrosslinkingCrosslinking is the formation of intermolecular bondsamong the chains of a polymer. Researchhas examined

Box 2. Polymer abbreviationsCMC:EVOH:HDPE:HPC:HPMC:LDPE:MC:PEG:PMMA:PVDC:PVOH:VOH:

CarboxymethylcelluloseEthylene vinyl alcohol copolymerHigh-density polyethyleneHydroxypropylcelluloseHydroxypropyl methylcelluloseLow-density polyethyleneMethylcellulosePoly(ethylene glycol)Poly(methyl methacrylate)Poly(vinylidene chloride)Poly(vinyl alcohol)Vinyl alcohol

Table 2. The effects of free volume on permeability

Fractional free Oxygen permeabilityPolymer volumeb at 25CPoly(4-methyl pentene-1) 0.204 1.56Polystyrene 0.176 0.17Polycarbonate 0.168 0.097Poly(methyl methacrylate) 0.132 0.0065Nylon 6 (a= 1 .O) 0.120 0.0029 (dry)Poly(vinylidene fluoride) 0.098 0.0019Poly(acrylonitrile) 0.080 0.000039Poly(acrylonitrile) (annealed) 0.050 0.000016Poly(vinyl alcoh ol) (a= 1) 0.030 0.0000 016 (dry)Adapted from Ref. 8; reproduced with permission from Technom ic Publishing Co., Inc.Fractional free volume is the ratio of the interstitial space behveen molecules to thevolume of the polymer at a temperature of absolute zero-Units for permeability are cm pm/(mV kPa) (see Table 1)~1, Amorphous volume fraction (the ratio of the volume of the polymer that exists in anamorphous state, as opposed to a crystalline state, to the total volume of the polymer)

the effects on mass ransport of polymer crosslinkinginduced by heat curing and irradiation of a variety ofpolymers and by enzymatic treatment of protein-basededible polymers &-] Heat curing of biopolymers re-sulted in decreasedwater vapor permeability for soyproteinI and whey protein isolate15.These effects wereattributed to an increase n intermolecular crosslinkingamong he protein strandsduring heating.Polymer chemists have made great advances in pro-ducing synthetic polymers that have very specific prop-erties and characteristics; however, predicting and con-trolling the structure of biopolymer films are both verydifficult tasks. Food scientists have begun fleshing outthe properties and characteristics of edible films, butmany significant topics pertaining to the application of

Table 3. The effects of crystallinity on permeability

Polyethylene (d = 0.92) 43 0.19Polyethylene (d = 0.955) 74 0.038Poly(ethylene terephthalate) 40 0.0049Poly(ethylene terephthalate) 30 0.0024Poly(ethylene terephthalate) 45 0.0014Nylon 6 0 0.0029 (dry)Nylon 6 60 0.00045dry)Polybutadiene 0 0.97Polybutadiene 60 0.27

Adapted from Ref. 8; reproduced with permissron from Technom ic Publishing Co., Inc.Units for permeability are cm3 p,m/(m d kPa) (see Table 1)d, Density

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edible films remain unexplored. Examination of the in-fluences of the composition of synthetic polymers onoxygen and aroma barrier properties suggests that thepolar nature of edible polymer films should yield excel-lent oxygen and aroma barrier properties.Edible polymer film composition and structureEdible polymer films include polysaccharides and/orproteins. Kester and Fennema? have produced an ex-cellent overview of the types, methods of preparation,properties and applications of all types of edible poly-mers, and pointed out the rationale for developing thesefilms as packaging supplements. The authors noted thatpossible functional properties include the retardation ofmoisture migrat ion, gas transport (oxygen and carbondioxide), oil and fat migration and solute transport, aswell as improved mechanical handling properties, ad-dit ional structural integr ity, use as a vector for foodadditives, and retention of volatile flavor compounds.

