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Factors Affecting the Moisture Permeability of Lipid-Based Edible Films: A ReviewValérie Morillon a d , Frédéric Debeaufort a e , Geneviève Blond b , Martine Capelle f &Andrée Voilley ca ENS.BANA, Université de Bourgogne, 1 Esplanade Erasme, 21000 Dijon, France, Tel: +33 (0)3 80 39 66 59 - Fax: +33 (0) 3 80 39 66 11b ENS.BANA, Université de Bourgogne, 1 Esplanade Erasme, 21000 Dijon, France, Tel: +33 (0)3 80 39 66 59 - Fax: +33 (0) 3 80 39 66 11, e-mail: blond@u-bourgogne.frc ENS.BANA, Université de Bourgogne, 1 Esplanade Erasme, 21000 Dijon, France, Tel: +33 (0)3 80 39 66 59 - Fax: +33 (0) 3 80 39 66 11, e-mail: voilley@u-bourgogne.fr.frd Chamtor, route de Pomacle, BP 20, 51110 Bazancourte Institut Universitaire de Technologie - Département Génie Biologique, Bd du Dr. Petitjean,BP 17867, 21078 Dijon Cedex, France, Tel: +33 (0) 3 80 39 68 43 - Fax: +33 (0) 3 80 39 66 11,e-mail: debeaufo@u-bourgogne.frf Nestlé, Centre R&D, rue Charles Tellier, Z.I., 60 000 Beauvais, France, Tel: +33 (0)3 44 1212 12 - Fax : +33 03 44 05 13 75, e-mail: Martine.Capelle@rdbv.nestle.comVersion of record first published: 03 Jun 2010.

To cite this article: Valérie Morillon , Frédéric Debeaufort , Geneviève Blond , Martine Capelle & Andrée Voilley (2002):Factors Affecting the Moisture Permeability of Lipid-Based Edible Films: A Review, Critical Reviews in Food Science andNutrition, 42:1, 67-89

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Critical Reviews in Food Science and Nutrition, 42(1):67–89 (2002)

Factors Affecting the Moisture Permeability ofLipid-Based Edible Films: A Review

Valérie Morillon,1,2 Frédéric Debeaufort,1, 3* Geneviève Blond,1 MartineCapelle,4 and Andrée Voilley1

1ENS.BANA, Université de Bourgogne, 1 Esplanade Erasme, 21000 Dijon, France, Tel: +33 (0) 3 80 39 6659 - Fax: +33 (0) 3 80 39 66 11, e-mail: voilley@u-bourgogne.fr and blond@u-bourgogne.fr; 2Chamtor, routede Pomacle, BP 20, 51110 Bazancourt; 3Institut Universitaire de Technologie - Département Génie Biologique,Bd du Dr. Petitjean, BP 17867, 21078 Dijon Cedex, France, Tel: +33 (0) 3 80 39 68 43 - Fax: +33 (0) 3 8039 66 11, e-mail: debeaufo@u-bourgogne.fr; 4 Nestlé, Centre R&D, rue Charles Tellier, Z.I., 60 000 Beauvais,France, Tel: +33 (0)3 44 12 12 12 - Fax : +33 03 44 05 13 75, e-mail: Martine.Capelle@rdbv.nestle.com

Referee: Dr. Aris Gennadios, Senior Manager, Materials Science, Research and Development, Banner Pharmacaps Inc., P.O. Box

2210, 27261-2210, 4125 Premier Drive, High Point, NC 27265-8144

* Corresponding author: Lab. GPAB, ENSBANA, 1 Esplanade Erasme, F-21000 Dijon, Tel : +33 (0)3 80 39 68 43, Fax : +33 (0)380 39 66 11, E-mail: debeaufo@u-bourgogne.fr

ABSTRACT: Moisture transfers inside food products could be controlled or limited by the use of edible films.These are usually based on hydrophobic substances such as lipids to improve barrier efficiency. Water permeabilityof films is affected by many factors, depending on both the nature of barrier components, the film structure(homogeneous, emulsion, multilayer, etc.), crystal type, shape, size and distribution of lipids, and thermodynamicssuch as temperature, vapor pressure, or the physical state of water in contact to the films. After a brief presentationof lipids and hydrophobic substances used as moisture barrier, cited in the scientific literature, this article reviewsall of the parameters affecting barrier performances of edible films and coatings.

Key Words: composition, structure, physico-chemistry, transfer, water.

I. INTRODUCTION

Food quality deterioration due to physico-chemical changes or chemical reactions are oftencaused by mass transfer or between food and thesurrounding medium or inside the product. Com-pounds whose migration has to be controlled arewater, oxygen, flavor, the main compound beingwater. The traditional packaging is sufficient toretard moisture loss or gain in food but cannot beapplied to isolate or separate different compo-nents varying in water activity within a compositefood. So edible barriers were developed, espe-cially based on hydrophobic substances such aslipids.

Because of their apolar nature, hydrophobicsubstances are used mainly as a barrier againstmoisture migration.1 The study of the scientific,

patent, and technical literature over the last 15years permits the listing of all hydrophobic sub-stances used as components of edible films andcoatings (Table 1). A very wide range of com-pounds could be used in the formulation of ed-ible packaging and, then, their choice dependsmainly of the target application. So, many stud-ies deal with the use of coatings on fresh fruitsand vegetables to control their desiccation. Thesubstances the most often used for this applica-tion are waxes.2,3,4 In the oenology domain, corkfor wine bottles could be coated by beeswax andparaffin wax to prevent cork soaking and therelease of off-flavor from the cork to the wine.This is an alternative way to avoid cork substi-tution by plastic.5 Cocoa butter and cocoa-basedfilms are widely used in the confectionery andbiscuit industries, other films in pizza, and dough

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TABLE 1Hydrophobic substances (lipids, lacs, varnishes, resins, essential oils and emulsifiers)potentially used as film-former or barrier compound

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products to prevent moisture absorption by crispyand dry cereal-based products to prevent the lossof flavor and water from fat or aqueous stuffingand filling.6,7,8 Most of the food-grade lacs andvarnishes are applied in pharmaceutical domainas a protective layer for active substances, or inthe food industry to improve the surface appear-ance (color, shininess, glossiness) and feeling(nonsticky) and in the flavor industry. Few stud-ies deal with the applications of lac and varnishfrom natural origin but many food additive sup-pliers propose many of them as coating agents.9

Emulsifiers and surface active agents are some-times used as a barrier to gases or moisture onfish or meat products,4,10,11 but they are added incoating recipe to improve their adherence on thefood to be coated.

