Oxygen and aroma barrier properties of ediable films

A Review

Oxygen and aroma barrier

properties of edible films:

A review

K.S. Miller and J.M. Krochta

Interest in maintaining food quality while reducing packaging

waste has encouraged the exploration of the oxygen and

aroma transport properties of edible films. This review article

introduces the theoretical basis for oxygen and aroma barrier

property determination and presents a brief historical per-

spective of the development of barrier polymers. The effects

of structure and composition on mass transport in edible

films are examined and compared with those of the more

thoroughly investigated synthetic polymers. A survey of edible

film oxygen and aroma barrier research is presented; areas re-

quiring additional investigation are suggested, for applications

as well as basic research. The potential of edible films and

coatings to provide excellent aroma retention and superior

oxygen barrier properties makes this quite a promising area of

research for both food and packaging scientists.

Food quality is easily diminished by the deleterious transport of aroma compounds and oxygen. Food is required to satisfy the biological need for a source of nu- trition; however, it is the flavor and aroma of a food that provide the impetus for its consumption. In fact, a large segment of commercial manufacturing deals with the production of packaging that extends the shelf life of food by controlling oxygen and aroma transport. A food’s characteristic flavor and aroma are the result of a complex construct of hundreds of individual constituent compounds interacting to produce a recognizable taste and aroma. Thus, if one or more flavor constituents are altered or diminished, food quality may be reduced. A reduction in food quality may result from the oxidation of aroma components due to the ingress of oxygen, or it may be the result of the loss of specific aroma compounds to the packaging material or environment. Therefore, it is critical to identify both the oxygen and aroma mass transfer properties of food packaging.

K.S. Miller (formerly of the Department of Biological and Agricultural

Engineering, University of California, Davis, CA 95616, USA) is now at

Frito-Lay, Inc., 7701 Legacy Drive, Plano, TX 75024, USA. J.M. Krochta (corresponding author] is at the Departments of Biological and Agricultural

Engineering 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~$17.00 PI,: 50924.2244(97~01O51-0

Origin and definition of edible polymer films Foods such as fruit and nuts have natural built-in

packaging protection in the form of skins and shells. These natural barriers regulate the transport of oxygen, carbon dioxide and moisture and also reduce flavor and aroma loss. However, processed foods dominate today’s diet; and no such natural barriers exist for processed foods.

Humankind’s instinct to cover food (perhaps stem- ming from a desire to hide this precious commodity) may have inadvertently led to the implementation of food packaging. The very first package probably con- sisted of leaves, animal skin or the shell of a nut or gourd’. Around SOOOBC, the different types of packaging materials that were available included sacks, baskets and bags made from plant or animal material, as well as primitive pottery and ceramic vessels’.

By -15OOBC, hollow glass objects had begun to appear, but it was not until -AD~OO that the woven, pressed sheets that eventually became known as paper appearedr. Lard or wax was used to enrobe fruit and other food items in 16th-century England2. The first plastic, a cellulose-based polymer, was introduced in 1856; then in 1907, phenol formaldehyde plastic (Bakelite) was discovered’. From then on, a series of discoveries and inventions led to today’s multitude of primarily synthetic polymer packaging materials.

Polymer scientists have produced a variety of synthetic polymers and polymer laminates that are excellent barriers to both oxygen and aroma compounds. How- ever, despite the availability of these synthetic barriers, the food industry is now considering natural packaging biopolymers such as edible and biodegradable polysaccharide or protein films. Although these biopolymers share their origins with the early, all-natural packaging materials, they have many of the same properties and are as convenient as the synthetic polymers that they augment. Environmental and economic reasons as well as product development and consumer trends have pushed food and packaging scientists along this cyclic path.

Edible polymer films may be formed as either food coatings or stand-alone film wraps and pouches. These biopolymer films have potential for use with food as oxygen and/or aroma barriers2. This reduces the re- quirements of the synthetic polymer to the provision of a barrier to moisture loss and protection of the food from external contamination. Thus, the amount of synthetic packaging is reduced and recyclability is increased because the need for synthetic laminates, often used to improve oxygen and aroma barrier properties, is diminished.

Regardless of whether it is a synthetic polymer or biopolymer, a polymer’s mass transport properties are influenced by similar factors; these include composition and structure, which directly affect a film’s performance as a barrier to quality loss. For these reasons, environmental and processing conditions that affect the composition and structure of polymer films are of great interest to both food and polymer scientists.