Recently, Krochta and De Mulder-Johnston provideda synopsis of the research on edible polymer films andtheir potentia l applications. They also touched on nutri-tional, safety and heal th issues associated with ediblepolymers. Edib le polymer films prepared from celluloses,starches, other polysaccharides (alginates, carrageenansand pectinates) and proteins (collagen, gelatin, ze in, glu-ten, soy protein, casein and whey protein) were reviewed.Water-insoluble cellulose is brought in to aqueous so-lution by etherification with methyl chloride, propyleneoxide or sodium monochloroacetate to yield the non-ionicmethylcellulose (MC), hydroxypropyl methylcellulose(HPMC) and hydroxypropylcellulose (HPC) films and theionic sodium carboxymethylcellulose (CMC) filmst9. Thedegree of substitution that occurs during these etherif i-cation reactions affects the properties of a film such aswater retention, sensitivity to electrolytes and other solutes,dissolution temperatures, gela tion properties and solubil-ity in non-aqueous systems. Cellulose ether films are resist-ant to fats and oils, and are therefore likely to be goodaroma barriers. The cellulose ethers produce moisture-sensitive films that are effective oxygen barriers, andwhen appl ied to various fresh commodities, they havebeen shown to retain flavor components during storage,thus indicating their potential aroma barrier properties.The linear starch polymer amylose produces a hy-drophilic film with low oxygen permeabili ty; hydroxy-propylated amylose also yields films with very low oxygenpermeability19. Plasticization, chemical crosslinking andesterification al l affect the final structure of the starchfilm to varying degrees. Coating apple slices and driedapricots with starch hydrolysates resulted in a better fla-vor, indicating their potential aroma barrier properties.

Alginate films are composed of polymer segments ofpOly@-D-InaIIUUrOniC acid), poly(a-r.-guluronic acid)and of a segment of alternating D-mannuronic andL-guluronic acid residents2. Alginate films have beenshown to reduce oxygen transport and aroma loss in vari-ous food products 19.Alginate film structure is affected bythe concentration of polyvalent cations in the gel (such ascalcium), rate of cation addition, time of cation exposure,

pH, temperature and presence of other constituents such ashydrocolloids2. The calcium ions pull the alginate polymerchains together v ia ionic bonding and thus allow for in-creased hydrogen bonding. The same effect occurs withpectin films. Carrageenan films are thought to form a three-dimensional polymer structure via the formation of adouble-helix structure, which is also thought to be ef-fected by inter-chain salt bridges. The oxygen and aromabarrier properties of films from pectins, carrageenansand other polysaccharides have not been examined in theliterature.Krochta? discussed the effects of protein structure andcomposition on edible film barrier properties. The proteinsmust be in an open or extended form to allow the mol-ecular interaction that is necessary for film formation.The extent of this interaction depends on the proteinstructure (degree of chain extension) and the sequenceof hydrophobic and hydrophilic amino acid residues inthe protein. Increased molecular interaction results in afilm that is strong but less flexible and less permeable.The degree of hydrophilicity of the amino acid residuesin a protein controls the influence of moisture on the pro-tein films mass transport properties*. Most edible filmsare quite moisture sensitive, but this inherent hydrophilicitymakes them excellent barriers to nonpolar substances suchas oxygen and some aroma compounds. As mentionedpreviously, an increase in crystallinity, density, orientation,molecular weight or crosslinking results in a decrease inpolymer permeability. Complicated protein structuresmake the control of these factors quite chal lenging.

Researchers studying edib le polymers have signifi-cant obstacles to surmount in simply producing a usablefilm. Only of late have investigations of edible polymersincluded the examination of barrier properties for per-meants other than moisture. The promise of using a re-newable resource to simplify packaging and extend foodshelf life has encouraged researchers to explore the oxy-gen and aroma barrier properties of edible polymers.Oxygen and aroma barrier properties of ediblepolymer filmsOxygen barrier properties

Oxygen permeability is the next most commonly stud-ied transport property of edible polymer films after watervapor permeability. Commercial data2 on MC and HPMCfilms indicate that they are moderate barriers to oxygen;their oxygen permeability is approximately an order ofmagnitude lower than that of low-density polyethylene(LDPE), but two to three orders of magnitude greater thanthat of poly(vinylidene chloride) (PVDC) and ethylenevinyl alcoho l copolymer (EVOH) at -24C and 50%relative humidity (Table 4). Although cellulose etherspossess a chemical formula similar to that of EVOH, theirrepeating ring and side-group structures probably producea smaller cohesive energy density, larger free volumeand smaller crystallinity relative to those of the linearEVOH. The higher oxygen permeabi lity of HPMC com-pared with that of MC can probably be attributed to thelarger HPMC side group, which results in HPMC havinga smaller cohesive energy density, larger free volume and