Table 2 gives the permeability of some lipid-based edible films as a function of their composi-tion or structure. Some of the lipid and food-gradehydrophobic substances have permeability valuesclose to that of plastic films, such as low-densitypolyethylene or polyvinyl chloride. The perme-ability of lipids having high solid fat content isusually much lower than that of liquid lipids.However, most of solid-lipid-based films are brittlewhen used alone, so they are often combined withhydrocolloids to form bilayer or emulsion films.Their structure and barrier properties depend onthe preparation technique used. The most com-mon techniques are

• deeping support in molten lipid to have anhomogenous fat layer,12–13

• casting and drying a film-forming emulsion inwhich fats are dispersed in a hydrocolloid aqueoussolution to obtain an emulsified edible film,14–18

• deposition of a lipid layer previously moltenor solubilized in an adequate solvent on ahydrocolloid-based film used as a mechani-cal support to obtain bilayer or multilayerfilms.9,19

The barrier efficiency of bilayer films is of thesame order of magnitude that that of pure lipid orplastic films and is much lower than that of emul-sion-based films. The permeability of the later isoften very close to that of protein or polysaccha-ride based edible films as shown in Table 2. Watervapor permeability values could vary in a widerange according to many parameters related to theexternal conditions (temperature, relative humid-ity, pressure, …) or to the film.

The composition and structure of edible filmsaffect water transfer mechanism and thus barrierperformances, as much as the physico-chemicalproperties of the penetrant such as concentrationor moisture affinity. Both edible films and coatedor wrapped food products are exposed to variousfactors such as variable temperatures during pro-cessing, carrying, and storage. So, the knowledgeof the behavior of the barrier layer as a functionof physical and chemical parameters is needed forimproving the formulation of edible packaging.Many factors affect functional properties of ed-ible films, which can be categorized in two groups:those concerning the composition and the othersdealing with the structure.

TABLE 1 (continued)

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TABLE 2Water vapour permeability (WVP) of lipid-based edible films and plastic packaging

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TABLE 2 (continued)

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TABLE 2 (continued)

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TABLE 2 (continued)

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II. INFLUENCE OF THE FILMCOMPOSITION

A. Effect of Chemical Nature andConcentration of Ingredients

Lipid-based edible films have a low affinityfor water, which explains why they have lowmoisture permeability.

Each hydrophobic substance has its ownphysico-chemical properties, and thus edible filmsbased on lipids have variable behavior againstmoisture transfer. Polarity of lipid constituentshave to be considered, that is, the distribution ofelectrostatic potentials on the molecules that de-pends on the chemical group, aliphatic chainlength, and on the presence of unsaturation.

Waxes are the most efficient substances toreduce moisture permeability because of a highhydrophobicity due to their high content in longchain fatty alcohols and alkanes with long chains.Indeed, paraffin wax composed of alkanes andcandelilla wax containing 57% of alkanes havevery low water vapor permeability.20 Kester andFennema12 compared fats having different chemi-cal groups applied on a polar support (cellulose-based filter paper) and classified them accordingto their barrier efficiency. The most efficient isthe beeswax, followed by the stearyl alcohol, acetylacyl glycerols, hexatriacontane (C36H74) tristearin,and stearic acid. This classification can be ex-plained by the hydrophobicity of the moleculesthat interacts more or less with water. Stearylalcohol seems to be a good barrier compared totriglycerides or fatty acids because the hydroxylgroup has less affinity for water than carbonyland carboxyl groups.12 Considering only the po-larity, alkanes should be always the best, but thiswas not observed in the case of hexatriacontanebecause another factor such as the structure oc-curs.

For components having the same chemicalnature, the chain length modifies the barrier prop-erties. McHugh and Krochta16 stated that mois-ture barrier efficiency of fatty alcohols and fattyacids increases with carbon number (from 14 to18) because the apolar part of the molecule in-creases and does not favor water solubility in thefilm and thus moisture transfer. Many authors

also showed that among the carboxylic acids,stearic and palmitic acids present the lowest watervapor permeability.15,21–23 However, when the car-bon chain length is higher than 18 atoms, filmscontaining arachidonic or behenic acids havehigher permeability. This is explained by verylong chains inducing an heterogeneous structureof the polymer network.15 These results are shownin Figure 1. The behavior of all films is similar forthe three relative humidity gradients studied ex-cept for lauric acid, for which authors did notprovide any explanation. Moreover, it should benoted that very little data are available for triglyc-erides. Talbot6 displayed that fats with short chainlength (from 8 to 14 carbons) are more efficient toreduce moisture transfers through films than reci-pes containing 16 or 18 carbon atoms. However,this author made permeability measurement atdifferent temperature with the aim to keep con-stant the solid fat content (SFC) of the films.

Unsaturated fatty acids are less efficient tocontrol moisture migration because they are morepolar than saturated lipids. Indeed, films contain-ing stearic or palmitic acids have better perfor-mances to reduce desiccation of oranges thancoatings composed of oleic acid.23 The water va-por permeability of an emulsified edible film basedon oleic acid is about 80 times higher than that ofthe one composed of stearic acid, but oleic acid isin a liquid state at 25°C contrary to stearic acid.24,25

When the unsaturation degree increases, the melt-ing point of lipid decreases; for instance, it is 71,13, –5, and –11°C for stearic, oleic, linoleic, andlinolenic acids, respectively. The increase of theliquid content modifies both physical state andstructure of films that affects strongly their barrierproperties. The conformation of unsaturated fattyacids is important too because their chemical char-acteristics are different. For instance, elaidic acid(trans conformation) has a melting point at 44°C,which is an intermediate value between the one ofcis conformation (oleic acid) and that of the cor-responding saturated fatty acid (stearic acid).26

Besides the physical state, crystallization changesaccording the unsaturation degree and conse-quently the conformation modify the film densityand permeability. Kamper and Fennema24 reportedthat the area occupied by a molecule of oleic acidin a monolayer film is 0.48 nm2, whereas it is only

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0.23 nm2 for the stearic acid. Gontard et al.17

reported that oleic acid has a greater mobility dueto the double bond that favors water moleculediffusivity.

