Jung Han. Innovations in Food Packaging

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Edible films and coatings from non-starch polysaccharides

345

coliform bacteria on beef cuts (Williams

al., 1978; Cuq

al., 1995), which was

attributed to the presence of calcium chloride (Williams

al., 1978; Cuq

al., 1995).

Nisperos-Carriedo (1994) and Williams

al. (1978) reported that alginate coatings

did not affect cooking losses, flavor, odor or overall acceptability of coated beef and

poultry products. Alginate coatings have been used to retard oxidation off-flavors in

re-heated pork patties (Wanstedt

al., 1981), precooked beef patties (Wu

al.,

2001), and cooked pork chops (Hargens-Madsen, 1995). Coated pork patties and pork

chops exhibited improved flavor and juiciness, with no lipid oxidation. Coated pre-

cooked beef patties with an alginate-based coating containing stearic acid and toco-

pherol were effective in controlling moisture loss and lipid oxidation. Coatings made

from pectin have similar beneficial properties (Kester and Fennema, 1986).

Alginate coatings also improve adhesion between batter and product, thus reducing

the loss of batter from the surface of meats and fish (Fischer and Wong, 1972).

Calcium-alginate films can also have beneficial effects on the quality of fruits and

vegetables

for example, coated mushrooms with calcium alginate were found to

have a better appearance and color, and an advantage in weight, in comparison with

uncoated ones (Hershko and Nussinovitch, 1998).

Carrageenan

Carrageenan is derived from red seaweed, and is a complex mixture of several poly-

saccharides. The three principal carrageenan fractions are kappa (K), iota (L) and

lambda

(A),

which differ in the sulfate ester and 3,6-anhydro a-D-galactopyranosyl

content (Morris

al., 1980; Glicksman, 1982). K-carrageenan fractions contain the

lowest number of sulfate groups and the highest concentrations of the 3,6-anhydro-

a-D-galactopyranosyl units. The L-carrageenan fractions have an additional sulfate

group at the 2-position.

Gelation of both fractions occurs in the presence of both monovalent and divalent

ions (Nisperos-Carriedo, 1994). Gels prepared with alginates rich in L-glucopyranosyl-

uronic acid tend to be stronger, more brittle, and less elastic than those prepared with

alginates rich in D-mannopyranosyluronic acid (Nisperos-Carriedo, 1994). Commercial

carrageenans are mixtures of these three fractions. In the presence of locust bean

gum

an association with the double helix structure is formed, increasing the elasticity of

the gels. Pectin, guar, CMC, and starches could also be used to modify the gel struc-

ture (Nisperos-Carriedo, 1994).

Carrageenan-based coatings have been applied for a long time to a variety of foods

to carry antimicrobials or antioxidants, and to reduce moisture loss, oxidation, or dis-

integration. Defrosted coated whole chickens have a better resistance against off-flavor

development during storage at 4OC (Pearce and Lavers, 1949). The incorporation of

gallic acid or ascorbic acid as an antioxidant in a carrageenan-based coating was able

to keep the sensorial qualities of frozen mackerel fillets over 8 months of storage at

18°C (Stoloff

al., 1949). Food-grade antimicrobial agents (lysozyrne, nisin, grape

hit seed extract, and EDTA) incorporated with Na-alginate and K-carrageenan films

can be used effectively in a wide range of food packagings (Choi

al., 2001).

Carrageenan protects against moisture loss by acting as a sacrificial moisture layer

(Kester and Fennema, 1986). A carrageenan-based coating applied on cut grapefruit

346

Innovations in

FooA

Dackagin-

halves resulted in less shrinkage, leakage or deterioration of taste after 2 weeks of

storage at 4OC (Bryan, 1972).

Agar

Agar is a

gum

that is derived from a variety of red seaweeds,

and,

like carrageenan, it

is a galactose polymer (Sanderson, 1981). It forms strong gels characterized by melt-

ing points far above the initial gelation temperature (Whistler and Daniel, 1985). Like

carrageenan, antibiotics, bacteriocins or natural antimicrobial compounds can be

incorporated in agar-based films. These films can be used in order to improve shelf

life and to control pathogenic bacterial growth. Natrajan and Sheldon (1995) observed

a reduction in Salmonella typhimurium population of

1.8

to 4.6 log cycles after

96 hours of storage at 4OC on coated fresh poultry where nisin was incorporated.

