Jung Han. Innovations in Food Packaging

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Agro-polymers for edible and biodegradable films

265

materials are used in various forms (simple or composite materials, single-layer or

multi-layer films).

Polysaccharides used for material formulations are generally the same ones as

those used as stabilizing, thickening, and gelling agents. These polysaccharides are of

various origin: plant polysaccharides such as cellulose and derivatives; starches and

derivatives; pectin or arabinoxylanes; algae gums such as alginates or carrageenans;

and microbial gums, pullulan, xanthan and gellan.

Many plant and animal proteins have been studied as raw materials for films and

coatings. These proteins are generally characterized by having interesting functional

properties (Guilbert and Cuq, 2002).

Lipids and derivatives are used due to their good water-barrier properties. The use

of lipids (or derivatives) with a polysaccharide or protein-based matrix or support is

generally advised.

A few examples of applications of edible films or coatings, used to improve prod-

uct appearance or conservation, include sugar and chocolate coatings for candies or

icecreams, wax coatings for fresh hits, and oil or fat coatings for raisins and dry

hits. Edible films become a part of the whole food product; their composition is

dependent on the product nature, and must conform to regulations that apply to the

respective food product. Therefore, edible film and coating technologies are regarded

as a "problem" for food formulations (Kester and Fennema, 1986; Krochta

al.,

1994; Guilbert and Cuq, 1998).

Biodegradable materials

As far as biodegradable materials are concerned, starch is the most commonly used

agricultural raw material. Starch is inexpensive, widely available, and relatively easy

to handle. "All-starch" bio-plastics are made from thermoplastic starches, which are

produced by standard techniques of synthetic polymer films, such as extrusion or

injection molding (Colonna, 1992). The use of thermoplastic proteins has also been

investigated (Gontard and Guilbert, 1994; Guilbert and Cuq, 2002), but its commercial

applications are still expected. Among the proteins, milk proteins (casein, whey pro-

teins), soy proteins and cereal proteins (wheat gluten, zein) have been more extensively

studied (Genadios

al., 1994; Redl

al., 1996; Guilbert

al., 2001).

Materials based on hydrocolloids constitute effective oil or fat barriers, but they are

generally not very resistant to water and their moisture-barrier properties are poor.

However, in some cases, water solubility or the sensitivity to water is a functional

advantage

for example, in the formulation of soluble sachets of edible films or coat-

ings (less perceptible in the mouth) or in the formulation of "active" materials, where

the water swelling is used to induce a drastic change in the properties. However, in

general, improving water resistance and water-barrier properties is of most impor-

tance. Therefore, chemical modifications of the biopolymers and the development of

specific additives (cross-linking agents or plasticizers) adapted to the polymer

structures are proposed. Regarding these developments, proteins that have a "non-

monotonous" complex structure and many potential functional properties are promising

(Cuq

al., 1998).

Commercial water-resistant starch-based bio-plastics (for non-edible applications)

are produced by using fine molecular blends of biodegradable synthetic polymers and

starches. These composite materials are made with gelatinized starch (up to 60-85%),

hydrophilic synthetic polymers (e.g. ethylene vinyl alcohol copolymer) or hydrophobic

synthetic polymers (e.g. polycaprolactone), and with compatibility agents (Fritz

al.,

1994). The most important starch-based material on the market is ~aterbi@, proposed

by Novamont.

Microbial polymers (e.g.

poly(3)-hydroxybutyrate-hydroxyvalerate)

are excreted

or stored by micro-organisms cultivated on starch hydrolysates or lipid mediums. The

isolation and purification costs of these products, which are obtained from complex

mixtures, can be high. Monsanto stopped the commercialization of their product

"~iopol@"

1999. Coopeazucar, in Brazil, has built new facilities for pilot plant

production of these polyhydroxyalkanoates.

Polylactic and polyglycolic acids are mainly produced by chemical polymerization

of lactic acid and glycolic acid, which are obtained by Lactobacillus fermentation.

Commercial applications of these materials are rapidly increasing under the trade-

marks of

copl la@

from CargilVDow Chemical, or ~acea@ from Mitsui.