The concentration of hydrophobic substanceis a key factor for the barrier efficiency. Watervapor permeability of films composed of stearicacid and hydroxypropyl methyl cellulose (HPMC)depends on the fatty acid concentration, and itstrongly decreases for concentration values higherthan 14%.21 As shown in the Figure 2, the opti-mum stearic acid concentration is between 40 and50% with the lowest permeability. For higherconcentrations of stearic acid, films become tooheterogeneous to form a continuous barrier.

Koesch and Labuza15 displayed the same re-sults for films composed of methylcellulose andsome fatty acids with the lowest permeability for40 to 50% of stearic acid, but permeability is 1.5times lower when the film-forming solution con-tains ethyl alcohol. From these authors, mechani-cal resistance and cohesiveness of films havingstearic acid contents higher than 46% is notablydecreased. Moreover, the addition of waxes de-creases water vapor permeability of whey pro-tein-lipid-based films up to a 35 to 40% concen-tration of lipid.27 Increasing the amount ofcarnauba wax increases the barrier efficiency

against moisture transfer of films containing res-ins, shellac, or oxidized polyethylene waxes.2

McHugh and Krochta16 also observed a weakincrease of the barrier performance of whey pro-tein films when the content of beeswax, palmiticacid, or stearyl alcohol increases. In the case ofstearic acid, Sapru and Labuza28 explained thatthe increase of the permeability for concentrationhigher than a critical value is due to the formationof large fat crystals allowing interstitial zones freeof lipids that favor moisture migrations.

Moreover, the physical structure of all ingre-dients is also responsible of the barrier propertiesof composites lipid-based edible films.

B. Role of the Physical State andStructure of Ingredients

The solid or liquid state of lipid compoundsstrongly influences the barrier efficiency of the film.The permeability of a hydrogenated cottonseed oil-based film increases by 300 times when the liquidfraction of the lipid varies from 0 to 40%.7 MartinPolo et al. 29 showed that the permeability of aparaffin wax increases 100 times when the paraffinoil content increases from 75 to 100%. Several au-thors showed that the increase of the solid fat con-

FIGURE 1. Water vapour permeability of some fatty acids at three relative humidity gradients and 22.6°C(from Koelsh and Labuza15).

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tent especially between 0 and 30% allows for theimprovement of the barrier efficiency.6,24,30 Indeed,CH2 groups of liquid aliphatic chains have a greatervolume, from 4 to 5.10–3 nm3, than when they arecrystallized.31 So, the solid structure of fats is moredense and limits the diffusion of water. Moreover,the solubility of water in solid lipids is also re-duced.1,24 However, for solid fat contents higherthan a critical value that depends of the lipid nature,permeability could increase due to structure defectswithin the film. Martin-Polo et al.29 observed forcoatings composed a mixture of pure alkanes (C16H34

and C28H58) that the permeability increases for solidfat content higher than 50%, which was not ob-served in the case of paraffin wax and oil blends asshown in Figure 3. The scanning electron micros-copy analysis displayed an heterogeneous structurewith ‘needle-like’ crystals for 100% octacosane(C28H58), whose melting point is 65°C. This “po-rous-structure” and heterogeneity of the high solidcontent lipid distribution explains the decrease ofbarrier efficiency of these films compared with thosecontaining hexadecane (C16H34), which is liquid at25°C and whose crystals looks like platelets and aremore dense.29 A more complex mixture of alkaneslike in paraffin permits masking the defects in thelipid by filling all the cracks due to the retractionduring the crystallization of high solid content al-kanes.

Fat polymorphism, that is, their ability to crys-tallize under several forms, also affects the barrierefficiency of lipid-based edible films, particularlyfor hydrogenated oil and cocoa butter-based films.Triglycerides crystallize under three main typescalled, respectively, α (hexagonal), β (triclinic)and β′ (mainly orthorhombic, but other forms arealso found). In the case of β and β’, crystals arestabilized by London forces occurring betweenthe aliphatic chains, which give dense networks.On the contrary, these forces do not exist in the αform and carbon atom could rotate (small angles)to lead to an hexagonal non-stable structure. Theorder α, β, and β′ corresponds to an increasingstability of the crystal and to a higher density;indeed, the average volume of the CH2 groupsare, respectively, 30, 25, and 24 Å for each crystalspecies.32,33

Kester and Fennema34 studied the effect ofthe polymorphism of a mixture of fully hydroge-nated soya and canola oils on their barrier perfor-mances. The molten lipids were deposited on afilter paper used as a support and rapidly cooledat room temperature to obtain the α form. Thenthey tempered the α films at 53°C for 24 h toobtain the β′ form, and then they tempered the β′film at 58°C for 336 h to obtain the β type. Theresistance to the water vapor transfer of thesefilms is 40200, 30600, and 38700 s.m–1 at 25°C

FIGURE 2. Influence of stearic acid concentration on a hydroxypropyl methylcellulose-based ediblefilm at 25°C and ∆RH = 0 to 85% (from Hagenmaier and Shaw21).

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for the α, β′, and β forms, respectively. Even ifthe β′ form is lightly more dense than the α one,for instance, 1.014 and 1.017 g.cm–3 in the case oftristearin, aliphatic chains in β′ crystals are lessflexible and should limit water diffusivity. How-ever, the higher flexibility in hexagonal crystalsprovides plasticity to the film and prevents defectformation during crystallization and thus improvesthe moisture permeability.34 Crystallization in theβ form increases the mass transfer resistance ofthe films compared with crystallization in the β′form. This is mainly explained by the greaterdensity (1.043 g.cm–3 for tristearin) of the β crys-tals but also by a very slow crystallisation rategiving less structure defects. It is a little surpris-ing to note that the water vapor transfer resistanceof the β form is not higher than that of α one.Some authors supposed that hydration of crystalsvary according to the polymorphism. The size ofcrystals could also play an important role becausethey are usually as much bigger as they are stablewith time.