Pectin

Pectin is a complex anionic polysaccharide composed of P-1, Clinked D-galacturonic

acid residues, where the uronic acid carboxyls are either klly (high-methoxy pectin)

or partially (low-methoxy pectin) methyl esterified. High-methoxy pectin forms

excellent films. Low-methoxyl pectin, derived by controlled de-esterification, forms

gels in the presence of calcium ions and can also be used for edible film develop-

ments. Blends of pectin and starch can be used to make strong, self-supporting films.

Pectin has been used in making biodegradable drinking straws

which coloring and

flavoring substances incorporated in a pectin layer are released when liquids pass

through the straw (Kasten, 1989). Plasticized blends of citrus pectin give strong, flex-

ible films, which are thermally stable up to 1 80°C (Tharanathan and

Kittur,

2003).

Pectin can also form cross-links with proteins under certain conditions (Thakur

et al., 1997). Autoclaving enhances pectin-protein interactions, resulting in a three-

dimensional network with improved mechanical and barrier properties. Films obtained

from irradiated solutions generated films with improved puncture strength (Letendre

et al., 2002a). Pectin is also miscible with poly(vinylalcohol) in all proportions. These

films are solution-produced by air-drying after casting at ambient temperatures.

Laminated films formed from pectin and chitosan with either glycerol or lactic acid as

a plasticizer have been prepared (Fishman et al., 1994). Although pectinate coatings

have poor moisture barriers, they can retard water loss from food by acting as a sacri-

ficial agent. Pectin coatings have been investigated for their ability to retard moisture

loss and lipid migration (Brake and Fennema, 1993), and improve handling and appear-

ance of foods. However, the addition of lipids may increase their resistance to water

vapor transmission (Schultz et al., 1949). Calcium pectinate gel coatings reduced

shrinkage and bacterial growth on beef patties (Stubbs and Cornforth, 1980).

Microbial polysaccharides

Pullulan, levan, and elsinan are extracellular microbial polysaccharides that are edible

and biodegradable. Pullulan films cast from aqueous solutions are clear, odorless, and

Edible films and coatings from non-starch polysaccharides

347

tasteless, and have good oxygen barriers (Yuen, 1974; Conca andYang, 1993). Pullulan

coatings have been used successfully as oxygen barriers to prolong food shelf life. The

polymorphic Aureobasidium pullulans secretes a polysaccharide pullulan, which is

commercially a useful hydrocolloid (Seviour et al., 1992). It is an a-glucan consisting of

repeating maltotriose residues joined by 1,6-linkages. It can be extruded

films that are

biodegradable, resistant to oils and grease, have excellent oxygen permeability rates,

and are non-toxic. Pullulan is of use in the biodegradable packaging films industry.

Levan and elsinan can also be used as edible coating materials for foods and phar-

maceuticals due to their low oxygen permeability properties (Kaplan et al., 1993).

Xanthan

gum

is produced by fermentation from the organism Xanthomonas

campestris. It contains five sugar residues: two P-D-glucopyranosyl, two P-D-

mannopyranosyl, and one (3-glucopyranosyluronic acid residue (Jansson et al., 1975;

Melton et al., 1976). Xanthan gum is soluble in both cold and hot water and has a high

viscosity. It is used for its thickening, suspending and stabilizing effects (Nisperos-

Carriedo, 1994).

Gellan

gum

is produced by fermentation of a pure culture of Pseudomonas elodea.

It is a linear tetrasaccharide with repeating units of

(1->3)-P-D-glucopyranosyl,

(I->4)-P-D-glucopyranosylwonic

acid,

(I->4)-P-D-glucopyranosyl,

and (1->4)-

a-L-rhamnopyranosyl units.

gel is formed after it is heated in the presence of cations

and then allowed to cool. It can be used to improve the texture and the stability of foods.