Processing

Two general process pathways for agro-materials are distinguished:

1. The

"dry

process", such as thermoplastic extrusion, is based on the biopolymers'

thermoplastic properties when plasticized and heated above their glass transition

temperature under low water content conditions

The "solvent process" or "casting" is based on the drying or dispersion of the film-

forming solution.

The casting process is used to form edible preformed films, or to apply coatings

directly onto the products (Guilbert and Cuq, 1998). This process is generally adapted

for coating seeds and foods, for making cosmetic masks or varnishes, and for making

pharmaceutical capsules. Heat processing of agro-polymer based materials, using

techniques usually used for synthetic thermoplastic polymers (e.g. extrusion, injec-

tion, molding etc.) is more cost effective. This process is often used for making flexi-

ble films (e.g. films for agricultural applications, packaging films, and cardboard

coatings) or objects (e.g. biodegradable materials) that are sometimes reinforced with

fibers (composite bio-plastics for construction, automobile parts etc.).

The material characteristics (e.g. polysaccharide, protein, polyester or lipid; plasti-

cized or not; chemically modified or not; used alone or in combination) and the fabri-

cation procedures (e.g. casting of a film-forming solution, thermoforming) must

be adapted to each specific food product and the conditions in which it will be used

(relative humidity, temperature).

Agro-polymers for edible and biodegradable films

Edible and biodegradable films must meet a number of specific functional require-

ments (e.g. moisture barrier; solute andlor gas barrier; water or lipid solubility; color

and appearance; mechanical and rheological characteristics; non-toxicity, etc.). These

properties are dependent on the type of material used, its formation, and its applica-

tion. Plasticizers, cross-linking agents, antimicrobials, anti-oxygen agents, texture

agents etc. can be added to enhance the functional properties of the films.

The properties of edible or biodegradable films depend on the type of film-forming

materials used, and especially on their structural cohesion characteristics. Cohesion

depends on the structure of the polymer, its molecular length, geometry, and molecular

weight distribution, and on the type and position of its lateral groups. Film properties are

also linked to the film-forming conditions (e.g. type of process and process parameters).

The properties of amorphous or semi-crystalline materials are seriously modified when

the temperature of the compounds rises above the glass transition temperature (T,).

The glass transition phenomenon separates materials into two domains, according

to clear structural and property differences, thus dictating their processing conditions

and potential applications (temperature and water resistance). Generally, fully arnor-

phous bio-plastic applications are limited by the fact that a polymer's

is highly

affected by the relative humidity (especially for hydrophilic polymers). Below the T,

the material is rigid, and above the T, it becomes visco-elastic or even liquid. Below

this critical threshold, only weak, non-cooperative local vibration and rotation move-

ments are possible. Film relaxation in relation to temperature follows an Arrhenius

time course. Above the T, threshold, strong, cooperative movements of whole mole-

cules and polymer segments can be observed.

Organoleptic properties

Edible films and coatings must have organoleptic properties that are as neutral as pos-

sible (clear, transparent, odorless, tasteless etc.) so as not to be detected when eaten.

Enhancing the surface appearances (e.g. brilliance) and the tactile characteristics (e.g.

reduced stickiness) can be required. Hydrocolloid-based films are generally more

neutral than those formed from lipids or derivatives and waxes, which are often opaque,

slippery, and waxy tasting. It is possible to obtain materials with ideal organoleptic

properties, but they must also be compatible with the food's filling

for example,

sugar coatings, chocolate layers (or chocolate analogs), and starch films for candies,

biscuits, some cakes and icecream products (wafer coatings) etc.

Films and coatings can also help to maintain desirable concentrations of coloring,

flavor, spiciness, acidity, sweetness, saltiness etc. Some commercial films, especially

Japanese pullulan-based films, are available in several colors, or with spices and sea-

sonings included. This procedure could be used to provide nutritional improvement

without destroying the integrity of the food product

for example, by using edible

films and coatings enriched with vitamins and various nutrients.