Edwards35 observed that paraffin and micro-crystalline waxes could present both small crys-tals looking like needles in the hexagonal formand big and flat orthorhombic crystals. Greener-Donhowe and Fennema20 showed that beeswax,candellila, carnauba, and microcrystalline waxeshave all the two typical X-ray diffraction peaks(at 0.413 to 0.414 nm and 0.372 to 0.373 nm) of

the orthorhombic form. These waxes are goodbarrier against moisture transfers, but their effi-ciencies are different and cannot be related topolymorphism and size of crystals. Indeed, car-nauba wax presents the most heterogeneous sur-face with pores, whereas microcrystalline waxhas the smoothest surface. However, films com-posed of carnauba or microcrystalline wax haveneither the best nor the worth barrier performances.Their permeability is probably more affected bytheir affinity for water.

The α form of the monoglyceride ester mix-tures is very stable except for the 1,2-diacetyl-3-stearyl glycerol whose permeability decreases from103 to 0.75 ×10–12 g.m–1.s–1.m–1.Pa–1 after 12 daysat 21°C because of a change of polymorphismtowards a more stable and dense crystal.26

Landmann et al.7 observed that the tempering ofcocoa butter induces a decrease of the moisturepermeability of films by 10 times. However, dur-ing aging of cocoa butter containing high liquidfractions of lipid, the water vapor permeabilityincreases continuously with time probably be-cause of a growing of crystals, which let freeinterstitial spaces where water molecules couldmigrate.7,35,37 So the size and distribution of fatcrystals seem to affect the barrier efficiency asmuch as their polymorphism.

Kester and Fennema83 studied the effect ofaging on the moisture permeability of stearyl

FIGURE 3. Influence of the solid fat content of lipids on the water vapor permeability of lipid coatingsat 25°C (from Martin-Polo et al.29,30).

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alcohol-based films without changing the poly-morphism, but only the size and shape of crys-tals were modified. Stearyl alcohol crystal-lizes in a orthorhombic form (β′) whosestructure remains stable up to 54°C. The mois-ture transfer resistance of coatings stored at48°C for 0, 14, and 35 days increases from 11to 50% with time. These results are in agree-ment with those reported by Fox,39 who dis-played a significant rise of transfer resistanceof paraffin wax films after they have beentempered at high temperatures. Stearyl alco-hol crystal shape is like platelets horizontallyoriented to the film surface, which reducesmoisture migration. The number and size ofcrystals increases with aging at 48°C becausebig crystals are thermodynamically morestable. The mechanism responsible of improv-ing barrier properties with time is not fullydescribed and explained, but it seems that twoparameters are important: the decrease of struc-ture defects when recrystallization occurs withlow rate, and the increase of the solid fat con-tent as long as a part of liquid lipid crystal-lizes.38 Fox 39 explained the better barrier effi-ciency of aged paraffin waxes by large crystalsdisposed parallely to the film surface. X-raydiffraction patterns of films composed of al-kanes or mixtures of paraffin oil and waxshowed they are all in orthorhombic form, buttheir shape and size are very different as shownby scanning electron microscopy.29 Aliphaticchains of paraffins are parallely oriented tothe support, whereas they are perpendicularlyorganized in the case of pure alkanes as ob-served by Fox.39 So, pure alkane crystals ag-gregates as needles, whereas those of paraffincannot be clearly observed by SEM, even for1000-fold magnification. Paraffin gives morea homogeneous structure, which probably ex-plains their great performance to reduce mois-ture transfer.29

The chemical nature of ingredients contrib-utes to the barrier efficiency because it partiallyaffects the affinity of the film for water. Physicalstructure of lipids determines the film structureand then the permeability even if the process ofassociation differs. The film structure is then aparameter that has to be considered.

III. INFLUENCE OF THE FILMSTRUCTURE ON THE BARRIERPROPERTIES

A. Role of the Preparation Technique

Edible films containing lipids could be ob-tained from different techniques, which determinethe barrier structure as previously described.

Several authors studied the influence of thefilm preparation technique on its barrier proper-ties.24,40,41 Kamper and Fennema24 made filmscomposed of hydroxypropylmethyl cellulose andstearic and palmitic acids from emulsions withpermeability 40 times lower than bilayer films,respectively, 0.5 10–12 and between 19 and 28×10–12 g.m–1.s–1.Pa–1, whereas the amount of lipidwas 10 times lower in the emulsified films. Mar-tin-Polo et al.40 and Debeaufort et al.41 displayedopposite results for methylcellulose-paraffin waxfilms. For these films, the water vapor permeabil-ity of emulsified film is 40 times higher than thebilayer films. These contradictory results couldbe explained by considering film preparation con-ditions. Although the initial emulsion prepared byKamper and Fennema24 was homogeneous, a phaseseparation is observed during drying that leads toan apparent bilayer structure for the emulsifiedfilm. Debeaufort et al.41 explain observed differ-ences by the distribution of lipid globules in themethylcellulose matrix. Scanning electron micros-copy observation reveals irregular, rough, andheterogeneous surface of emulsified films, whereassmooth and homogeneous surface are observedfor bilayer films. So a better distribution of lipidover the surface is more efficient to control watertransfers.

In emulsified films, water migrates preferen-tially through the continuous hydrophilic matrix,and the dispersed lipid phase only modifies the“apparent tortuosity”. So, McHugh and Krochta42

observed a positive linear correlation betweenpermeability and the mean diameter of beeswaxglobules in a whey protein matrix. Moreover,they showed that immobilized proteins at the lipidparticle interface induce a decrease of film per-meability. In this type of film, drying conditionsare key parameters because they influence thestability of the film-forming emulsion and thus

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the final film structure. In methylcellulose andparaffin wax films, high air-drying rates favorcreaming of the lipid phase, whereas temperaturetends to increase aggregation and coalescence.43,44

The hydrophobic fraction moves to the surface toform a homogeneous second layer as shown byKamper and Fennema24 when films are dried at90°C for 15 min. As displayed by Martin-Polo etal.,40 it seems that the most efficient barriers areobtained when the film structure contains a con-tinuous lipid layer obtained either from bilayertechnique or after destabilization of emulsionduring drying. In chocolate, the continuous phaseis composed of fats and due to conching, hydro-philic particles (sucrose crystals, cocoa powder)do not significantly affect the water vapor perme-ability except for a high relative humidity forwhich film structure is modified.8,45 Hoskin andDimick46 observed structural changes duringconching due to fats that fully enrobed hydro-philic particles. These phenomena are slightly im-proved by the addition of lecithin. So, chocolateor cocoa-based coatings are solid suspensionswhose continuous phase is hydrophobic and pro-vides good barrier performances.