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lower crystallinity than MC. Donhoweand FennemaZ2 found that comparedwith other water or water-ethanol sol-vents, oxygen permeability was mini -mized when an MC f ilm was formedfrom a water-ethanol solvent in theratio of 75% : 25% at elevated tempera-ture (Table 4). Films formed in thismanner also had greater crystallinity,lower water vapor permeabil ity, highertensile strength and higher elongation.Donhowe and Fennema31 found thatglycerol, added at 30% (w/w), was amore effective plasticizer than propyl-ene glycol in decreasing the tensilestrength and increasing the elongationof MC films. Both approximately dou-bled the oxygen permeability at -25Cand 50% relative humidity. Lowermolecular weight (molecular weightof 400 and 1450) poly(ethylene gly-~01)s (PEGS) were also good plasticiz-ers but increased oxygen permeabil ityby a factor of 4-5. Park et aL3? foundthat at 0% relative humidity, the oxy-gen permeability of MC and HPC filmsincreased as their molecular weight in-creased. Propylene glycol was shownto be a relatively poor plasticizer andproduced a large increase in oxygenpermeability at 0% relative humidity.Interestingly, although glycerol andPEG-400 were found to be good plas-ticizers for MC and HPC, they had lit-tle effect over a range of concen-trations on oxygen permeability at 0%relative humidity. On the other hand,Park and Chinnan found that thequantity of PEG-400 greatly affectedthe oxygen permeability of MC andHPC at 0% relative humidity. Rico-Peiia and Torres3 found that oxygentransmission through an MC-palmitic

Film type Test conditionsCellulose-based:MCHPMCMCStarch-based:Amylomaize starchHydroxypropylated amylom aize starchProtein-based:CollagenCollagenCollagenZein : PEG t glycero l (2.6 : I)Gluten :glycero l (2.5 : 1)Soy protein isolate : glycerol (2.4 :l)Whey protein isolate : glycerol (2.3 : 1)Whey protein isolate : sorbitol (2.3 : 1)Synthetic:LDPEHDPEPolyesterEVOH (70% VOH)EVOH (70% VOH)PVDC-based films

See Box 2 for polymer abbreviationshUnits for oxygen permeability are cm~~m/(m*~d kPa) (see Table 1) Based on a percentage of the oxygen permeability of PVDC-based film; Ref. 6RT, Room temperatureI H, Relative humidity

acid compositefilm increased rapidly with relative humidity >57%,correlating well with moisture content. Park et al.35studied MC films laminated with a corn zein-fatty acidlayer. They found that oxygen permeability increasedas the concentration and chain length of the fatty acidsincreased.

Table 4. Comparison of the oxygen permeability of edible polymer films and conventional syntheticpolymer films

Permeabilityb Ref.

24C, 50% RH 97 2124C, 50% RH 272 2125C, 52% RH 90 22

25C,


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Films that are based on corn zein, wheat gluten, soy proteinor whey protein appear to possess an oxygen permeabil itythat is greater than that of collagen-based films (at 0%relative humidity)hm29. This is probably due to the factthat these globu lar proteins have a less linear structure anda greater percentage of larger amino acid side groups thancollagen, resulting in a smaller cohesive energy densityand larger free volume. However, proper selection of plas-ticizer appears to reduce the oxygen permeability whilemainta ining the mechanical properties, presumably byaffecting the polymer free volume (Table 4)29.Gennadios et aL2 investigated the effect of tempera-ture on the oxygen permeability of corn zein, wheat glutenand wheat gluten-soy protein isolate films at 0% rela-tive humidity. Results showed good agreement with theArrhenius activa tion energy model. Based on the lack ofbreaks in the Arrhenius plots, no structural transitions wereidentified in the 7-35C temperature range. Brandenburget af.8 discovered that the oxygen permeabil ity of soyprotein films decreased as the pH of the film solutionpreparations increased from 6 to 12. Gennadios et al.found that replacing glycerol plasticizer with triethyleneglycol in wheat gluten films produced a large increasein oxygen permeability . This effect was attributed to thelarger size and less polar nature of triethylene glycol,which would also correlate with an increased free vol-ume and reduced cohesive energy density.McHugh et aL4 studied the properties of films madefrom fruit purCes. Peach puree films were found to bebetter oxygen barriers than MC and other polysaccharidefilms and comparable to whey-protein-based films.In general, the oxygen permeability of edible polymerfilms, especially protein films, appears to be quite low.Optimization of polymer structure by increasing crystal-lini ty, orientation or crosslinking in pre-processing steps orduring film formation may result in further reductions inthe oxygen permeability of a film. Modification of polymerstructure combined with optimized selection of plasticizermay produce edible films with oxygen barrier propertiesthat are as good as those of PVDC and EVOH films.Aroma barrier properties