Innovations

Food Packaging

The optical properties of films depend on the film formulation and fabrication

procedures.

Biodegradability

Biodegradation kinetics are dependent on the type of polymer used (molecular weight,

structure, crystallinity) and on the additives used (e.g. plasticizers, fillers etc.). The

methods used to evaluate biodegradability are generally based on Sturrn's procedure

(Sturm, 1973)

i.e. international standard IS0 14852

which measures the ultimate

aerobic biodegradability of materials. Another approach consists of the evaluation of

the biodegradability in soil.

The agro-polymer based materials are generally fully biodegradable (apart from

when some very severe chemical modifications are applied). They are non-ecotoxic.

Polylactic acid is biodegradable, but at temperatures above its

(>45-55OC)

that

is, in a composting medium.

It is often interesting to adjust biodegradation rates to each desired type of applica-

tion. This can be achieved by applying formulation andlor chemical modifications.

Mechanical properties

Mechanical properties are also dependent on the type of polymer and the additives

used. For many biodegradable polyesters, the tensile strength is often similar to that of

polyethylene or PET, but the elongation is often much lower (without external plasti-

cization). Hydrocolloids, which generally require plasticization (e.g. using low molec-

ular weight hydrophilic molecules such as glycerol or with amphipolar molecules

such as fatty acid derivatives) show lower tensile strength, but their elongation is mainly

dependent on the plasticizer's content. The mechanical properties of various biodegrad-

able, edible or conventional materials are illustrated in Figure 16.1. It is clear from this

figure that biodegradable synthetic and conventional materials have very similar prop-

erties (high tensile strength and high elongation), while microbial bio-plastics and

agricultural bio-plastics are either highly deformable or highly resistant, but not both

simultaneously.

Water vapor transfer

Owing to their relatively low water-vapor barrier properties, agro-polymer based

materials and edible films can only be used as protective barriers to limit moisture

exchange for short-term applications. However, they can be of considerable interest

for numerous applications where very high water vapor permeability is required, such

as in the case of modified atmosphere packaging of fresh, minimally processed or fer-

mented foods (fish, meat, fruits, vegetables, and cheeses). Biodegradable polyesters

(lipids or waxes for edible applications) show better water-vapor barrier properties

compared to starch or protein-based materials, but still have significantly lower water-

vapor barrier properties than most conventional materials. As far as edible applications

Agro-polymers for edible and biodegradable films

269

0 200 400 600 800 1000

Elongation

break

(%)

Figure

16.1

Mechanical properties ofvarious biodegradable, edible and conventional materials

(adapted from Guilbert et a/., 2001).

(a) Syntheticmaterials (closedymbols): thermoplastic polyurethane elastomer (Dow Chemical); Polyvinylchloride;

plasticized with Di-2-ethylheylphtalate; Polypropylene; low-density polyethylene and biodegmdable synthetic

materials (open ymbols): Bak 1095, polyester amide (Bayer, Germany); Ecoflex: 1,4 Butandiol adipinic-

dicarbonic and terephtalate copolyester (Basf, Germany); Eastar 14766: poly (tetramethylene adipate-

coter6phtalate) (Eastman, USA); Bionolle 3000: polybutylene succinateladipate (Showa, Japan).

(b) Biodegmdable materialsfmm micmbialorigin: Biopol: polyhydroxybutyrate (Monsanto, Italy); Lacea:

Polylactic acid (Mitsui, Japan); Ecopla, Polylactic acid (CargillIDow, USA).

starch/polycaprolactone (Novamont, Italy) and edible films: wheat gluten films; molded soy protein isolate

films, alginate based films; chitosan based film; pullulan based films.

are concerned, lipid compounds such as animal and vegetable fats (natural waxes and

derivatives, acetoglycerides, surfactants, etc.) are generally proposed for their excel-

lent moisture-barrier properties, but can cause textural and organoleptical problems

due to oxidation and a waxy taste.