Because of their poor mechanical properties,lipids are often used with a support, which can besometimes considered a model food to be coated.However, this support could strongly influencefilm properties by modifying the distribution ofhydrophobic substances. The more porous is thesupport, the less efficient is the barrier becausethe surface to be protected is much greater andbecause the rough surface made less easy theapplication of a continuous and regular coatinglayer. In the case of highly porous support, suchas filter paper, lipids fill the pores but moisturecan migrate through the hydrophilic fiber net-work. Besides, if the support is too polar, hydro-phobic compounds do not adhere and emulsionsbecome unstable when applied on the support.40

Moreover, a high affinity of the support for watercould induce a loss of the barrier properties of thecoating as observed by Kester and Fennema.12

However, the support can greatly improve themechanical resistance of fats when it containshigh amount of fibers.12 Greener-Donhowe andFennema47,48 applied a methylcellulose film tobrownies to fill pores of these biscuits with the

aim of depositing the lipid moisture barrier on asmooth surface. The efficiency of this beeswax-based coating is four times greater when the me-thylcellulose support is used. These authors ex-plain this better performance by less structuredefects initially caused by an irregular and hetero-geneous surface.

The adhesion of a film on food products is aphysical phenomenon that could be controlledeither by the overlapping of rough surfaces, bywettability forces due to interfacial forces be-tween materials whose at least one is liquid, or byelectrostatic forces between charged polymers, orby chemical bonds when reaction occurs.49

Hershko et al.50 studied the interactions betweenfood and coatings, particularly the effect of thesurface roughness and the penetration rate of thecoating in the garlic skin. Nussinovitch andHershko51 improved the adhesion of coatings com-posed of alginates and gellan gum on garlic byusing naturally present molecules from garlic skinas the β-sitosterol. The adhesion strength of thesecoatings is increased by 2 to 3 times when 0.2%of β-sitosterol is used and their barrier efficiencyis also improved. The adhesion of the batter onpoultry meat pieces is significantly increased byusing proteins in the mix; egg albumen and gela-tin are then the most efficient.52 Hagenmaier andBaker3 showed that the surface tension of liquidcoatings containing shellac or waxes has verylittle effect on their barrier properties when ap-plied on orange skin. Indeed, all these coatingsform a continuous layer on the fruit surface be-cause the contact angle is lower than 90°, and thebarrier performances mainly depend on the natureof the lipid. Barron53 displayed that chocolatecoating, whose adhesion on a biscuit is very good,tends to crack because the latter swell when mois-ture is absorbed. So in this case, a too good adhe-sion of the coating on the support is not an advan-tage.

The interaction occurring between the sup-port and the coating has to be taken into accountbecause it could modify its barrier properties ei-ther because of a partial absorption of the barrierlayer by the support or by a heterogeneous lipiddistribution. Indeed, barrier coating can only beefficient if it forms an homogeneous and continu-ous layer on all the surface of the food product to

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be protected. When cracks appear, all barrier prop-erties are lost. The continuity and the homogene-ity of the barrier strongly depend on the filmthickness.

B. Influence of the Film Thickness

Film thickness could modify some of theirproperties such as the barrier performance. Ac-cording to the following equation, the WVTR(water vapor transfer rate) correspond to theamount of water vapor (∆m) transferred througha film area (A) during a definite time (∆t) wherepermeability is WVTR normalised by thickness(x) and partial vapor pressure gradient (∆p):

WVTRm

A t= ∆

∆. (g.m–2.s–1) and P WVTR

x

p= .

∆

(g.m–1.s–1.Pa–1)

Martin-Polo et al. 40 noted that the water vaportransfer rate of cellophane films coated with paraf-fin wax or oil decreases when thickness increasesfrom 30 to 60 µm but remains quasiconstant up to120 µm thick. In the latter case, permeability in-creases with thickness. These results could be dueto film discontinuities, which are more importantfor thin films. Debeaufort et al.44 showed that wa-ter vapor transfer rate and permeability exponen-tially decrease when the triglyceride layer thick-ness increases from 0 to 60 µm and remain constantfor higher thickness, although the critical value isabout 300 µm in the case of paraffin-based films.In chocolate, Biquet and Labuza54 observed a de-crease of the water vapor transfer rate and perme-ability through films whose thickness varied from0.6 to 0.9 mm, whereas water vapor transfer ratestill decreases between 0.9 and 1.2 mm but perme-ability increases. Landmann et al.7 also displayedan increase of the water vapor permeability of thecocoa butter when the film thickness rises becausethe water vapor transfer rate does not vary signifi-cantly. The increase of the film permeability withthickness indicates a water affinity of the film thatcould be dedicated to hydrophilic compounds. Thenit seems that the influence of thickness varies withthe lipid nature or the film composition. Lovegrenand Feuge36,55 observed that thickness of acetyl

stearyl glycerols strongly affects the water vaporpermeability but not oxygen, carbon dioxide, ornitrogen permeabilities. The transfer rates of thesenoncondensable gases decreases proportionally withthe thickness variation according to Fick’s andHenry’s laws. The deviating behavior observed forwater vapor transfer should be explained by a highersorption phenomenon.

Mass transfer resistance of lipid compoundsagainst gas migration such as oxygen is mainlydue to their structure, the more the crystals aredense, that is, compact, homogeneously distrib-uted and oriented, the less gas diffusion is easyand then the lower is the permeability. In the caseof water migrations, the phenomenon is morecomplex. These factors are still acting, but theaffinity for water is often as important as the filmstructure. So, the water molecules transfer de-pends both on diffusivity (kinetic factor) and sorp-tion (thermodynamic factor) even in the case ofhydrophobic substances.