Although a significant body of work concerned with theoxygen barrier properties of edible films exists, the aromabarrier properties of edible films have not been thoroughlyexamined. Recent reviews of the use of proteins as ediblefilms and coatings indicate that the literature is somewhatlacking in research pertaining to the aroma barrier prop-erties of edible films20s4. However, reviews of the literatureon synthetic polymers are valuable resources to the re-searcher studying the aroma transport properties of ediblefilms [Refs 42 and 43, and K.S. Mille r (1997) PhysicalProperties of Whey Protein Isolate Films: d-LimonenePermeability, Water Vapor Permeability and MechanicalProperties (PhD thesis), University of California, Davis,CA, USA].In fact, Debeaufort and Voilley4 were the first to ex-amine aroma permeability in edible polymers. Theyexamined the co-permeation of moisture and l-octen-3-01(mushroom aroma) in LDPE, cellophane, MC and gluten

(wheat protein) films. An isostatic gas chromatographtechnique was used with a dual-detection scheme for meas-uring moisture and aroma transport simultaneously. Thegluten film was a better barrier to 1-octen-3-01 than eitherthe LDPE or MC fi lm, but not as good a barrier as thecellophane film.

Continuing this work, Debeaufort et al. attempted toexpla in the differences in l-octen-3-01 transport amongLDPE, cellophane, MC and gluten films. However, theywere unable to correlate aroma flux to the amount ofaroma absorbed, the hydrophobicity of the polymer, or totrends in the diffusion coefficient. It was concluded thatthe sorption-diffusion model, alone, cannot describe thearoma or moisture permeability in edible films. Further-more, i t was suggested that the variations in aroma per-meability were due to a moisture plasticization phenom-enon and the sweeping action of water vapofls.Whey protein films have excellent oxygen barrier prop-ertiesz9. However, DeLassus has shown that a polymersoxygen barrier properties are not necessarily a reliableindicator of its aroma barrier properties. The author cau-tioned that oxygen and aroma compounds behave quitedifferently in glassy versus rubbery polymers. Glassypolymers have medium to high oxygen diffusion coeffi-cients but very low aroma diffusion coefficients (at lowpermeant concentrations) rl. Rubbery polymers exhibit dif-fusivities for oxygen and aroma compounds that are ofthe same order of magnitude (i.e. permeant size is not asinfluential a factor). DeLassus stated that trends in oxy-gen and aroma permeability are comparable within therubbery or glassy polymer categories, but not betweenthem. Recent work by Miller et a14 indicates whey pro-tein isolate films to be excellent barriers to d-limonene.Miller and Krochta47 found whey protein isolate filmsconta ining 25% glycerol (dry basis) plasticizer to be com-parable to EVOH films as a barrier to d-limonene undersimilar temperature and humidity conditions. Additionally,d-limonene permeability in 25% glycerol whey proteinisolate films was found to be significantly affected bytemperature and relative humidity but not by permeantconcentrations in the range of 62-226 ppm (v/v).Existing commercial applications of edible films includecol lagen as a casing for sausages and a wrap for smokedmeats, and gelatin and corn zein as encapsulating agentsfor food ingredients and pharmaceuticals20. Evaluation ofthe basic barrier properties of edible polymers will pavethe way for additional app lied research dea ling with spe-cific food applications. Such applied studies of the oxy-gen and aroma barrier properties of edible polymers willaid in defining the limits of specific food applications.Current research on edible biopolymers allows for specu-lation on several food-polymer applications.Gas and aroma barrier food applications of edible filmsOxygen barrier applicationsApplications that take advantage of the beneficia l oxy-gen barrier properties of edible polymer films have beenexplored for many years. Ganz4x found that HPC fi lmcoatings provided peanuts with some protection from oxy-gen, but the effect wasnot well quantified. MC andHPMC