Dextrans are microbial gums composed solely of a-D-glucopyranosyl units, but with

varying types and amounts of glycosidic linkages mstler and Daniel, 1985). Dextran

coatings have been proposed to preserve the flavor, color, and freshness of shrimp

(Toulmin, 1956a, 1956b), fish (Novak, 1957), and meat products (Toulmin, 1957).

Exudate gums

Gum arabic or acacia is the dried, gummy exudate from the stems or branches of

Acacia Senegal and other related species of Acacia (Balke, 1984). Gum arabic is a

neutral or slightly acidic salt, and is a complex polysaccharide containing calcium,

magnesium, and potassium ions. It is a D-galactopyranosyl, L-rhamnopyranosyl,

L-arabinopyranosyl, L-arabinofuranosyl and D-glucopyranosyluronic acid containing

residues (Prakash et al., 1990; Whistler and Daniel, 1990).

Seed gums

Guar

gum

and locust bean

gum

are extracted from leguminous seeds (Glicksman,

1969; Whistler and BeMiller, 1973). Locust gum consists of P-D-mannopyranosyl and

a-D-galactopyranosyl units in a ratio of 4

1. This

gum

has a high viscosity, is soluble

in hot water (95OC), and is compatible with other hydrocolloids, carbohydrates, and pro-

teins (Nisperos-Carriedo, 1994). It is used as a thickener, a binder of free water, and a

suspending or stabilizing agent. Guar

gum

is soluble in cold and hot water, and has a

high viscosity. It is also compatible with other plant hydrocolloids, chemical modified

starches or cellulose, and water-soluble proteins.

348

Innovations

Food

Packaging

Applications

edible films and coatings

The successful development of edible coatings and films for food applications is

based on specific functional properties that the polymer should meet for each type of

food, due to the different properties of the various components (plasticizers, stabiliz-

ers, lipids, proteins etc.). The addition of these compounds in the optimal concentra-

tions and in the correct order after the appropriate treatment (cross-linking with

proteins, extrusion, coacervation, functionalization, bi-layer coatings etc.) can be ben-

eficial in developing films with superior functional properties.

Composition

The addition of plasticizers can improve the flexibility and reduce the brittleness of films

(Brault

al., 1997; Mezgheni

al., 1998a; Ressouany

al., 1998; Le Tien

al., 2000;

Letendre

al., 2002a). Plasticizers such as glycerol, sorbitol, propylene glycol, triethylene

glycol, polyethylene glycol, and ethylene glycol are commonly used.

Certain

plasticizers

can inhibit hgal growth. For example, lactic acid was substituted for glycerol in pectin

films and no further fungal growth was observed. The pectin films containing lactic

acid had properties similar to those made with pectin, glycerol and lactic acid.

sig-

nificant differences in the storage moduli (a measurement of stiffness) were observed

in the films containing lactic acid instead of glycerol (Hoagland and Parris, 1996).

The addition of stabilizing agents in coating solutions has been shown to preserve

the film integrity and, in certain cases, improve the mechanical properties and the

transparency of the films. Stabilizing agents are generally proteins such as gelatin,

collagen, prolamine etc. There are other known stabilizing agents, but the preferred

ones are those of a proteinaceous nature. These can be associated with polysaccharide

matrices. Furthermore, stabilizing agents that are included in edible coating formula-

tions can in certain cases increase the biofilm stability (at a certain pH limit) and the

water resistance (at elevated temperatures), and can limit film shrinkage (at a low or

intermediate relative humidity) (Lacroix

al., 2001).

Edible lipid components can be incorporated into coating formulations (Greener

and Fennema, 1989; Martin-Polo

al., 1992; Debeaufort and Voilley, 1994; Lacroix

al., 2001). Such lipids include oils (canola oil), fatty acids (palmitic acid), fatty

alcohols (stearyl alcohol), monoglycerides, and triglycerides having long carbon

chains from 10 to 20 carbon atoms, which are either saturated or unsaturated.

Composite films of proteins and lipids have also been formulated with the aim of

decreasing their solubility. However, protein-lipid films are often difficult to obtain.

For example, bi-layer film formation requires the use of solvents or high temperatures,

making the production of such films more costly. Furthermore, with time, separation

of the layers may occur. For films cast from aqueous-lipid emulsion solutions, the

process is more complex and the incorporation of the lipids is limited.