IV. INFLUENCE OF THE PENETRANTPROPERTIES AND STATE OF WATER

In the case of dense materials in opposition toporous ones, properties of the penetrant such asits concentration, polarity, or physical state affectthe interactions with the film components but alsothe kinetic phenomenon of the transfer. The sizeand shape of the diffusing molecules influencesthe diffusivity, whereas polarity and its ability tocondense modify the sorption.56 Water is a polarcompound having small size that tends to favorboth sorption and diffusivity in dense materials,which are then more permeable to moisture thanto noncondensable gases. In this article, only waterwill be considered.

A. Effect of Water Concentration Gradient

The driving force of the transfer is the vaporpressure difference between the two sides of thefilm. Without interaction, the higher the gradient,the greater the transfer rate, but permeability re-mains constant as observed for paraffin wax films,40

hydrogenated cottonseed oil,7 or dark chocolate8,54

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as long as the higher humidity is lower than 80%.However, other behaviors were observed, particu-larly an increase of the permeability for paraffin oilcoatings,40 cocoa liquor and butter, milk choco-late,7 or acetyl-stearyl-glycerols.7 Water vapor per-meability of milk chocolate increases from 10.7 ×10–12 to 892 × 10–12 g.m–1.s–1.Pa–1 when the humid-ity gradient varies from 22 to 75% to 0 to 100%.This phenomenon is attributed to the moisture sorp-tion by hydrophilic particles (milk powder, sucrosecrystals, and cocoa powder) of the coating, whichleads to a loss of the film integrity.

However, it is well known that the permeabil-ity of hydrophilic films depends both on the rela-tive humidity (RH) difference and on the absolutehumidity values. For instance, for the same RHgradient, the permeability increases with the va-por pressure value.

Concerning hydrophobic films, few studieswere published and display the same behaviors,but the effect of RH gradient is less important.The barrier efficiency could be decreased by 15times for bilayer films composed of fatty acidsand hydroxypropylmethylcellulose when RH gra-dient varies from 33 to 65% to 65 to 97%.57

Greener-Donhowe and Fennema58 showed thatpermeability of film, beeswax or beeswax-monoglyceride ester mixtures, increases signifi-cantly when RH the lower RH value of the gradi-ent is higher than 65% at 25°C (the high RH valuewas 100%). The permeability obtained for 80 to100% RH gradient represents 83% of the perme-ability obtained for 0 to 100% RH and can reach257% when coatings contain 20% acetyl acylglycerols. These films only contained lipids, andthen were not affected by a hydrophilic support.This behavior is attributed to the sorption of waterby polar groups of the molecules more numerousin the case of monoglyceride than in waxes. More-over, the water content of the beeswax is 0.35g/100 g dm when exposed to 80% RH and only0.17 g/100 g dry matter between 30 and 65%RH.58,59 This phenomenon was also displayed fordark chocolate, which permeability increased twicewhen the RH gradient varied from 0 to 84% to 54to 84% RH at 20°C.8 Tiemsra and Tiemsra60 es-tablished a direct relationship between water va-por transfer rate and humidity for films composedof peanut oil, peanut butter, or cocoa liquor:

WVTRk a a

xw w= . .1 ∆

where WVTR is the water vapor transfer rate(g.m–2.s–1), aw1 is the higher water activity valueof the gradient ∆aw, x the thickness (m), and k anadjusted mass transfer coefficient (g.m–1.s–1). Thismodel should be more interesting if the k param-eter could be correlated to the films physico-chemical and/or structural characteristics.

In hydrophilic films, the permeability in-creases at high relative humidity levels is oftendue to swelling and plasticization of the polymernetwork by moisture sorption that induce a lessdense structure where chains extremities are moremobile. So, water diffusivity and permeation ofwater become easier. Hydrophobic substances pos-sess hydrophilic groups such as the ester groupsin fatty alcohols or in glycerides, hydroxyl groupsin fatty alcohols, or carboxyl groups in fatty ac-ids, which can be hydrated and then modify thefilm permeability. This is confirmed by the sorp-tion isotherm of the beeswax, which displays asignificant increase of the water content at rela-tive humidity higher than 80% at 23°C.59 Greener-Donhowe and Fennema59 proposed a schematicrepresentation of the water content profile in hy-drophobic films containing polar groups for twodifferent gradients as shown in Figure 4. At a 80to 100% RH gradient, the whole film matrix ishydrated and could swell in all the thickness range,whereas for 0 to 100% RH gradient, only a smallpart of the film structure (side exposed to themoist compartment) is changed. So in the lattercase, film can remain a relatively good barrieragainst the moisture transfer.

Even for hydrophobic substances that theo-retically have low affinity for moisture, wateraffects the barrier efficiency of edible films andcoatings when it contains hydrophilic particles orpolar groups as in chocolate or cocoa-based coat-ings. Most of the published work deals with trans-fers of water vapor. However, films and coatingscould be used for highly hydrated products inwhich water could be in a liquid state such as gelsoften used in stuffed foods. Water in contact withthe barrier layer could also be in a solid state inthe case of frozen foods. Therefore, it is interest-ing to consider the effect of the physical state ofwater on its transfer.

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FIGURE 4. Moisture concentration profiles in hydrophobic films containing hydrophilic groups as a function ofthe relative humidity gradient (from Greener-Donhowe and Fennema59).