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coatings are commonly used for pharmaceutical tablets,providing protection from oxygen, aroma and moisturetransport. Several researchers have found that CMC-basedcoatings can delay ripening and improve the quality offresh fruit and vegetables by retarding the transport ofoxygenJ9-5.

Park et al.s investigated the application of MC filmlaminated with a corn zein-stearic acid-palmitic acidblend for the packaging of potato chips. Acceptable chipquality was main tained for up to 25 d at 25C. The com-position of the corn zein-stearic acid-palmitic acid blendlayer had no effect on the results.Jokay et aL5 concluded that sensory tests on storedalmond nut meats coated with hydroxypropylated high-amylose starch indicated considerable protection againstthe development of oxidative rancidity. However, quanti-tative data were not presented. Murray and Luft5s foundthat starch hydrolysate coating app lied to apple slices be-fore drying maintained whiteness more effectively than 2%ascorbic acid solution, but not as effectively as sulphurdioxide. However, slices coated with starch hydrolysatewere judged superior in flavor. Murray and Luft5s alsoreported that almonds coated with the starch hydrolysatehad improved flavor and shelf life, indicating oxygenbarrier attributes for the coating; however, they did notpresent any data.

Wanstedt et a1.j found that coating ground pork pat-ties with calcium alginate either before or after precook-ing improved the quality of the final cooked product, asmeasured by the development of oxidative rancidity.Earle and Snyder 57 found that an alginate coating im-proved the flavor and color of frozen shrimps, probablybecause of a reduction in rancidity. Earle and McKeej8developed an alginate-based coating with oxygen barrierproperties for breaded and filled-dough food products.Meyer et a1.s9 found that carrageenan coatings extendedthe shelf life of poultry pieces by acting as an oxygenbarrier. Chitosan coatings were found to be effective inextending the life of fresh fruit by modification of oxy-gen and carbon dioxide transfer60.h.Collagen casings for sausages are known to providesome protection from oxygen:. Gelatin coatings havebeen found to be effective in protecting several meatproducts from oxygen h3.M Zein-based coatings have beenused to reduce rancidity in nuts and confections65,6h. Cornzein films were also shown to affect oxygen and carbondioxide exchange in fresh tomatoes, as evidenced by a de-lay in color change, firmness loss and weight loss dur-ing storage6. The result was an extension of shelf li feby 6d. Coatings based on whey protein were shown toreduce the oxygen uptake by dry-roasted peanuts68, de-laying oxidative rancidity, as measured by the peroxidevalue and hexanal content of the peanuts6.Aroma barrier applications

Edib le films can be used as flavor carriers in additionto providing a barrier to aroma 10~s~~~.AndresO alsopointed out that flavor quality deterioration can includethe loss of characteristic flavor owing to oxidation orpoor oxygen barrier properties. Thus, an edible film can

assist in retaining the characteristic food flavor via itsaroma barrier properties and also limi t quality deterio-ration due to oxidation via its oxygen barrier properties.Researchers have examined the ability of edible coat-ings app lied to harvested fruit to prevent the loss ofcharacteristic flavor. The use of edible coatings on cit-rus fruit resulted in an increase in desirable flavor com-pounds after storage, as compared with uncoated fruits2.Cellulose-based composite films including wax seemedto provide the best balance between flavor retent ionand the prevent ion of weight loss due to moisturetransport5.