Trends in Food Science &Technology July 1997 [Vol. 81

Box 1. Polymer film mass transport properties

The diffusion coefficient describes the movement of permeant mol- When S is independent of the sorbed permeant concentration ecules 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 becomes linear and sion process used to describe permeant movement in a polymer. S is referred to as the Henry’s law solubility coefficient. This re-

Activated diffusion is described as the opening of a void space lationship is often used to calculate the solubility coefficient from among a series of segments of a polymer chain due to oscillations sorption isotherms, which are plots of the permeant concentration of the segments (an ‘active state’), followed by translational motion in the headspace above a polymer versus the concentration of the of the permeant within the void space before the segments return permeant within the polymer. to their ‘normal state’3. DiBenedetto pointed out that both the ac- The permeability coefficient incorporates both kinetic and tive and normal states are long-lived, as compared with the trans- thermodynamic properties of the polymer-permeant system, and lational rate of the permeant. thus provides a gross mass transport property. The permeability

Fick’s 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 by c 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 dissolution of a permeant (dM/dt) ;;$dy L

in a polymer, and thus represents a thermodynamic property of the P= (4)

polymer-permeant system. The solubility coefficient may be de- ASP

fined 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 and and may be a function of the vapor pressure (or concentration of is required to be at steady state for the permeability coefficient

dissolved permeant). calculation.

1

This article first reviews the parameters that are used essential for activated diffusion. Factors affecting a poly- to characterize mass transport in polymer films, includ- mer’s structure have a direct effect on segmental mobility ing the relationship between polymer structure and and, therefore, influence its mass transport properties. those mass transport 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 research on 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 have shown that an increase in crystallinity, basic and applied research needs are identified. density, orientation, molecular weight or crosslinking

results in decreased polymer permeability5x6. Structural influences on polymer mass transport A barrier polymer inhibits permeant progress, thereby properties presenting a greater barrier to mass transport than the

A film’s mass transport properties are often described by permeant would otherwise meet in the absence of the three common coefficients: diffusion (the rate of move- polymer. The necessary characteristics of a barrier polymer ment of a permeant molecule through the tangled polymer include: a degree of polarity, high chain stiffness, inertness matrix, based on, for example, the size of the permeant molecule and the struc- Permeant molecule ture of the polymer matrix), solubility (the partitioning behavior of a permeant I of polymer

2L

Segments I I

molecule between the surface of the chains I I I I

polymer and the surrounding headspace) I and permeability (the rate of transport of

4

a permeant molecule through a polymer l

=$x-- gg%J-;

I I

gg as a result of the combined effects of dif- I Reference I I

fusion and solubility). These are for- I ----- position I I

mally defined in Box 1. Figure 1 depicts the activated diffu- Normal Activated Normal state after

sion process and clearly shows the im- state state one diffusional jump

portance of polymer structure for 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 of 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 to newly formed void spaces, is A.T. DiBennedetto, and reproduced with permission from John Wiley & Sons, Inc.

Trends in Food Science & Technology July 1997 [Vol. 81 229

to permeants, high chain-to-chain packing, some inter- them open to engage in hydrogen bonding even when the molecular crosslinking and a high glass transition tem- cohesive energy density is relatively high. In the case of perature’. The effects of the previously mentioned poly- a polymer with a simple carbon repeating 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 same diversity of aroma compounds has impeded the thorough backbone with a hydroxyl group substituent. One would investigation of their myriad polymer-permeant interac- also expect polymers with a higher cohesive energy den- tions and of the associated effects on aroma permeability. sity to be better barriers to nonpolar aroma compounds.

Chemical structures Free volume Knowledge of the effects of differing chemical struc-

tures on a polymer’s mass transport properties is important for today’s packaging industry. The types of substituent groups present in a polymer can have a tremendous effect on the variability of the permeability coefficient by influencing two main factors: how tightly the polymer chains are bound together and how much free volume exists between the chain9.

Cohesive energy density Cohesive energy density is a measure of the polarity of

a polymer and of the energy binding the polymer chains together. In general, the higher a polymer’s cohesive energy density, the more difficult it is for the polymer chains to open and allow a permeant to pass (highly polar permeants such as water being an exception to this rule). An empiri- cal correlating parameter, dubbed the Permachor value, can be used to predict gas permeation, if free volume and cohesive energy density are known8. The effects of various substituent groups on polymer permeability are shown in Table 1. As increasingly polar substituents are added to the same carbon backbone (thus increasing the cohesive energy density), oxygen permeability decreases by five orders of magnitude. However, water, being highly polar, does not rely on the polymer chains to ‘open’ and can force

Free volume is a measure of the degree of interstitial space between the molecules in a polymer. The diffusion coefficient and the permeability coefficient both decrease with a decrease in free volume for carbon dioxide, he- lium and methane in various polymers’. Maeda and Paul’ pointed out that the addition of plasticizers to increase the free volume resulted in lower glass transition temperatures, whereas the addition of anti-plasticizers to decrease the free volume increased the glass transition temperature. Table 2 shows the dramatic effect of free volume on the permeability of oxygen. As the fractional free volume decreases from 0.204 for poly(4-methyl pentene-1) to 0.03 for poly(viny1 alcohol) (PVOH; see Box 2 for all polymer abbreviations), the oxygen permeability diminishes by six orders of magnitude. Stiff-chained polymers that have a high glass transition temperature generally have low gas permeability, unless they also have a high free volumea. These results suggest that nonpolar aroma compounds would also have low permeability coefficients in polymers with a low free volume.

