Jung Han. Innovations in Food Packaging

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Edible films and coatings: a review

245

and safety, up to the level where the additives interfere with physical and mechanical

properties of the films (Kester and Fennema, 1986; Baldwin

al., 1995,1997; Guilbert

al., 1996; Howard and Gonzalez, 2001; Han, 2002,2003). Emulsifiers are surface-

active agents of amphiphilic nature able to reduce the surface tension of the water-lipid

interface or the water-air surface. Emulsifiers are essential for the formation of protein

or polysaccharide films containing lipid emulsion particles. They also modify surface

energy to control the adhesion and wettability of the film surface (Krochta, 2002).

Although many biopolymers possess certain levels of emulsifying capacity, it is nec-

essary to incorporate emulsifiers into film-forming solutions

produce lipid-emulsion

films. In the case of protein films, some film-forming proteins have sufficient emulsi-

fying capacity due to their amphiphilic structure. Antioxidants and antimicrobial

agents can be incorporated into film-forming solutions to achieve active packaging or

coating functions (Han, 2002, 2003). They provide additional active functions to the

edible film and coating system to protect food products from oxidation and microbial

spoilage, resulting in quality improvement and safety enhancement. When nutraceutical

and pharmaceutical substances are incorporated into edible films and coatings, the sys-

tem can be used for drug delivery purposes (Han, 2003). Incorporated flavors and col-

orants can improve the taste and the visual perception of quality, respectively. Because of

the various chemical characteristics of these active additives, film composition should

be modified to keep a homogeneous film structure when heterogeneous additives are

incorporated into the film-forming materials (Debeaufort

al., 1998).

Functions

and

advantages

Edibility and biodegradability

The most beneficial characteristics of edible films and coatings are their edibility and

inherent biodegradability (Guilbert

al., 1996; Krochta, 2002). To maintain edibility,

all film components (i.e. biopolymers, plasticizers, and other additives) should be

food-grade ingredients and all process facilities should be acceptable for food pro-

cessing (Guilbert

al., 1996). With regard to biodegradability, all components should

be biodegradable and environmentally safe. Human toxicity and environmental safety

should be evaluated by standard analytical protocols by authorized agencies.

Physical and mechanical protection

Edible films and coatings protect packaged or coated food products from physical

damage caused by mechanical impact, pressure, vibrations, and other mechanical fac-

tors. Standardized mechanical examinations of commercial film structures are also

applied to edible film and coating structures. Such tests may include tensile strength,

elongation-at-break, elastic modulus, compression strength, puncture strength, stiffness,

tearing strength, burst strength, abrasion resistance, adhesion force, folding endurance

and others. Table 15.2 shows the tensile properties of various edible films and common

246

Innovations in Food Packaging

Table

15.2

Tennlc

and

gas

buner

characmnshcs

edible

films

and

common

plastks

Tensile strength

(MPa)

PPC: Cly

Elongation

(96)

Ca-caseinate

Cly

Marginal

Good

Superior

1-10

Coll

Cell

Cly

Na-caseinate

Gly

Ca-caseinate

Cly

WPI

Sor

EWP: Gly

SPI

Cly

PEG

Cly

WPI

BW: Gly

SPI: FA: Cly

Pea protein

Gly

10-100

WPI

Gly

FMP: Cly

CZ:Cly

HAPS

Cly

LDPE

DPE

>I00

Cellophane

HPMC

Amylose

OPP

WDC

PET

1-70

HPMC

WPI

BW: Cly

10-100

corl :cell

cly

Na-caseinate

Cly

WPI

Cly

WPI

Sor

FMP:Gly

EWP:Cly

EWP

PEG

SPI

Gly

PEG

Pea protein

Cly

Cellophane

Amylose

OPP

PET

>loo

CZ: Cly

Cly

SPI :FA: Gly

HAPS

Cly

LDPE

HDPE

Own permeabil$

(cdPm me2d-'

k~a-'

)

>loo0 1000- 100

LDPE HPMC

Starch

Cly MC

Shellac

Beeswax

Most waxes

HDPE

Water vapor

permeability

(gmm m-2d-'kPa-')

>10 10-1

Na-caseinate

Cly

Ca-caseinate

Gly

EWP

Cly

WPI

Sor

WPI

Cly

SPI

Cly

PPC

Cly

SPI

FA: Cly

Gly

WC:BW:Gly

Ca-caseinate

WPI

BW: Sor

WPI

BW: Gly

Cellophane

100-10

Collagen

Cly

WPI

Cly

EWP

Cly

PPC

Cly

Cellophane

Polyester

1-0.1

Shellac

Chocolate

<10

Cly

SPI

Cly

WPI

Sor

HAPS

Gly

NOH

WDC

<o.