Control of gas exchange

Agro-polymer based materials have impressive gas-barrier properties in

dry

condi-

tions, especially against oxygen. For instance, the oxygen permeability of a wheat

gluten film was

800

times lower than that of low-density polyethylene, and two times

lower than that of polyamide

a well-known high oxygen-barrier polymer. Increasing

the water activity promotes both the gas difisivity (due to the increased mobility in the

hydrophilic macromolecule chains) and the gas solubility (due to the water swelling

in the matrix), leading to a sharp increase in the gas permeability. With carbon dioxide,

a sharp increase in the permeability is more important than with the oxygen per-

meability. The selectivity coefficient between carbon dioxide and oxygen (Gontard

al.,

1996;

Mujica

Paz

and Gontard,

1997)

is very sensitive to moisture and temperature

(for example, fi-om

up to

for gluten films), whereas the selectivity coefficient for

synthetic polymers remains relatively constant, at

44.

This could be explained by the

270

Innovations in Food Packaging

differences in the water solubility of these gases (e.g. carbon dioxide is very soluble), but

also by specific interactions between carbon dioxide and the water-plasticized polymer.

Very high gas selectivity is particularly interesting for modified atmosphere pack-

aging of cheeses (to control the proliferation of the microflora) and fresh fruit and

vegetables (to control the respiration rates). Some selective gluten-based films have

been shown to lead to the production of new modified atmospheres (e.g. low oxygen

and carbon dioxide concentrations at equilibrium) when used to wrap fresh vegetables

(Guilbert

al., 1996, 1997; Barron

al., 200 1).

Modification of surface conditions and controlled

release of additives

"Active" edible or biodegradable films can be applied to foods to modify and control

the atmosphere of food surface conditions (Cuq

al., 1995). Improvement in food

microbial stability can be obtained using edible active layers as surface retention

agents to limit the diffusion of food additives into the food core, or they can be used

in combination with treatments such as refrigeration and a controlled atmosphere.

Maintaining a high concentration of an effective preservative in a local area may allow

a reduction

the total amount of the preservative while sustaining the same effec-

tiveness. Improvement of food microbial stability can also be obtained by reducing the

surface pH. This can be achieved using films that immobilize specific acids or charged

macromolecules (Torres and Karel, 1985). It is important to be able to predict and

control preservative release (Torres

al., 1985; Red1

al., 1996). Microbiological

analysis generally confirms the efficiency of the preservative retention within surface

coatings that are able to increase the shelf life of intermediate moisture foods (Torres

and Karel, 1985; Guilbert, 1988).

Applications

agro-polymer based materials

Applications

biodegradable plastics

Biodegradable plastics can be classified into three categories (Guilbert, 1999):

1. Plastics to be composted or recycled (i.e. where reuse or fine recovery is difficult)

Plastics to be used in the natural environment (i.e. where recovery is not econorni-

cally or practically feasible)

Specialty plastics (with specific features where bio-plastics possess preferential

properties).

Since agro-polymers are relatively costly to produce, actual applications are limited

to special niches with environmental considerations. Loose-fill packaging and com-

post bags are the two major end uses, constituting nearly 90% of the demand

1996

(Bohlmann and Takei, 1998). However, new applications in agriculture and food

packaging are emerging.

the domain of plastics for agriculture, the demand for

Agro-polymers for edible

and

biodegradable

films

271

biodegradable mulching plastics is increasing because biodegradability is a real func-

tional advantage. For food packaging products such as plastic films, nets and small

containers or trays are of concern, even if they are more expensive than the traditional

products. The principal applications of agro-polymers are listed in Table 16.1.

Plastics to be composted or recycled

Loose-fill packaging

Waste and carrier bags

Dishes and cutlery

Hygiene disposables

Miscellaneous short-life goods

Food packaging

Paper or cardboard

Plastia used in natuml environments (no recovery)

Biodegradable/soluble/controlled-release

materials for agriculture and fisheries

Civil engineering, car industry and

construction materials

Disposable leisure goods

Speciality ingredients or materials

Functionalized nanoparticles

Medical goods

Edible films/coatings

O2 barrier, selective 02/C02 barrier,

aroma barrier

Matrix for controlled-release systems

Super-absorbents

Adhesives

Paints

Dyes and pigments

Applications

Shock absorbers

Compost bags

Trays, spoons, cups

Diapers, sanitary napkins, sticks for cotton swabs,

razors, toothbrushes

Pens, tees, toys, gadgets, keyrings

Dried foods, short lifecycle food packaging (e.g.