Pervaporation, the removal of organics from anaqueous solution through a separating membrane, hasbeen successfully util ized to enrich and recover fla-vor volatiles. Understanding the behavior of flavors inaqueous solutions, such as the systems used in thesepervaporation studies, provides insight into the potentialapplicat ions for edible films in environments with ahigh water activity.The sorption characteristics of edible films may allowthe incorporation of desirable flavors and aromas intoa coating for delayed release, thereby enhancing thefoods flavor prof ile. Encapsulated flavors and aromascould be released by heat ing and/or rehydration, as wellas by mastication. Hydrophilic edible films can be ap-plied to any low-moisture food with a sensitive charac-teristic flavor to aid in aroma retention. An examplewould be fruit-flavored chewing gums, which often losetheir characteristic aroma with time. Dry, fruit-flavoredcereal would be another potential application for ediblefilms to prolong a products shelf life by limiting aromatransport.Basic and applied research needsThe effects of factors, identified by Banker?, that in-fluence film mass transport - polymer structure and orien-tation, salt concentration, ion ratios, polymer-permeantinteractions, acid and base concentrations, addition ofdispersed solids, and permeant boundary layer thickness- provide the edible film researcher with boundless av-enues of research. Specifical ly, no work has been doneto optimize the influences of free volume, crystallinityor orientation on the oxygen and aroma barrier proper-ties of edible polymers.Before a packaging specialist can take advantage of anedible polymers barrier properties, the polymer must besuccessfully applied to the desired food system. Guilbertexamined the factors influencing the food film coating op-eration and concluded that the degree of cohesion (inter-actions among the polymer molecules) and the degree ofadhesion (interactions between the polymer and the foodmolecules) are of critica l importance to the successfulapplication of an edible packaging. The author men-tioned several formulation and processing parametersthat influence cohesion and adhesion, including solutiontemperature, solvent evaporation rate, solvent character-istics and the concentration of the film-forming polymermolecules in the solution. Few researchers have focusedon the effects of these parameters on both the degree of

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adhesion and the degree of cohesion during food filmcoating. Understanding these basic effects is critical tothe successful application of an edible coating to a food.Gaseous diffusion through polymers has long beenstudied by polymer scientists. DiBenedetto3 concludedthat models of such diffusion depend on knowledge of

the physical properties of the polymer and the geometryof the permeant. Lack of knowledge about these poly-mer and permeant properties restricts the applicabilit yof many of the models that have been proposed to pre-dict oxygen and aroma transport.Knowledge about the physical properties of ediblefilms is even more limited. Kester and Fennema* con-cluded that much of the edible fi lm and coating work re-ported in the literature is of limited value owing to thelack of quantitative data on barrier characteristics of thecoatings. It is only through the compilation of barrierproperties and their correlation with edible polymerstructure and composition that it will be possible to applygeneralized theories explaining oxygen and aroma masstransfer behavior to solve food packaging problems.Finally, microbial stability is an area that will be-come more important as more edible polymers approachcommercial viability. This will be especially importantfor higher-water-activity applications. The addition ofantimicrobial agents and their migration in MC andHPMC mul ti-layer polysaccharide films have been ex-amined with respect to their effect on oxygen perme-ability34*73. However, other antimicrobial agents andtheir effects on both aroma and oxygen permeability inedible polymers have not been examined.

References1 Soroka, W. (1995) Fundamentals of Packaging Technology, institute ofPackag mg Professionals, Herndon, VA, USA

2 Kester, J.J. and Fennema, O.R. (1986) Edible Films and Coatings: A Review inFood Jechnol. 40,47-59

3 DiBenedetto, A. T. (1963) Molecular Properties of Amorphous High Polymers.Il. An Interpretation of Gaseous Diffusion Through Polymers in 1. Polym. SC;.:Part A 1, 3477-3407

4 Jasse, B., Seuvre, A.M. and Mathloutht, M. (1994) Permeability and Structurein Polymeric Packaging Materials in Food Packaging and Preservation(Mathlouthr, M., ed.), pp. l-22, Blackie

5 Cuilbert, S. 11986) Technology and Application of Edrble Protective Films infood Packaging and Preservation: Theory and Practice (Mathlouthi, M., ed.),pp. 371-394, Elsevier

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27 Gennadios. A., Weller, CL. and Te stm , R.F. (1993) Temperature Efiect onOxygen Permeabrlity of Edible Protein-based Films in 1. FoodSo. 58,212-214

28 Li, H., Ghorpade, V. and Hanna, M.A. (1993) Effects of ChemicalModrfications on Soy and Wheat Protein Films (Paper No. 936529), ASAE,St Josephs, Ml, USA