Crystallinity Crystallinity is a measure of the degree of order of the

molecules in a polymer. Polymer properties that affect crystallinity include the structural regularity of the poly-

, mer chains; polymer chain mobility,

Table 1. The effects of cohesive energy density on permeability”

Backbone: CCH,-CHj I X Permeability at 25”Cb

Substituent CED

group C4 Polymer (cal/cm3) Oxygen Water

-H Polyethylene 66 0.188 100 -

-0 \ / Polystyrene 85 0.168 1100

-OCOCH, Polybinyl acetate) 88 0.023 8500

-Cl Poly(vinyl chloride) 94 0.0036 250

-CN Polyacrylonitrile’ 180 0.000039 300

-OH Poly(vinyl alcohol) 220 0.0000016 (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 a

specified 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 thickness

‘Unannealed film

dPoly(vinyl alcohol) is soluble in water

CED, Cohesive energy density

which allows variable conformation; the repeating presence of side chains, which engage in intermolecular bonding; and the absence of bulky side chains, which interfere with the crystal lattice formation”‘. The mass transfer of a gas or aroma in a semi-crystalline polymer is primarily a function of the amorphous phase, because the crystalline phase is usually assumed to be impermeable. Table 3 illustrates the effects of crystallinity on oxygen permeability. As the percent crystallinity of a polymer increases, the oxygen permeability decreases. The degree to which oxygen permeability is affected is highly dependent on polymer structure. An increase in the crystallinity of polyethylene from 43% to 74% results in a five- fold decrease in oxygen permeability, whereas an increase in the crystallinity of poly(ethylene terephthalate) from ~10% to 45% yields a threefold decrease in oxygen permeability. The low

230 Trends in Food Science & Technology July 1997 [Vol. 81

diffusion coefficients for aroma compounds in glassy polymers suggest that the permeability coefficients for polymers with a high crystallinity would be correspond- ingly low”.

Orientation Orientation refers to the alignment of the polymer chains

in the plane of the polymer backbone, and is a by-product of the processing operation. Sha and Harrison’” mentioned several mechanisms for these orientation effects. They reported that the decrease in the fractional free volume of the amorphous region with orientation correlated well with the decrease in permeability, solubility and diffu- sivity coefficients. However, others contend that the alignment of the polymer’s crystallites increases the tortuosity of the permeant’s path, thus significantly reducing the permeability only in the case of semi-crystalline polymer?. The minimal reduction in oxygen permeability following 300% orientation of completely amorphous polystyrene is cited in support of this observationx.

Tacticity Tacticity refers to the stereochemical arrangement of the

substituted groups in relation to the plane of the polymer backbone. Isotacticity occurs when all of the substituent groups lie on one side of the plane of the main chain. If substituent groups alternate on either side of the plane, the polymer is considered to be syndiotactic, and atactic if the substituent groups are randomly configured. Min and PaulI examined the influence of tacticity on the permeability of carbon dioxide, oxygen and nitrogen in poly(methy1 methacrylate) (PMMA). It was concluded that permeability increased as the percentage of syndiotactic substituents increased. Jasse et ~1.~ suggested that these results might be indicative of a more densely packed polymer structure for isotactically substituted polymers.

Crosslinking Crosslinking is the formation of intermolecular bonds

among the chains of a polymer. Research has examined

Box 2. Polymer abbreviations

CMC:

EVOH:

HDPE:

HPC:

HPMC:

LDPE:

MC:

PEG:

PMMA:

PVDC:

PVOH:

VOH:

Carboxymethylcellulose

Ethylene vinyl alcohol copolymer

High-density polyethylene

Hydroxypropylcellulose

Hydroxypropyl methylcellulose

Low-density polyethylene

Methylcellulose

Poly(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 permeability Polymer volumeb at 25°C’

Poly(4-methyl pentene-1) 0.204 1.56

Polystyrene 0.176 0.17

Polycarbonate 0.168 0.097

Poly(methyl methacrylate) 0.132 0.0065

Nylon 6 (a= 1 .O) 0.120 0.0029 (dry)

Poly(vinylidene fluoride) 0.098 0.0019

Poly(acrylonitrile) 0.080 0.000039

Poly(acrylonitrile) (annealed) 0.050 0.000016

Poly(vinyl alcohol) (a= 1) 0.030 0.0000016 (dry)

‘Adapted from Ref. 8; reproduced with permission from Technomic Publishing Co., Inc.