HPMC

PEG

HPMC

BW: PEG

Beeswax

Paraffin

wax

Most waxes

LDPE

HDPE

WDC

(Continued)

Edible films and coatings:

review

247

Marginal

Good

Superior

'ea protein

Cly

iAPS

Gly

NOH

PET

ily, glycerol; Sor, sorbitol; PEG, polyethylene glycol; PPC, peanut protein concentrate; Coll, collagen;

:ell, cellulose; WPI, whey protein isolate; EWP, egg white protein; SPI, soy protein isolate;

corn

ein; WC, wheat gluten; FMP, fish myofibrillar protein; MC, methylcellulose; HPMC, hydroxypropyl

nethylcellulose; HAPS, high-amylose pea starch; LDPE, low-density polyethylene; HDPE, high-density

~olyethylene; PP, polypropylene;

PS,

polystyrene; OPP, oriented polypropylene; NOH, ethylene vinylal-

:ohol; WC, polyvinyl chloride; WDC, polyvinylidene chloride; PET, polyethylene terephthalate; BW,

peeswax;

FA,

fatty acids.

-est conditions for tensile test and water vapor permeability are at approximately 50% RH and

25°C.

Iata collected from Gennadios

eta/.

(1 994), McHugh and Krochta (1 994), Guilbert

et a/.

996),

kochta (1997,2002), Choi and Han (2001), Wu

eta/.

(2002), Mehyar and Han (2003).

plastic films. Edible films have lower tensile strength than common plastic films,

while their elongation-at-break varies widely. Some edible films have elongation val-

ues comparable to those of common plastic films. Many edible film and coating mate-

rials are very sensitive to moisture (Guilbert and Gontard, 1995; Guilbert

al., 1996;

Krochta, 2002). At higher relative humidity conditions, their physical strength is

lower than that at lower relative humidity since absorbed moisture bctions as a plas-

ticizer. Temperature is also

important variable affecting the physical and mechanical

properties of edible films and coatings (Guilbert

al., 1997; Miller

al., 1998; Wu

al., 2002). The physical strength of materials dramatically decreases when temperature

increases above the glass transition temperature. High relative humidity and large

amounts of plasticizers lower the glass transition temperature of film-forming materials.

Migration, permeation, and barrier functions

The quality of most food products deteriorates via mass transfer phenomena, including

moisture absorption, oxygen invasion, flavor loss, undesirable odor absorption, and

the migration of packaging components into the food (Kester and Fennema, 1986;

Debeaufort

al., 1998; Miller

al., 1998; Krochta, 2002). These phenomena can occur

between food and the atmospheric environment, food and packaging materials, or among

heterogeneous ingredients in the food product itself (Krochta, 1997). For example,

atmospheric oxygen penetration into foods causes oxidation of food ingredients; inks,

solvents and monomeric additives in packaging materials can migrate into foods;

essential volatile flavors of beverages and confections may be absorbed into plastic

packaging materials; and pielpizza crusts absorb moisture from fillings/toppings,

leading to the loss of crispness. Edible films and coatings may wrap these food prod-

ucts or be located between heterogeneous parts of food products to prevent these

migration phenomena and preserve quality (Guilbert

al., 1997; Krochta, 2002).

To characterize the barrier properties of edible films and coatings, the transmission

rates of specific hazardous migrants should be determined using stand-alone edible

nnovations in Food Packaging

Temperature increase

Effects of phase transition temperature on tensile properties and mass transfer. Tgand

5,dss cr,,isition temperature and melting temperature, respectively. Y-axis has a logarithmic scale.

films. Most research has dealt with water vapor permeability, oxygen permeability,

carbon dioxide permeability, flavor permeability, and oil resistance of edible films.