foast-food packaging, egg boxes

Fresh or minimally processed fruits and vegetables,

dairy products, organically grown products

Accessories or windows for paper envelopes or carc

board packaging, coating for paper or cardboarc

Mulching plastic, films for banana culture, twine,

nursery pots

Materials for controlled-release fertilizers or

agrochemicals

High water-retention materials for planting

Soluble sachets, biodegradable containers for

fertilizers or agrochemicals

Fishing lines and nets

Heat insulators, noise insulators, form wares

Car interior door casings

Retaining walls or bags for mountain areas or sea,

protective sheets and nets for tree planting

Golf tees, miscellaneous goods for marine or

mountain sports

Nanoparticles for rubber reinforcement

Bone fixation, sutures, films, non-woven tissues

Barrier internal layers, surface coatings, "active"

superficial layers

Soluble sachets for instant dry food and beverages,

for food additives

Component of simple or multi-layer packaging

Slow release of fertilizers, agrochemicals,

pharmaceuticals, food additives

Innovations in Food Packaging

Application of edible

films

Edible films and coatings provide an easy means of structurally strengthening certain

foods, reducing particle clustering and improving visual and tactile features on product

surfaces (Cuq

al.,

1995).

Edible films can also be used to package components or

additives that are to be dissolved in hot water or food mixes, and also act as an addi-

tional parameter for improving overall food quality and stability. They represent one

way to apply hurdle technology to solid foods without affecting their structural

integrity (Guilbert,

1986;

Guilbert

al.,

1997).

Many functions of edible films are the

same as those of synthetic packaging, but they must be chosen according to a specific

application and type of food product, and its main deterioration mechanisms.

Films that have a significant gas- and moisture-barrier properties are required for

many applications. For example, they are used to control gas exchange for fresh or

oxidable foods, and to reduce moisture exchange with the external atmosphere, etc.

The retention of specific additives in edible films can lead to a functional response.

This is generally confined to the surface of the product, where they are used to mod-

ify and control "surface conditions7'. Edible coatings can also limit oil and solute pen-

etration into foods during processing. Figure

16.2

schematic representation of a

Food additives (e.g. antioxygen

and antifungic agents)

Diffusion

........

..............

of food additives

...............

(a) Without active layer.

Food additives in

Surface retention

of food additives

Storage

Control

of water transfer

(b)

With active layer.

Figure

16.2

Schematic representation of food preservation with or without edible films and coatings as

active layers, when the first mode of deterioration results from respiration (a), from dehydration or mois-

ture uptake (b), or from surfice microbial development or oxidation (c), adapted from Cuq

etal.

995).

Agro-polymers for edible and biodegradable films

273

food preservation method using agro-polymers as ediblehiodegradable active layers

when the food product undergoes three types of deterioration, due to (a) respiration,

(b)

dehydration or moisture uptake, or (c) surface microbial development or oxidation.

The protective feature of the

film

is dependent on gas- and water-vapor barrier prop-

erties, on surface condition modifications, and on its own antimicrobial properties.

Market

opportunities

Edible films and coating are generally not commercialized as preformed self-supporting

materials, but as pre-mix ingredients. This makes it very difficult to estimate the

demand

the market, especially when this is not significant compared to demand for

other specialty ingredients and additives. Companies in this type of market are usually

suppliers of cellulose and derivatives, gums, food-texturing agents, food-grade sur-

factants, waxes, and lipid derivatives. Recently, lipid materials specifically designed

to be used as edible water barriers have been proposed for commercial use. Most of

them are composed of specifically designed acetoglycerides, either alone or mixed

with waxes.

The market for biodegradable plastic materials is currently expanding, but it

remains a niche market. The total world production capacity of "modern thermoplas-

tic bio-plastics" is approximately 300 000 tons (estimation for 2003). However, the

2003 world consumption is estimated to be only between 70 000 and 90 000 tons.