‘Fractional free volume is the ratio of the interstitial space behveen molecules to the

volume of the polymer at a temperature of absolute zero

-Units for permeability are cm’ pm/(mVkPa) (see Table 1)

~1, Amorphous volume fraction (the ratio of the volume of the polymer that exists in an

amorphous state, as opposed to a crystalline state, to the total volume of the polymer)

the effects on mass transport of polymer crosslinking induced by heat curing and irradiation of a variety of polymers and by enzymatic treatment of protein-based edible polymers I&-]’ Heat curing of biopolymers re- . sulted in decreased water vapor permeability for soy proteinI and whey protein isolate15. These effects were attributed to an increase in intermolecular crosslinking among the protein strands during heating.

Polymer chemists have made great advances in producing synthetic polymers that have very specific properties and characteristics; however, predicting and controlling the structure of biopolymer films are both very difficult tasks. Food scientists have begun fleshing out the properties and characteristics of edible films, but many significant topics pertaining to the application of

Table 3. The effects of crystallinity on permeability”

! Polyethylene (d = 0.92) 43 0.19

Polyethylene (d = 0.955) 74 0.038

Poly(ethylene terephthalate) 40 0.0049



Nylon 6 0 0.0029 (dry)

Nylon 6 60 0.00045 (dry)

Polybutadiene 0 0.97

Polybutadiene 60 0.27

“Adapted from Ref. 8; reproduced with permissron from Technomic Publishing Co., Inc.

‘Units for permeability are cm3 p,m/(m’d kPa) (see Table 1)

d, Density

Trends in Food Science & Technology July 1997 [Vol. 81

edible films remain unexplored. Examination of the influences of the composition of synthetic polymers on oxygen and aroma barrier properties suggests that the polar nature of edible polymer films should yield excellent oxygen and aroma barrier properties.

Edible polymer film composition and structure Edible polymer films include polysaccharides and/or

proteins. Kester and Fennema? have produced an excellent overview of the types, methods of preparation, properties and applications of all types of edible polymers, and pointed out the rationale for developing these films as packaging supplements. The authors noted that possible functional properties include the retardation of moisture migration, gas transport (oxygen and carbon dioxide), oil and fat migration and solute transport, as well as improved mechanical handling properties, additional structural integrity, use as a vector for food additives, and retention of volatile flavor compounds.

Recently, Krochta and De Mulder-Johnston’” provided a synopsis of the research on edible polymer films and their potential applications. They also touched on nutritional, safety and health issues associated with edible polymers. Edible polymer films prepared from celluloses, starches, other polysaccharides (alginates, carrageenans and pectinates) and proteins (collagen, gelatin, zein, gluten, soy protein, casein and whey protein) were reviewed.

Water-insoluble cellulose is brought into aqueous solution by etherification with methyl chloride, propylene oxide or sodium monochloroacetate to yield the non-ionic methylcellulose (MC), hydroxypropyl methylcellulose (HPMC) and hydroxypropylcellulose (HPC) films and the ionic sodium carboxymethylcellulose (CMC) filmst9. The degree of substitution that occurs during these etherification reactions affects the properties of a film such as water retention, sensitivity to electrolytes and other solutes, dissolution temperatures, gelation properties and solubility in non-aqueous systems. Cellulose ether films are resist- ant to fats and oils, and are therefore likely to be good aroma barriers”. The cellulose ethers produce moisture- sensitive films that are effective oxygen barriers, and when applied to various fresh commodities, they have been shown to retain flavor components during storage, thus indicating their potential aroma barrier properties”.

The linear starch polymer amylose produces a hydrophilic film with low oxygen permeability; hydroxypropylated amylose also yields films with very low oxygen permeability19. Plasticization, chemical crosslinking and esterification all affect the final structure of the starch film to varying degrees. Coating apple slices and dried apricots with starch hydrolysates resulted in a better flavor, indicating their potential aroma barrier properties”.