Table 15.2 shows oxygen permeability and water vapor permeability values of edible

films and common plastic films. Edible films possess a wide range of oxygen perme-

ability values. Certain edible films are excellent oxygen barriers. Except for lipid-based

materials, the water vapor permeability of most edible films is generally higher than

that of common plastic films. All barrier properties of edible films and coatings are

affected greatly by film composition and environmental conditions (relative humidity

and temperature). Plasticizers in edible film-forming materials reduce glass transition

temperatures and increase the permeability of most migrants. Oxygen permeability is

very sensitive to relative humidity (Guilbert

al., 1997; Mat6 and Krochta, 1998). At

higher relative humidity conditions, oxygen permeability increases substantially.

Therefore, it is very important to maintain low relative humidity environments to

maximize the effectiveness of edible films as gas barriers.

Temperature is also an important factor of migration (Guilbert

al., 1997; Amarante

and Banks, 2001; Wu

al., 2002). A temperature increase provides more energy to

the migrating substances and increases the permeability. At temperatures far distant

from the phase transition, changes of migration coefficients such as permeability and

diffisivity follow the Arrhenius equation (Guilbert

al., 1997; Miller

al., 1998). At

the glass transition and melting temperatures of film materials, most mass transfer

coefficients change substantially (Figure 15.3).

Convenience and quality preservation

Edible films and coatings provide many benefits

terms of convenience. Reinforced

surface strength of fi-agile products makes handling easier. Coated fruits and vegeta-

bles have much higher resistance against bruising and tissue damage caused by phys-

ical impact and vibration. Besides this protective function for foods, edible films and

coatings are utilized

the food and pharmaceutical industries to develop single-dose

Edible films

and coatings:

review

249

(a) Uncoated crust

(b)

Coated crust

Rguro

15.4

Bottom of apple pies after 7-day storage. (a) Regular apple pie; (b) Inside piecrust had been

sprayed with edible coating materials before filling. The edible coating layer prevented the migration of

moisture from the filling to the crust, and maintained crispiness ofthe crust. (Courtesy of BioEnvelop Agro

Technologies,

St.

Hyacinth, Quebec, Canada.)

pre-measured pouches of food ingredients and drugs, as well as to mask the eventual

undesirable taste of medicines (Gennadios and Weller, 1990).

Quality maintenance and enhancement are also very significant functions of edible

films and coatings (Krochta, 1997). They can retard surface dehydration, moisture

absorption, oxidation of ingredients, aroma loss, fiying oil absorption, ripeninglaging,

and microbial deterioration of food products.

addition to the physical and chemical

quality enhancement, edible films and coatings contribute to visual quality, surface

smoothness, flavor carriage, edible color print, and other marketing related quality

factors. Edible coatings maintain the quality of foods even after the package is opened

(Krochta, 1997).

Edible films and coatings may be used to preserve the quality of several food com-

modities. The oxygen-barrier properties of film and coating layers can prevent oxidation

of lipid ingredients, colorants, and flavors of food products such as nuts, confectionary,

fried products, and colored produce (Baldwin

al., 1997). The oxygen-barrier prop-

erty is also quite usell for retarding the respiration rate of fresh produce (Baldwin

al.,

1995). Many climacteric fruits and vegetables can be coated with edible film-forming

materials to slow down their respiration rate (Park, 2000; Amarante and Banks, 2001).

High-fat meat and fish products, such as sausages, jerky, and fillets, can be protected

from oxidation after an edible coating process. Moisture-barrier properties of edible

films and coatings can protect fresh fruits and vegetables from dehydration. Moisture

loss is the most critical quality degradation factor of fresh produce (Guilbert

al., 1997).

The moisture barrier property can also be utilized to prevent moisture migration

between heterogeneous food product ingredients, for example between raisins and

breakfast cereals (Kester and Fennema, 1986), pie fillings and crusts (Figure 15.4),

hit chips and baking dough (Buss, 1996), and the like. The active ingredient carrier

function is very useful for the addition of quality preserving agents as well as nutrients

and nutraceuticals, resulting in an upgraded quality level of products such as col-

oredlflavored confectionary, glazed bakery, flavored nuts, and vitamin-enriched rice.

nnovations in Food Packaging

Shelf-life extension and safety enhancement

The enhancement and maintenance of quality are directly related to shelf-life extension

and safety improvement.

increased protective function of food products extends

shelf life (Kester and Fennema, 1986; Krochta, 1997) and reduces the possibility of

contamination by foreign matter (Han, 2002, 2003). The market for minimally

processed foods and fresh produce has recently seen a significant increase, and accord-

ingly there is a requirement to secure the safety and extend the shelf life of the prod-

ucts involved (Baldwin

al., 1995, 1997; Park, 2000). The massive scale of modern

food manufacturing, distribution systems, and fast-food business also dictates the

need for improved systemic procedures for maintaining safety and shelf life.