Eighty-seven percent of biodegradable materials will result from renewable

resources. This market still has great diversity in producers and materials, but attempts

are being made to decrease this variety. The major companies involved are Cargill Dow,

Novamont, Basf, Eastman, Mitsui, and Solvay. These corporations have a double

fimction, acting both as purchasers and as suppliers.

The total target market is 500000-900000 tons in 2005-2007. Today, these film

applications account for about 30% of the sales, and this percentage is increasing,

with estimates in the range of 30-50% by 2010. They are not meant to replace the

traditional plastics. The applications that require biodegradability as a fimctional

characteristic currently account for about 75% of sales, and it is expected that this

proportion will decrease to 60-50% in the future. Currently, the largest market share

for bio-plastics is provided by blends of thermoplastic starch and synthetic biodegrad-

able polyesters, representing about 65% of the total market. They contain

average

starch content of about 60-70%. Novamont is the leader in this market, with the

~aterbi~ products. Polylactic acid is the other major bio-plastic produced commer-

cially at the moment (mainly by Cargill Dow with ~co~la@).

The industrial fhture for biodegradable plastics is still under discussion, owing to

numerous problems. One of the main obstacles to worldwide use of bio-plastics is the

cost of commercially available products (which is similar to that of speciality plastics,

e.g. 1.5 to 10 eurolkg). Bio-plastics are still unknown to the processing industry, and

their performances (e.g. the water sensitivity of agricultural-based plastics) and pro-

cessibility often remain a problem. In terms of the marketing point of view, the major

274

Innovations in

Food

Packaging

hurdles are the absence of well-identified demands, the lack of eligibility of the offer,

the risk in terms of the image, consumer perception, and the complexity of the whole

system. Other major obstacles are the absence of a recovery chain, and of the infia-

structure of nationwide disposal in most developed countries.

New legislations in Western Europe and in Japan, which are in favor of using

biodegradable plastics, has helped to increase the demand, but developing interna-

tional standards for biodegradable polymers is difficult and the competition regarding

the recycling of plastics is still an obstacle.

addition to being biodegradable, the

plastics must now also be non-ecotoxic. However, with the development of a new

range of functionalities and the benefits to the environment, topics that are

line with

international objectives, a dynamic market is predicted.

After a long period of latency, biodegradable plastics have now become credible.

Major polymer manufacturers are entering the market, material costs are quickly

falling, and performances and processibility are improving significantly. Important

niche markets are opening due to consumer demands, which are considered impor-

tant. In addition, new legislations and standards in favor of the use of biodegradable

plastic are helping to encourage market growth. More significant markets are

expected to be located in Western Europe and in Japan.

Among the different categories of biodegradable plastics obtained hm agro-polymers,

the starchlpolyester blends and the "microbial" biodegradable plastics satisfy the

majority of the requirements proposed by plastic packaging industries (material qual-

ities, processibility, performance, etc.). Interest

has

been shown

other bio-plastics based

on natural polysaccharides or proteins, due to their low costs; however, their non-

reproductive quality and their lower performance are still problems.

Edible films and coatings provide an added and at times necessary means to control

physiological, microbiological, and physicochemical changes in food products.

Recently, the concept of "edible active layers" (Cuq

al.,

1995) has been introduced.

These films or coatings contribute to food preservation, for instance by controlling

mass transfer of water vapor, oxygen, carbon dioxide, ethylene, etc., or by modifying

and controlling the food surface conditions (e.g. pH, level of functional agents, slow

release of additives etc.).

Edible and/or biodegradable packages formed with several compounds (composite

or complex materials) have been developed to take advantage of the complementary

functional properties of the different constituent materials, and to overcome their

respective drawbacks. Most composite films studied to date combine one or several

polyester (or lipid) compounds with a hydrocolloid-based one. The future of edible

and/or biodegradable materials is therefore probably dependent on the development

of applications where some of their preferential properties (such as gas selectivity) are

enhanced, or on the development of composite materials.