Alginate films are composed of polymer segments of pOly@-D-InaI’IUUrOniC acid), poly(a-r.-guluronic acid) and of a segment of alternating D-mannuronic and L-guluronic acid residents2. Alginate films have been shown to reduce oxygen transport and aroma loss in various food products 19. Alginate film structure is affected by the concentration of polyvalent cations in the gel (such as calcium), rate of cation addition, time of cation exposure,

pH, temperature and presence of other constituents such as hydrocolloids2. The calcium ions pull the alginate polymer chains together via ionic bonding and thus allow for increased hydrogen bonding. The same effect occurs with pectin films. Carrageenan films are thought to form a three- dimensional polymer structure via the formation of a double-helix structure, which is also thought to be ef- fected by inter-chain salt bridges’. The oxygen and aroma barrier properties of films from pectins, carrageenans and other polysaccharides have not been examined in the literature.

Krochta?” discussed the effects of protein structure and composition on edible film barrier properties. The proteins must be in an open or extended form to allow the molecular interaction that is necessary for film formation. The extent of this interaction depends on the protein structure (degree of chain extension) and the sequence of hydrophobic and hydrophilic amino acid residues in the protein. Increased molecular interaction results in a film that is strong but less flexible and less permeable.

The degree of hydrophilicity of the amino acid residues in a protein controls the influence of moisture on the protein film’s mass transport properties*“. Most edible films are quite moisture sensitive, but this inherent hydrophilicity makes them excellent barriers to nonpolar substances such as oxygen and some aroma compounds. As mentioned previously, an increase in crystallinity, density, orientation, molecular weight or crosslinking results in a decrease in polymer permeability. Complicated protein structures make the control of these factors quite challenging.

Researchers studying edible polymers have significant obstacles to surmount in simply producing a usable film. Only of late have investigations of edible polymers included the examination of barrier properties for permeants other than moisture. The promise of using a re- newable resource to simplify packaging and extend food shelf life has encouraged researchers to explore the oxygen and aroma barrier properties of edible polymers.

Oxygen and aroma barrier properties of edible polymer films Oxygen barrier properties

Oxygen permeability is the next most commonly studied transport property of edible polymer films after water vapor permeability. Commercial data2’ on MC and HPMC films indicate that they are moderate barriers to oxygen; their oxygen permeability is approximately an order of magnitude lower than that of low-density polyethylene (LDPE), but two to three orders of magnitude greater than that of poly(vinylidene chloride) (PVDC) and ethylene vinyl alcohol copolymer (EVOH) at -24°C and 50% relative humidity (Table 4). Although cellulose ethers possess a chemical formula similar to that of EVOH, their repeating ring and side-group structures probably produce a smaller cohesive energy density, larger free volume and smaller crystallinity relative to those of the linear EVOH. The higher oxygen permeability of HPMC compared with that of MC can probably be attributed to the larger HPMC side group, which results in HPMC having a smaller cohesive energy density, larger free volume and

232 Trends in Food Science & Technology July 1997 [Vol. 81

lower crystallinity than MC. Donhowe and FennemaZ2 found that compared with other water or water-ethanol sol- vents, oxygen permeability was mini- mized when an MC film was formed from a water-ethanol solvent in the ratio of 75% : 25% at elevated temperature (Table 4). Films formed in this manner also had greater crystallinity, lower water vapor permeability, higher tensile strength and higher elongation. Donhowe and Fennema31 found that glycerol, added at 30% (w/w), was a more effective plasticizer than propylene glycol in decreasing the tensile strength and increasing the elongation of MC films. Both approximately doubled the oxygen permeability at -25°C and 50% relative humidity. Lower molecular weight (molecular weight of 400 and 1450) poly(ethylene gly- ~01)s (PEGS) were also good plasticizers but increased oxygen permeability by a factor of 4-5. Park et aL3? found that at 0% relative humidity, the oxygen permeability of MC and HPC films increased as their molecular weight increased. Propylene glycol was shown to be a relatively poor plasticizer and produced a large increase in oxygen permeability at 0% relative humidity. Interestingly, although glycerol and PEG-400 were found to be good plasticizers for MC and HPC, they had lit- tle effect over a range of concentrations on oxygen permeability at 0% relative humidity. On the other hand, Park and Chinnan found that the quantity of PEG-400 greatly affected the oxygen permeability of MC and HPC at 0% relative humidity. Rico- Peiia and Torres3” found that oxygen transmission through an MC-palmitic

Film type” Test conditions

Cellulose-based:

MC

HPMC

MC

Starch-based:

Amylomaize starch

Hydroxypropylated amylomaize starch

Protein-based:

Collagen

Collagen

Collagen

Zein : PEG t glycerol (2.6 : I)

Gluten :glycerol (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:

LDPE

HDPE

Polyester

EVOH (70% VOH)

EVOH (70% VOH)

PVDC-based films

“See Box 2 for polymer abbreviations hUnits 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. 6 RT, Room temperature

I RH, Relative humidity

acid composite film increased rapidly with relative humidity >57%, correlating well with moisture content. Park et al.35

studied MC films laminated with a corn zein-fatty acid layer. They found that oxygen permeability increased as the concentration and chain length of the fatty acids increased.