The use of biodegradable materials for food packaging in the food service business

is attractive because it reduces the total amount of synthetic materials and appeals to

environmentally conscious consumers (Krochta and De Mulder-Johnston, 1997;

Arnarante and Banks, 2001). However, it is obvious that the period of disintegration

of edible films and coatings through biodegradation mechanisms should be longer

than the expected shelf life of the packaged products.

Active substance camers and controlled release

Edible films and coatings can carry food ingredients, pharmaceuticals, nutraceuticals,

and agrochemicals in the form of hard capsules, soft gel capsules, microcapsules, solu-

ble strips, flexible pouches, coatings on hard particles and others (Kester and Fennema,

1986; Gennadios and Weller, 1990; Baldwin

al., 1996). The most important pararne-

ter to judge the effectiveness of these applications is the ability of controlled release

(Han, 2003). Depending on the nature of the application, various release rates are

required, which may include immediate release, slow release, specific release rate, or

non-migration of active substances. Edible films and coatings should control the release

rates of the incorporated active substances to the surrounding media as specifically as

possible to achieve maximum effectiveness of the function(s) of the active substances.

It is also important to consider chemical interactions between the active substances and

the film-forming materials, and between the former and the environmental conditions.

Many different active substances can be incorporated into film-forming materials

to design controlled release systems. Such active substances requiring specific migra-

tion rates are antimicrobials, antioxidants, bioactive nutraceuticals, pharmaceuticals,

flavors, inks, fertilizers, pesticides, insect repellents, and medicaVbiotechnology diag-

nostic agents.

Non-edible product applications

Edible films and coatings may substitute for conventional synthetic packaging mate-

rials since they have comparable material properties (Krochta, 1997; Krochta and De

Mulder-Johnston, 1997). Because edible films and coatings are biodegradable, their

mechanical and physical properties do not last for a very long time. Their properties

should remain effective during the whole period of shelf life. However, because of

Edible films and coatings: a review 251

BBMBa||MgM Uncoated | yiK^^'mMggfiiii

f^^^^^^^^^f^^^^^^^

i ^m^^^^^M

Figure 15.5 SEM image of cross-sections of edible protein coated pulp-papers. Right-hand side, paper

was coated by whey protein isolate at the level of 10g of protein per m^ of paper surface. Edible coating

resulted in smoother surface as well as stronger resistance to moisture, oil, and oxygen transmissions.

environmental sensitivity (such as to humidity) and relatively inferior mechanical and

optical properties compared to those of conventional packaging materials (e.g. paper,

paperboard, plastics, glass, and metals), complete substitution of the latter with edible

film and coating materials may not be suitable to maintain the fimctional requirements

of packaging systems. Therefore, partial replacement is recommended to reduce the

use of synthetic materials (Han and Krochta, 2001).

Whey protein coatings on paper and paperboard can improve the barrier properties

of paper-based packaging materials by slowing even fiirther the moisture and oil pen-

etration, as well as improving surface smoothness (Han and Krochta, 1999, 2001; see

Figure 15.5). Edible film layers on conventional packaging materials could improve

their optical properties and modify their hydrophilic/hydrophobic nature, including

surface energy and water/oil absorption of material surfaces.

Edible films and coatings may be used in non-food situations, such as for agricul-

tural applications, paper manufacturing, plastic modification, consumer products, and

in the painting industry. Edible films could replace synthetic films, maximizing

biodegradation activity. Coating applications may modify the surface properties of

any compatible materials. Both applications can carry active substances and prevent

moisture and gas migration through the materials. In agricultural uses, they can be

used for mulching, wrapping, seed coating, and agrochemical delivery (Guilbert and

Gontard, 1995; Nussinovitch, 2003). Surface coating of low-grade papers with edible

biopolymers can upgrade their quality for printing and other special purposes (Han

and Krochta, 1999). Low-gauge thin paper can be transformed into heavy-gauge thick

and strong paper by adding an edible coating. The modified paper surface may accept

various inks and further lamination.