B. Influence of the Physical State ofWater on Barrier Performances

Water is a molecule that potentially can formhydrogen bonds. Oxygen atom possesses 4 sp3 or-bitals, which two of them are engaged in covalentbonds with hydrogen atoms allowing a tetraedricconformation able to form four hydrogen bonds.The vapor state corresponds to isolated moleculeswhose the angle between atoms is 104.5°C and thedistance of O-H is 0.096 nm.61 The V conformationof the molecule and the polarization of O-H bondlead to an asymmetric distribution of charges andhigh dipolar momentum responsible for great inter-molecular attraction forces. The liquid state corre-sponds to an irregular assembling of all moleculesbut homogeneous.61,62 This liquid state does notcontain crystalline zones but this is compatible tointermolecular interactions. In this, angle and bondsdisplay deviations compared to the tetraedral struc-ture.62 Figure 5 shows a snapshot arrangement thatchanges to an equivalent organization according a10–11 to 10–12 s period.61 In the Ice, corresponding tothe pressure and temperature ranges used for foods,the tetraedral structure of the molecule deals with athree-dimensional network with hexagonal units.The angle formed by three successive oxygen atomsis 109° that corresponds to the tetraedral conforma-tion (Figure 5). In this highly open structure, O-Hbonds are rectilinear and provide great stability tothe system, but water molecules are more distantthan in liquid water, and the distance between twooxygen atoms is 0.276 nm.62–64

In the field of edible films and coatings, mostof the work deals with the transfer of water vapor,but few authors consider the direct contact be-tween the barrier layer and the moist compart-ment.65–67 Indeed, Morillon et al.68 and Morillon69

studied the effect of the physical state of water,liquid or vapor, on its migration through lipidedible films and through hydrophilic and hydro-phobic synthetic films. They demonstrated thatthe water permeability of lipid-sugar-based ed-ible films is the same as long as water activity islower than 0.85 (Figure 6). At higher water activi-ties, the transfer of liquid water through lipid-based edible films and hydrophilic plastic films ismuch greater than the transfer of vapor water.Two phenomena occur: either a partial solubiliza-tion of the sucrose of the film and/or a swelling ofthe matrix in hydrophilic films. Other authorsmeasured the permeability of plastic packagingfilled with liquid water70 or the moisture sorptionof cereals immersed in water,71 but they nevercompared these transfer values to that of vaporones. Lebovits72 explains that the physical state ofwater in contact with a dense material has noinfluence on its permeability as long as there is nointeraction between the penetrant and the poly-mer. This is wrong in the case of porous materi-als. When penetrant and membrane moleculeshave the same chemical groups, cohesive forcesbetween parts are strong and favor the sorptionand then induce structure changes (swelling) andincrease the diffusivity. In a recent work,Grigoriew and Chmielewski73 showed that liquid

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FIGURE 5. Structure of liquid water (a) and ice Ih (b) (from Finney61).

FIGURE 6. Influence of the water activity gradient and physical state of water incontact to the lipid based and lipid-sucrose-based films on the water vapour transferrate at 25°C. The water activity of the dry compartment of the permeation cell is 0.22(from Morillon et al.68).

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water penetrates more rapidly in cellulosic mem-branes than the vapor and the amount of liquidwater absorbed is also greater, although the wateractivity is the same. Moreover, the water concen-tration equilibrium is reached more rapidly withliquid than vapor and forms nanometric inclu-sions called clusters. The molecular organizationof cellulose chains is more disturbed by liquidabsorption than by vapor water. However,Johansson and Leufven74 noted that the aromasorption in polypropylene films is from 100 to1000 greater when a vapor phase is in contactwith the polymer than a liquid one.

When the penetrant is partly in solid statesuch as water in frozen foods, its transferthrough edible or plastic films was scarcelystudied. Moisan75 observed a discontinuity ofboth solubility and diffusivity in polyethyleneof some molecules (ionol, BHT, santonox,irganox 1076, irganox 1010, dilauryl thiodipropionate and distearyl thio di propionate)in the melting range temperatures. These phe-nomena seem due to a modification of the en-ergy required for the deformation of moleculeswhen they diffuse and become very weak abovemelting point. Moreover, all molecules thatdiffuse in and through dense polymers are inliquid state,62,76 so when there is no state changesuch as melting or condensation, less energy isrequired for transfer. This could explain thatliquid penetrant in contact to membrane oftenfavors the transfer.

The study of the influence of the penetrantphysical state, and especially the water on trans-fer through dense films and coatings is a field thatis poorly considered up to now, but which isimportant to better understand the mechanism ofmoisture transfers in composite foods. However,physical state and structure of edible barrier alsostrongly depend on the temperature.

C. Influence of the Temperature

Food products could be exposed to tempera-ture variations in a wide range during processingand they are often stored at subambient tempera-tures. Temperature tremendously affects all ther-modynamic and kinetic phenomena and thus the

transfer. As long as there is no modification of thefilm or coating structure, permeability, diffusivity,and solubility coefficients vary according to theArrhenius law76 as described by the followingequation:

P P E RTa P= −0 exp( / ).

D D E RTa D= −0 exp( / ).

S S H RTS= −0 exp( / )∆

where Ea,P and Ea,D are the apparent activationenergies of permeation and diffusion phenomena(kJ.mol–1); ∆HS the enthalpy of sorption (kJ.mol–1);P0, D0, and S0 the reference values of permeabil-ity, diffusivity, and solubility; R the ideal gasconstant (8.31 kJ.mol–1.K–1) and T the tempera-ture (K).

From the definition of permeability, which isthe product of solubility and diffusivity coeffi-cients, the relationship between energies is fol-lowing:

E E Ha P a D S, ,= + ∆

∆HS is always a negative value for vapor such aswater, which means that solubility decreases whentemperature increases, whereas Ea,D is positiveand then diffusivity rises with temperature.76 Ac-cording to the transfer, which is mainly driveneither by kinetic or thermodynamic phenomena,permeability increases or decreases when tem-perature increases. For instance, water vapor per-meability of polyethylene waxes increases whentemperature varies from 25 to 40°C, and the acti-vation energy is comprised between 5 and 15kJ.mol–1.77 This indicates that moisture transfersseems to mainly depend on the diffusion. Kesterand Fennema12 studied the moisture barrier effi-ciency of some fats as a function of the tempera-ture. They displayed an increase of the transferwhen the temperature decreases, which could beexplained by a greater moisture sorption by thefilter paper used as a mechanical support. Toconfirm this hypothesis, they used an apolar sup-port (polytetrafluoroethylene) for acetylatedmonoglycerides. Other authors did not show any

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effect of the temperature because transfer ratevariations are directly related to the vapor pres-sure of water as observed for hydrogenated cot-tonseed oil and cocoa butter in a temperaturerange 3 to 26.7°C7 or between 21 and 27°C forpurified acetylstearyl glycerol.36