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

polymer films

Permeabilityb Ref.

24”C, 50% RH 97 21

24”C, 50% RH 272 21

25”C, 52% RH 90 22

25”C, <lOO% RH

25”C, ~78% RH

<65 23

-0 24

RT, 0% RH <0.04-0.5’ 25

RT, 63% RH 23.3 25

RT, 93% RH 890 25

25”C, 0% RH 38.7-90.3 26

25”C, 0% RH 6.1 27

25”C, 0% RH 6.1 28

23”C, 50% RH 76.1 29

23”C, 50% RH 4.3 29

23”C, 50% RH 1870 6

23”C, 50% RH 427 6

23”C, 50% RH 15.6 30

23”C, 0% RH 0.1 6

23”C, 95% RH 12 6

23”C, 50% RH 0.4-5.1 6

High-amylose amylomaize starch films are moderate to good oxygen barriersz3, with an oxygen permeability that is lower than that of the cellulose ethers, even at higher relative humidity (Table 4). Oxygen permeability is even higher for high-amylose films than for PVDC or EVOH films. However, hydroxypropylated starch films may have even lower oxygen permeability24. Apparently the starch structures in these films produce a combi- nation of higher cohesive energy density, lower free volume and higher crystallinity than occurs in cellulose ethers.

Butler et al.36 found that glycerol-plasticized chitosan films had extremely low oxygen permeability at 0% relative humidity. Increasing the plasticizer content increased the oxygen permeability. Wong et aL3’ found that adding lauric acid to a chitosan film more than doubled the oxygen transmission rate. However, pahnitic acid or acetylated monoacylglycerol reduced the oxygen transmission rate by an order of magnitude.

As a group, protein films appear to have lower oxygen permeabilities than non-ionic polysaccharide films. This may be related to their more polar nature and more linear (non-ring) structure, leading to higher cohesive energy density and lower free volume. At 0% relative humidity, collagen film has an oxygen permeability similar to that of PVDC and EVOH filmsZ. However, collagen fii is more sensitive to relative humidity; at -50% relative humidity, its oxygen permeability is one to two orders of magnitude greater than that of PVDC or EVOH films (Table 4).


Films that are based on corn zein, wheat gluten, soy protein or whey protein appear to possess an oxygen permeability that is greater than that of collagen-based films (at 0% relative humidity)‘hm29. This is probably due to the fact that these globular proteins have a less linear structure and a greater percentage of larger amino acid side groups than collagen, resulting in a smaller cohesive energy density and larger free volume. However, proper selection of plasticizer appears to reduce the oxygen permeability while maintaining the mechanical properties, presumably by affecting the polymer free volume (Table 4)29.

Gennadios et aL2’ investigated the effect of temperature on the oxygen permeability of corn zein, wheat gluten and wheat gluten-soy protein isolate films at 0% relative humidity. Results showed good agreement with the Arrhenius activation energy model. Based on the lack of breaks in the Arrhenius plots, no structural transitions were identified in the 7-35°C temperature range. Brandenburg et af.‘8 discovered that the oxygen permeability of soy protein films decreased as the pH of the film solution preparations increased from 6 to 12. Gennadios et al.‘” found that replacing glycerol plasticizer with triethylene glycol in wheat gluten films produced a large increase in oxygen permeability. This effect was attributed to the larger size and less polar nature of triethylene glycol, which would also correlate with an increased free volume and reduced cohesive energy density.

McHugh et aL4” studied the properties of films made from fruit purCes. Peach puree films were found to be better oxygen barriers than MC and other polysaccharide films and comparable to whey-protein-based films.

In general, the oxygen permeability of edible polymer films, especially protein films, appears to be quite low. Optimization of polymer structure by increasing crystallinity, orientation or crosslinking in pre-processing steps or during film formation may result in further reductions in the oxygen permeability of a film. Modification of polymer structure combined with optimized selection of plasticizer may produce edible films with oxygen barrier properties that are as good as those of PVDC and EVOH films.

Aroma barrier properties Although a significant body of work concerned with the

oxygen barrier properties of edible films exists, the aroma barrier properties of edible films have not been thoroughly examined. Recent reviews of the use of proteins as edible films and coatings indicate that the literature is somewhat lacking in research pertaining to the aroma barrier properties of edible films20s4’. However, reviews of the literature on synthetic polymers are valuable resources to the researcher studying the aroma transport properties of edible films [Refs 42 and 43, and K.S. Miller (1997) Physical Properties of Whey Protein Isolate Films: d-Limonene Permeability, Water Vapor Permeability and Mechanical Properties (PhD thesis), University of California, Davis, CA, USA].