Edible coatings can also modify plastic surfaces. Surface modification of plastics

has various benefits in many industrial applications. Hydrophilic edible coatings on

hydrophobic packaging material surfaces increase the surface energy and provide

great advantages in the printing and packaging processes (Hong et al, 2003). The

coated packaging materials with increased surface energy can accept various types of

inks and ink solvents, such as benzene-, alcohol-, and water-based (Han and Krochta,

1999,

2001; Hong et ai, 2003), and may eliminate electrostatic problems through a

lL1

Innovations in Food Packaging

fast and easy electrostatic discharge since the increased hydrophilicity raises the elec-

troconductivity of the material. The latter application has been studied frequently in

the clothing and textile area, but requires more experimental verification for the pack-

aging application.

Besides the benefit of surface modification, common plastic grocery bags may be

substituted with biopolymer bags. Many consumer plastic products may be replaced

by the edible biopolymer materials, which may be used for disposable plastic bags,

cups, plates, containers and utensils, and other plastic products. Edible biopolymers

may be incorporated in water-based paints to control paint viscosity and the proper-

ties of the paint surface after drying.

Edible biopolymers may also contain chemical cross-linking agents, resulting in a

stronger film structure with higher resistance and greater longevity. Even though the

cross-linked film structures are no longer edible, they still preserve their biodegrad-

ability, minimizing environmental impact (Rhim, 1998; Micard et al., 2000). Most

cross-linking agents that are commonly used for cross-linking the reactions of various

specific ligands in the column-packing matrix of affinity chromatography can be uti-

lized for the cross-linking of biopolymers, as long as they do not impart significant

toxicity to the corresponding non-food application.

Other process-aiding functions

Edible films and coatings can increase the effectiveness of some food processing unit

operations. For example, edible coatings on potato slices/strips or fish can reduce oil

absorption during fiying (Balasubramaniam

al., 1997; Sensidoni and Peressini,

1997). Edible coatings on fruits and vegetables can retard the oxidation of dried products

during dehydration and improve the shelf-life extension imparted by irradiation

(Lacroix and Ouattara, 2000). Edible coatings of plasticizers can also reduce the loss

of color, flavor, or nutrients in particulate fluid foods during processing and distribu-

tion (Buss, 1996). Edible coatings improve the effectiveness of the popping process of

popcorn (Wu and Schwartzberg, 1992), and act as adhesion agents between hetero-

geneous food ingredients (Anonymous, 1997).

Many advantages may result from the osmotic dehydration of fruits, vegetables, and

functional foods. Since the osmotic dehydration utilizes the migration of water caused

by osmotic pressure, many other water-soluble ingredients can be released from the

food into the dehydrating fluids (which are generally specific sugar solutions). Edible

coatings applied prior to the osmotic dehydration can prevent the migration of valu-

able ingredients into the dehydrating fluids during the dehydration process, as well as

minimizing the invasion of dehydrating agents into the food itself (Dabrowska and

Lenart, 2001). Therefore, edible coatings can broaden the selection of dehydrating agents

and optimize operation conditions. To maximize the benefit of edible coatings for the

osmotic dehydration process, the edible coating layer should have selective migration

rates. Water vapor permeability should be high to enhance dehydration; however, the per-

meability of valuable ingredients or dehydrating agents should be very low to protect

their migration during dehydration. Edible coatings on food products may be benefi-

cial to freeze-drymg processes, since the moisture-permeable coating can prevent the

Edible films and coatings: a review

253

evaporation of volatile flavors. Evidently, extensive experimental studies are required

to verify and validate the benefits of edible coatings as applied to food operations.

Scientific

parameters

Chemistry of film-forming materials

It is essential to understand the chemical properties and structure of film-forming mate-

rials, biopolymers as well as additives, to tailor them to specific applications (Han, 2002;

Nussinovitch, 2003). Solubility in water and ethanol is very important to select a solvent

for wet casting or active agent mixing. Thermoplasticity of biopolymers, including phase

transition, glass transition, and gelatinization characteristics, should be understood for

dry casting or thermoforming (Guilbert

al.,

1997). Many hydrophilic properties of

film-forming materials are also very important characteristics that should be identified.