Composite and bi-layer coatings are the edible coatings of the future. These two types

of coatings have combined the beneficial properties of the various coating ingredients

to create a superior film. Composite films of the future may consist of hydrophobic

Edible films and coatings from non-starch polysaccharides

particles within a hydrophilic matrix. Such a configuration could give a water-soluble

coating with good water vapor barrier properties (Baldwin, 1991). Bi-layer coatings

(which have already been used to a limited extent) contain the beneficial water-barrier

properties of the lipid coatings with the addition of the greaseless feel and good gas

permeability characteristics of polysaccharide coatings. Such a bi-layer coating has

been shown to reduce gas exchange in cut apple pieces (Wong

al., 1994). However, few

or no data exist showing the effects of bi-layer coatings on whole fruits or vegetables.

It may be desirable to include agents such as emulsifiers, lubricants, binders or

defoaming agents to influence the spreading characteristics of coating solutions. For

example, polysaccharide-based films can be applied as an emulsion to improve the

moisture-barrier properties of the films (Kester and Fennema, 1989; Hernandez,

1994; Baldwin, 1995) and reduce the water loss from foods (Baldwin

al., 1997).

Emulsions are colloidal systems containing two partially miscible fluids and, almost

invariably, a surfactant. Emulsions are comprised of fat droplets that are generally

greater than 0.2 pm and less than 5 pm in diameter. These droplets are enveloped by

polysaccharide-based films of surfactant material that stabilize the droplets, prevent-

ing their association and coalescence. Emulsifying agents, such as proteins, lower the

interfacial tension between the dispersed and the continuous phases. Emulsion stability

depends on the delicate balance between the attractive intermolecular forces between

molecules in the film and the repulsive forces on the outer surface of the film. Emulsion

formation involves an increase in free energy and in the interfacial area between the

two phases. Mechanical work is necessary to overcome the free energy barrier.

Emulsion systems are generally unstable, and therefore emulsifying agents or surfac-

tants are often necessary to improve the stability of the emulsions (Everett, 1989).

Composite edible films or coatings can be produced by coacervation. Coacervation

is based on the separation of a macromolecular solution into two immiscible liquid

phases; a dense coacervate phase, which is relatively concentrated in macromole-

cules, and a dilute equilibrium phase. This separation occurs when macromolecules

have an increased tendency to interact with one another, which may be caused by a

reduction in their ability to interact with the solvent (simple coacervation) or by an

ionic interaction between oppositely charged macromolecules (complex coacervation)

(Burgess and Carless, 1984; Burgess

al., 199 1; Burgess and Singh, 1993). Normally,

different polymers with different charges are used to form an aggregative complex.

This interaction is electrostatic in nature (Bungenberg de Jong, 1949). Chitosan-alginate

films were prepared by Yan

al. (2000, 2001) using coacervation reactions. These

films had good tensile properties, and were resistant to water and most solvents.

However, the films exhibited microscopic heterogeneity, which led to the formation of

porous films with high permeability to water vapor.

Property

modification

Generally, polymers in their native state are sensitive to humidity and are soluble to water;

consequently, certain modifications seem necessary to improve their properties. In

certain formulations of films or coatings, a functionalization agent is included to

increase the hydrophobicity of the polymer in order to improve its moisture-barrier

350

Innovations in Food Packaging

properties (gas and water vapor). It is also possible that this functionalization agent

may increase the mechanical properties of the films or coatings. This agent can be

defined as a substance that is covalently linked on the polysaccharide matrix, with or

without the help of a coupling agent. It generally includes glyceraldehyde, acyl chlo-

rides, fatty acids, and anhydrides. Covalent modifications of the polysaccharide for

the formation of coating formulations can be obtained by esterification, etherification,

carboxymethylation etc. (Rutenberg and Solarek, 1984; Brode, 1991; Brode

al.,

1991; Lacroix

al., 2001; Mulhacher

al., 2001).