29 Habig McHugh, T and Krochta, J.M. (1994) Sorbitol- Versus Glycerol-plasticized Whey Protein Edible Films : Integrated O xygen Permeabrhty andTenslIe Property Evaluation in /. Agric. Food Chem . 42, 841-845

30 Hanlon, J.F. , ed. (1992) Films and Foils in Handbook ofPackage Engrneering,pp. 3.1-3.59, Technom ic

31 Donhowe, LG. and Fennema, 0. 11993) The Effects of Plasticizers onCrystallin ity, Permeability and Mechanical Properties of Methylcellulose Film sin J. Food Process. Preserv. 17, 247-257

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33 Park, H.J. and Chinnan, M.S . (1995) Gas and Water Vapor Barrier Properties ofEdible Film s Fro m Protein and Cellulosic Materials in 1. Food Eng. 25, 497-507

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36 Butler, B.L., Vergano, P.J., Testin , R .F., Bunn, J.M. and Wiles. J.L (1996)Mechanical and Barrier Prope rties of Edible Chitosan Film s as Affec ted byComposition and Storage in ). Food Sci. 61, 953-955, 961

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51 Nisperos-Carriedo, M.O ., Shaw, P.E. and Baldwin, E.A. (1990) Changes inVolatile Flavor Components of Pineapple Orange Juice as Influenced by theApplication oi Lipid and Composite Films in 1. Agric. FoodChem. 38,1382-l 387

52 Baldwin, E.A. , Nisperos-Carriedo, M., Shaw, P.E. and Burns, J.K. (1995) Effectof Coatings and Prolonged Storage Conditions on Fresh Orange FlavorVolatiles, Degrees Brix, and Ascorbic Acid Levels In 1. Agric. Food Chem.43, 1321-1331

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54 Jokay, L. , Nelson, C.E. and Powell, E.L. (1967) Development of EdibleAmylaceous Coatings for Foods in Food Techno f. 21, 1064-1066

55 Murray, D.G. and Luft. L.R. (1973) Low-D.E. Corn Starch Hydrolysates inFood Technol. 27, 32-40

56 Wanstedt, K.G ., Seideman. S.C., Donnelly, L.S. and Quenzer, N.M. (19811Sensory Attributes of Precooked, Calcium Alginate-coated Pork Patties in1. Food Protect. 44, 732-735

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60 Elson, CM. and Hayes, E.R . (1985) Development of the Differentia llyPermeable Fruit Coating Nutri-Save@ fo r the Modified Atmosphere Storageof Fruit in Proceedings of the Fourth National Controlled AtmosphereResearch Conierence: Controlled Atmospheres for Storage and Transport o fPerishable Agricultural Commodities, North Carolna State University,Raleigh, NC, USA

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62 Baker, R.A., Baldwin, E.A. and Nisperos-Carriedo. M.O. (1994) EdibleCoatings and Films for Processed Foods in Edible Coafings and Films toimprove Food Quality (Krochta, J.M., Baldwin, E.A. andNisperos-Carriedo, M.O. , eds), pp. 89-104, T echnomlc

63 Klose, A.A., Mecchi, E.P. and Hanson, H.L. (1952) Use of AntioxIdants in theFrozen Storage o f Turkeys in Food Technol. 6, 308

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65 Alikonls, 1.1. (19791 Candy Technology, AVI Publishing, Westport. CT , USA66 Andres, C. (1984) Natural Edible Coating Has Excellent Moisture and Grease

Barrier Properties in food P rocess. 45, 48-4967 Park, H.J.. Chinnan, MS. and Shewfelt, R.L. (1994) Edible Coating Effects on

Storage Lie and Quality oi Tomato es in 1. Food Sci. 59, 568-57068 Mate, 1.1 , and Krochta, J.M. (1996) Whey Protein Coating Effect on the Oxygen

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Coatings: Effect on the Rancidity Process o f Dry Roasted Peanuts in I. Agric.Food Chem. 44,1736-l 740

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73 Rico-Peiia, D.C. and Torre s, J.A. I1 991) Sorbic Acid and Potassium SorbatePermeability of an Edible Methylcellulose-Palmitic Acid Film : Water Activityand pH Eifects in 1. Food Sci. 56, 497-499

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