These may be determined indirectly from water-related properties, such as hydrophilic-

lipophilic balance, hygroscopicity, water solubility, and solid surface energy of

films;

hydrodynamic radius of biopolymers and plasticizers; and surface tension and viscosity

of film-forming solutions. The chemical characteristics of plasticizers and any other addi-

tives should also be identified to verify their chemical compatibility with biopolymers

and to determine the changes in

film

structure caused by the addition of plasticizers and

additives. These investigations are very important to obtain critical infonnation related

film-forming mechanisms and film property modification.

Film-forming mechanisms

edible film is essentially a dried and extensively interacting polymer network of a

three-dimensional gel structure. Despite the film-forming process, whether it is wet

casting or dry casting, film-forming materials should form a spatially rearranged gel

structure with all incorporated film-forming agents, such as biopolymers, plasticizers,

other additives, and solvents in the case of wet casting. Biopolymer film-forming

materials are generally gelatinized to produce film-forming solutions. Further drying

of the hydrogels eliminates excess solvents from the gel structure. For example, whey-

protein films are produced from whey-protein gels by dehydration after heat-set or

cold-set gel formation. This does not mean that the film-forming mechanism during

the drying process is only the extension of the wet-gelation mechanism. The film-

forming mechanism during the drying process may differ from the wet-gelation

mechanism, though wet gelation is the initial stage of the film-forming process. There

could be a critical stage of a transition from a wet gel to a dry film, which relates to a

phase transition from a polymer-in-water (or other solvents) system to a water-in-polymer

system. The complete film-forming mechanisms of most biopolymers after gelation are

not clearly determined yet. Several polymer chemistry laboratory techniques are

required to identify them, including X-ray diffraction, FTIR spectrometry, NMR

spectrometry, electrophoresis, polarizing microscopy, and other polymer analysis

methodologies. For extrusion casting (dry process), many thermoplastic properties,

254

Innovations in Food Packaging

Film-forming

materiala

Films or

coating-

brication morphological treatments

ingredients to

process

modification (lamination,

film-forming condition! over-coating,

mechanisms*

'hysical methodr $EZi?r annealing)

Figurn

15.6 Various ways for modifying the characteristics of edible films and coatings.

indicates the addition of chemically or physically active ingredients, which may enhance or interfere with

the film-forming mechanisms;

includes any chemical cross-linking, chemical substitution of side chains

to create hydrophobic interactions or electrostatic interactions, and other extra mechanisms caused by

chemical modifications.

such as gelatinization, polymer melting, flow profile, polymer rearrangement and oth-

ers, should be investigated to predict the film-forming mechanisms. Figure 15.6

describes potential chemical and physical approaches to the modification of film-

forming mechanisms by altering film-forming raw materials, varying film-forming

processing conditions, and applying treatments on formed films.

As examples, potential chemical methods of modifying the film-forming mecha-

nisms of protein-based films include pH changes, salt addition, heat denaturation, sol-

vent changes, chemical modification of the side chains of peptides, cross-linking, and

hydrolysis of peptides (Yildirim and Hettiarachchy, 1997; Rhim, 1988; Were

al.,

1999), irradiation of peptides (Lacroix and Ouattara, 2000), and the addition of foreign

proteins. For polysaccharide-based films several chemical modifications are available,

including salt addition, solvent changes, heat gelatinization, pH changes, chemical

modification of hydroxyl groups, cross-linking of polysaccharides, hydrolysis of

polysaccharides, and the addition of foreign polysaccharides. Physical modifications

of edible films and coatings include lamination, formation of composites, addition of

particles or emulsions, perforation, over-coating, annealingheat curing (Gennadios

al., 1996; Miller

al., 1997; Micard

al., 2000), orientation, radiation (Gennadios

al., 1998; Micard

al., 2000), and ultrasound treatment (Banerjee

al., 1996).

Physical chemistry

films

While polymer structural chemistry is a very important tool for the determination of

film-

forming mechanisms, physical chemistry is essential to determine film characteristics.