Cross-linking between proteins and polysaccharides, or treatments used to improve

interactions between proteins and polysaccharides, may be used to improve the film's

functional properties and its resistance (Mezgheni

al., 1998b; Le Tien

al., 2000;

Ressouany

al., 2000; Letendre

al., 2002a). Pectin may form cross-links with pro-

teins under certain conditions

(Thakur

al., 1997). Autoclaving enhances protein-

polysaccharide, pectin-protein or agar-protein interactions, resulting in a three-

dimensional network with improved mechanical properties. Letendre

al. (2002b)

analyzed the interaction of pectin with calcium and whey proteins after certain heating

treatments.

Extrusion processes can also be used to preserve flavors during food processing.

This process consists of initially making a low-moisture carbohydrate melt (15%

moisture), adding flavor (10-20% dried weight basis), forming an emulsion, and then

extruding the melt under pressure. Gum arabic is normally used for the encapsulation

of food flavors (Reineccius, 1994). Encapsulation could also be performed using

functionalized polysaccharides. Le Tien

al. (2003) have demonstrated that lactic

acid bacteria viability could be preserved when encapsulated in functionalized algi-

nate or chitosan by succinylation, or in N-palmitoylaminoethyl alginate (Figure 20.1).

1.5%

2.5% SA SC

Matrix

type

kgurc

26.1

Viability

(%)

rhamnosus,

free and immobilized, on different functionalized matrices and

in simulated gastric fluid (pH

1.5)

for 30 minutes (FC, free cell; NA, native alginate; SA, succinylated

alginate; SC, succinylated chitosan and PA, N-palmitoylaminoethyl alginate). (Reproduced with permission

from LeTien

etal.

(2004) @Portland Press on behalf ofthe

IUBMB)

Edible films and coatings from non-starch polysaccharides

351

352

Innovations

Food Packaging

Controlled emulsion destabilization offers another alternative method for the for-

mation of bi-layer lipid-protein films. De-emulsification techniques are known in the

food industry. Agents capable of displacing the stabilizing surfactant have been devel-

oped, leaving an unstable surfactant (Everett, 1989). These technologies may be

applied in the future for the formation of effective edible films and coatings.

Carbohydrate chemistry

The structure of modified polymers can be characterized by several techniques such

as Fourier transform infrared (FTIR), nuclear magnetic resonance (NMR), X-ray dif-

fraction (XRD), and scanning electron microscopy (SEM). FTIR and NMR analysis

are frequently used to characterize the polymer because they are rapid and give ample

information. XRD and SEM are also good techniques, but are rarely used because

they are expensive.

The FTIR analysis could be useful to identify the functional groups of the polymers

after modifications. The important spectral regions at 3300-3000 cm-' are generally for

the elucidation of the free or the interacting hydroxyl

(-OH)

groups; at 2950-2850 cm-'

for the alkyl (-CH2-) chains; and at 1750-1650cm-', the bands represent the car-

bony1 groups

(-€*)

and the amides bands

(<-N-).

For example, Le Tien

al.

(2004) functionalized the chitosan by acylation in order to improve the hydrophobic-

ity of the polymer. FTIR analysis permitted the visualization of N-acyl chitosan

(Figure 20.2a). For the native chitosan (non-modified chitosan), the absorption peaks

at 1655 cm-' were assigned to the carbonyl stretching of secondary amides (amide I

band), at 1570 cm-' to the N-H bending vibrations of non-acylated 2-aminoglucose

primary amines, and at 1555 cm-' to the N-H bending vibrations of the amide I1 band

(Xu

al., 1996). After N-acylation, the vibration band corresponding to the primary

amino groups at 1570 cm-' disappeared (Figure 20.2), while prominent bands at 1655

and 1555 cm-' were observed.

addition, peaks at 295~2850cm-' represented

(<H2-), and their intensity was proportional to the acyl chain length. These analyses

clearly confirmed that the chitosan was substituted.