In fact, Debeaufort and Voilley4” were the first to ex- amine aroma permeability in edible polymers. They examined the co-permeation of moisture and l-octen-3-01 (mushroom aroma) in LDPE, cellophane, MC and gluten

(wheat protein) films. An isostatic gas chromatograph technique was used with a dual-detection scheme for measuring moisture and aroma transport simultaneously. The gluten film was a better barrier to 1-octen-3-01 than either the LDPE or MC film, but not as good a barrier as the cellophane film.

Continuing this work, Debeaufort et al.“’ attempted to explain the differences in l-octen-3-01 transport among LDPE, cellophane, MC and gluten films. However, they were unable to correlate aroma flux to the amount of aroma absorbed, the hydrophobicity of the polymer, or to trends in the diffusion coefficient. It was concluded that the sorption-diffusion model, alone, cannot describe the aroma or moisture permeability in edible films. Further- more, it was suggested that the variations in aroma permeability 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 polymer’s oxygen barrier properties are not necessarily a reliable indicator of its aroma barrier properties. The author cau- tioned that oxygen and aroma compounds behave quite differently in glassy versus rubbery polymers. Glassy polymers have medium to high oxygen diffusion coefficients but very low aroma diffusion coefficients (at low permeant concentrations) rl. Rubbery polymers exhibit dif- fusivities for oxygen and aroma compounds that are of the same order of magnitude (i.e. permeant size is not as influential a factor)“. DeLassus” stated that trends in oxygen and aroma permeability are comparable within the rubbery or glassy polymer categories, but not between them. Recent work by Miller et a14’ indicates whey protein isolate films to be excellent barriers to d-limonene.

Miller and Krochta47 found whey protein isolate films containing 25% glycerol (dry basis) plasticizer to be comparable to EVOH films as a barrier to d-limonene under similar temperature and humidity conditions. Additionally, d-limonene permeability in 25% glycerol whey protein isolate films was found to be significantly affected by temperature and relative humidity but not by permeant concentrations in the range of 62-226 ppm (v/v)““.

Existing commercial applications of edible films include collagen as a casing for sausages and a wrap for smoked meats, and gelatin and corn zein as encapsulating agents for food ingredients and pharmaceuticals20. Evaluation of the basic barrier properties of edible polymers will pave the way for additional applied research dealing with specific food applications. Such applied studies of the oxygen and aroma barrier properties of edible polymers will aid 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 films Oxygen barrier applications

Applications that take advantage of the beneficial oxygen barrier properties of edible polymer films have been explored for many years. Ganz4x found that HPC film coatings provided peanuts with some protection from oxygen, but the effect was not well quantified. MC and HPMC

Trends in Food Science & Technology July 1997 [Vol. 81

coatings are commonly used for pharmaceutical tablets, providing protection from oxygen, aroma and moisture transport. Several researchers have found that CMC-based coatings can delay ripening and improve the quality of fresh fruit and vegetables by retarding the transport of oxygenJ9-5’.

Park et al.s’ investigated the application of MC film laminated with a corn zein-stearic acid-palmitic acid blend for the packaging of potato chips. Acceptable chip quality was maintained for up to 25 d at 25°C. The composition of the corn zein-stearic acid-palmitic acid blend layer had no effect on the results.

Jokay et aL5’ concluded that sensory tests on stored almond nut meats coated with hydroxypropylated high- amylose starch indicated considerable protection against the development of oxidative rancidity. However, quantitative data were not presented. Murray and Luft5s found that starch hydrolysate coating applied to apple slices before drying maintained whiteness more effectively than 2% ascorbic acid solution, but not as effectively as sulphur dioxide. However, slices coated with starch hydrolysate were judged superior in flavor. Murray and Luft5s also reported that almonds coated with the starch hydrolysate had improved flavor and shelf life, indicating oxygen barrier attributes for the coating; however, they did not present any data.

Wanstedt et a1.j’ found that coating ground pork patties with calcium alginate either before or after precook- ing improved the quality of the final cooked product, as measured by the development of oxidative rancidity. Earle and Snyder 57 found that an alginate coating improved the flavor and color of frozen shrimps, probably because of a reduction in rancidity. Earle and McKeej8 developed an alginate-based coating with oxygen barrier properties for breaded and filled-dough food products. Meyer et a1.s9 found that carrageenan coatings extended the shelf life of poultry pieces by acting as an oxygen barrier. Chitosan coatings were found to be effective in extending the life of fresh fruit by modification of oxygen and carbon dioxide transfer60.h’.