In a wider range of temperature some authorsfirst observed a decrease of the permeability withtemperature and then a permeability increase forlow temperatures. Indeed, this was observed forplastic packaging,70,78 edible films composed offatty acid or fatty acid and beeswax mixtures,34

and for chocolate.8,54 Kester and Fennema79

showed a decrease of the water vapor permeabil-ity when temperature decreases from 40 to 15°Cand the activation energy is 59 kJ.mol–1, but per-meability increases at 4°C (Figure 7). This in-crease of permeability observed by several au-thors can be due to a constant or an increasingwater vapor transfer rate despite the temperaturedecrease. For lipid films with lipids, chocolate orcocoa butter, temperature modifies the solid fatcontent and thus affects both structure and barrierperformances. Indeed, the water vapor transferrate of chocolate decreases between 26 and 20°Cbut remains constant from 20 to 10°C, which isexplained by the SFC that increases from 80 at26°C to 90% at 20°C. The constant value of thetransfer rate between 20 and 10°C deals to anincrease of the permeability because the vaporpressure difference strongly decreases with tem-perature, whereas the water activity gradient re-

mains constant.54 For both plastic and edible pack-aging, authors explain this behavior by structurechanges due to phase transitions, defects appear-ing such as holes or cracks because of tempera-ture dilatation or contraction, or by a greatermoisture sorption at low temperatures.45,70,78–80 Thisparticular behavior of permeability when tem-perature decreases was also displayed byMorillon69 for hydrophilic and hydrophobic films,but these authors consider that the driving forceof the transfer is not the vapor pressure differencebut the chemical potential and they calculated a“corrected permeability”, which always decreaseswhen temperature falls.

Temperatures below 0°C allow a better preser-vation of foods, particularly by the inhibition ofmold growth, but chemical and physico-chemicalreactions always occur even if they are significantlyslowed down. Some chemical deleterious changesare caused by oxidation and protein degradations(denaturation, aggregation), whereas physico-chemi-cal alteration are mainly favored by moisture migra-tion. Water transfer is often responsible to ice crystalgrowth, to crispness loss in low hydrated products,highly moist food dehydration, etc.81 The effect oftemperature below 0°C was little studied in the fieldof edible barriers. Indeed, the few works publishedon moisture transfers at negative temperature onlyconsider the water amount gained or lost by thefood, but does not take into account the real barrierperformance or the permeation phenomena occur-ring in the edible film.82

FIGURE 7. Arrhenius representation of the water vapor permeability of a lipid based edible filmas a function of the temperature for a 0 to 97% RH gradient (from Kester and Fennema79).

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Kamper and Fennema65 used a bilayer filmcomposed of hydroxy propyl methylcellulose andstearic-palmitic acids mixtures to limit moisturetransfers between tomato paste and crackers,which simulated the pizza crust. Critical wateractivity corresponding to the loss of the crisp-ness is reached after 3, 5, and more than 10weeks at 25, 5 and -20°C, respectively. The con-trol samples (without a barrier layer) lost theircrispness after only few hour, 1 day and 3 weeksat these temperatures. However, at temperaturesabove 0°C, these edible films could only be usedfor water activities lower than 0.9. Kester andFennema79 measured the water vapor permeabil-ity at 25°C of a bilayer film composed of hy-droxy propyl methylcellulose, methylcellulose,stearic acid and palmitic acid blends, and bees-wax. They did not display any influence of thetemperature on film structure because perme-ability remains the same after the films havebeen stored 0, 3, or 9 weeks at –40°C. Whenthese films are applied on bread to reduce watermigration from tomato paste at -6.7°C, moisturetransfer is twice lower than without barrier.Moreover, it significantly improves the senso-rial characteristics.84 Rico-Pena and Torres 66

used an edible film based on methylcelluloseand palmitic acid associated to a chocolate layerto reduce moisture migrations between ice creamand sugar cone. They did not observe any in-crease of the biscuit water content after 4 weeksat –12°C, whereas the sugar cone had absorbedhigh quantity of water without the use of thefatty acid only after 18 days at this temperature.Some patents were published on barriers such asthe Loders Croklaan patent, which deals withflexible coating for ice cream applications whosespecificity is due to the particular triglyceridescomposition.85 Mars suggested a new recipe toimprove the adhesion of barrier coatings on fro-zen pastries without affecting organoleptic qual-ity.86 Nestlé proposed a patent on a protein-basedcoating containing saturated fats and emulsifiersto control moisture migration in an heteroge-neous product at temperatures both above andbelow 0°C.87

Other frozen foods could be protected by ediblefilms and coatings, especially meat, fish, and sea-food against moisture and oxygen transfers and loss

of flavors.88 Zabik and Dawson88 used acetylatedmonoglycerides because of their hydrophobic na-ture, low sensitivity to oxidation, and flexibility atlow temperatures. Coated poultry stored at –18°Cwith films have unchanged sensory characteristicsafter cooking than that wrapped in polyvinylidenechloride plastic films. Coatings containing acetylacyl glycerols having low melting points used aloneor as bilayers on a whey protein film allow to reduceby 65% the dehydration of frozen king salmon piecesafter 3 weeks of storage at -23°C, and significantlylimit fish lipid oxidation.89

V. CONCLUSION

Edible packaging based on hydrophobic com-pounds has complex recipes, making processes,structures, and physico-chemical properties thatdeal to highly variable barrier performances. Sotheir formulation and their use have to be chosenwith discernment in the aim to be well adapted tothe targeted objectives and to the food that haveto be protected. Indeed, a good food-edible bar-rier couple is a key parameter to improve foodquality. Krochta et al.,90 Gennadios et al.,91 andmore recently Debeaufort et al. 92 reviews all ap-plications of edible barriers in food industry what-ever the product. These papers display a widerange of substances and applications. In all cases,edible films and coatings cannot replace plastic ormore traditional packaging but are complemen-tary. They are crucial and irreplaceable insideheterogeneous foods to prevent migrations be-tween compartments or parts having differentwater activities. Their functional properties de-pends on many factors related to both film, pen-etrant, and external conditions. Definitively eachedible barrier has to be conceived for specificapplications and storage conditions.

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