In NMR, the structure of the polymer can be determined by

'H-

or 13c-nuclear

magnetic resonance spectroscopy (NMR). Using the same example (Figure 20.2b),

the

NMR spectra of native chitosan shows that peaks at 2.0-2.1 ppm represent the

three N-acetyl protons of N-acetylglucosamine (GlcNAc), and peaks at 3.1-3.2ppm

represent a H-2 proton of glucosamine (GlcN) residues. The ring protons (H-3,4, 5,

6,6' are considered to resonate at 3.&4.0ppm, and the peaks at 4.6 and 4.8 ppm were

assigned to the H-1 protons of the GlcN and GlcNAc residues, respectively (Xu

al.,

1996; Signini

al., 1999; Sashiwa

al., 2002). However, for the N-acyl chitosan,

new peaks at 0.7, 1.2 and 1.9 ppm were assigned to protons of CH3-, -€Hz- and

-CH2-(CO), respectively, of the alkyl residue (Le Tien

al., 2004). It is worth noting

that this method is not very useful for certain polymers due their high viscosity (i.e.

Edible films and coatings from non-starch polysaccharides

353

Kacyl chitosan

I I

4000 3500 3000 2500 2000 1500 1000

Wavenumber

(cm-')

HOD

(01"-pal mi toy la mine^

Figure

20.2

Characterization of N-acyl chitosan

FIR

(a)

and

NMR

(b)

analysis, according to

Tien

etal.

(2003).

alginate). Generally, it is then therefore necessary to reduce the viscosity and increase

the mobility of the molecules by depolymerization to obtain a degree of polymeriza-

tion of about 20-50%. The

NMR

methodology and the peak identifications were

based on data published by Grasdalen

al.

979, 198 1, 1983).

XRD

and SEM are rarely used to characterize the polymers.

XRD

is mostly used to

determine the degree of crystallinity of the polymer. Generally, polymers possess a

high

degree of crystallinity, forming a structure that is more organized and has good

mechanical properties (Le Tien

al.,

2004). For SEM, the analysis is essentially

based on the aspects of the polymer, such as uniformity and porosity.

Thermal and physicochernical properties

The thermal and mechanical properties of the polymeric materials are important for

processing, and for products derived from these materials. Most biobased polymer

materials perform in a similar fashion to conventional polymers. This indicates

that both polystyrene-like polymers (relatively stiff materials with intermediate serv-

ice temperatures) and polyethylene-like polymers (relatively flexible polymers

with intermediate service temperatures) can be found among the available biobased

polymers.

The mechanical properties (modulus and stiffness) of biopolymers are not very dif-

ferent compared to the conventional polymers. The modulus of biomaterials ranges

from 250&3000 MPa, and decreases for stiffer polymers such as thermoplastic

starches (50 MPa) and even more for rubbery materials like medium-chain polyhy-

droxyalkanoates. Furthermore, the modulus of most biobased and petroleum-derived

polymers can be tailored to meet the required mechanical properties by means of plas-

ticizing, blending with other polymers or fillers, cross-linking, or the addition of

fibers. Polymer such as bacterial cellulose could, for instance, be used in materials that

require special mechanical properties.

theory, biomaterials can be made having

similar strength to the ones that are used today (Iguchi

al., 2000).

major challenge for the material manufacturers is the improvement of the water

vapor barriers of films and coatings for food applications. For example, the water

vapor transmittance of various polysaccharide films, when compared to materials

based on mineral oil, is lower. It becomes clear that it is difficult to produce an edible

film or coating with water vapor permeability rates comparable to the ones provided

by some conventional plastics. Consequently, research is focusing on this problem in

order to produce future biomaterials that are able to mimic the water vapor barriers of

the conventional materials known today (Butler

al., 1996).

Conclusion

Polysaccharide-based films have the potential to be used in a variety of applications.

These films can be used to extend the shelf life of fruits, vegetables, seafood, meats,

and confectionery products by preventing dehydration, oxidation rancidity, surface

browning, and oil diffusion, as well as by their ability to modify the internal atmos-

phere in fruit and vegetable packaging. Moreover, when applied to fruits and vegeta-

bles, polysaccharide-based coatings can improve the physicochemical, nutritional,

and sensorial properties of the products. Several studies have demonstrated that the

incorporation of active compounds like antimicrobials, antioxidants or bacteriocins in

edible films and coatings can improve the microbial safety and the shelf life, and can

be used to stabilize the quality of the products during storage. The development of

new technologies (functionalization, cross-linking etc.) to improve the film properties

(control release, bioactivity protection, resistance to water etc.) of active packaging

and coatings is a major focus for future research.