Collagen casings for sausages are known to provide some protection from oxygen’:. Gelatin coatings have been found to be effective in protecting several meat products from oxygen h3.M Zein-based coatings have been . used to reduce rancidity in nuts and confections65,6h. Corn zein films were also shown to affect oxygen and carbon dioxide exchange in fresh tomatoes, as evidenced by a delay in color change, firmness loss and weight loss during storage6’. The result was an extension of shelf life by 6d. Coatings based on whey protein were shown to reduce the oxygen uptake by dry-roasted peanuts68, de- laying oxidative rancidity, as measured by the peroxide value and hexanal content of the peanuts6’.

Aroma barrier applications Edible films can be used as flavor carriers in addition

to providing a barrier to aroma 10~s~~~“. Andres’O also pointed out that flavor quality deterioration can include the loss of characteristic flavor owing to oxidation or poor oxygen barrier properties. Thus, an edible film can

assist in retaining the characteristic food flavor via its aroma barrier properties and also limit quality deterioration due to oxidation via its oxygen barrier properties.

Researchers have examined the ability of edible coatings applied to harvested fruit to prevent the loss of characteristic flavor. The use of edible coatings on cit- rus fruit resulted in an increase in desirable flavor compounds after storage, as compared with uncoated fruits2. Cellulose-based composite films including wax seemed to provide the best balance between flavor retention and the prevention of weight loss due to moisture transport5’.

Pervaporation, the removal of organics from an aqueous solution through a separating membrane, has been successfully utilized to enrich and recover flavor volatiles”. Understanding the behavior of flavors in aqueous solutions, such as the systems used in these pervaporation studies, provides insight into the potential applications for edible films in environments with a high water activity.

The sorption characteristics of edible films may allow the incorporation of desirable flavors and aromas into a coating for delayed release, thereby enhancing the food’s flavor profile. Encapsulated flavors and aromas could be released by heating and/or rehydration, as well as by mastication. Hydrophilic edible films can be applied to any low-moisture food with a sensitive characteristic flavor to aid in aroma retention. An example would be fruit-flavored chewing gums, which often lose their characteristic aroma with time. Dry, fruit-flavored cereal would be another potential application for edible films to prolong a product’s shelf life by limiting aroma transport.

Basic and applied research needs The effects of factors, identified by Banker’?, that in-

fluence film mass transport - polymer structure and orientation, salt concentration, ion ratios, polymer-permeant interactions, acid and base concentrations, addition of dispersed solids, and permeant boundary layer thickness - provide the edible film researcher with boundless av- enues of research. Specifically, no work has been done to optimize the influences of free volume, crystallinity or orientation on the oxygen and aroma barrier properties of edible polymers.

Before a packaging specialist can take advantage of an edible polymer’s barrier properties, the polymer must be successfully applied to the desired food system. Guilbert’ examined the factors influencing the food film coating operation and concluded that the degree of cohesion (interactions among the polymer molecules) and the degree of adhesion (interactions between the polymer and the food molecules) are of critical importance to the successful application of an edible packaging. The author mentioned several formulation and processing parameters that influence cohesion and adhesion, including solution temperature, solvent evaporation rate, solvent characteristics and the concentration of the film-forming polymer molecules in the solution. Few researchers have focused on the effects of these parameters on both the degree of


adhesion and the degree of cohesion during food film coating. Understanding these basic effects is critical to the successful application of an edible coating to a food.

Gaseous diffusion through polymers has long been studied by polymer scientists. DiBenedetto3 concluded that models of such diffusion depend on knowledge of the physical properties of the polymer and the geometry of the permeant. Lack of knowledge about these polymer and permeant properties restricts the applicability of many of the models that have been proposed to predict oxygen and aroma transport.

Knowledge about the physical properties of edible films is even more limited. Kester and Fennema* concluded that much of the edible film and coating work reported in the literature is of limited value owing to the ‘lack of quantitative data on barrier characteristics of the coatings’. It is only through the compilation of barrier properties and their correlation with edible polymer structure and composition that it will be possible to apply generalized theories explaining oxygen and aroma mass transfer behavior to solve food packaging problems.

Finally, microbial stability is an area that will be- come more important as more edible polymers approach commercial viability. This will be especially important for higher-water-activity applications. The addition of antimicrobial agents and their migration in MC and HPMC multi-layer polysaccharide films have been examined with respect to their effect on oxygen perme- ability34*73. However, other antimicrobial agents and their effects on both aroma and oxygen permeability in edible polymers have not been examined.

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Trends in Food Science & Technology July 19%’ [Vol. 81 237

Education

Oxygen and aroma barrier properties of ediable films