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Edible

films and

coatings from starches

Zhiqiang

Liu

Introduction

.....

Starch fundamentals

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

Filmformation ..........................................................

321

Mechanical properties ....................................................

328

Oxygen and carbon dioxide barrier

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

330

Summaryandconclusions

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

332

References

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

332

ntroduction

Starch is one of the most abundant natural polysaccharides (Narayan, 1994). The

world starch production in 2000 was 49 million tons, of which over 80% was from

corn starch (LMC International, 2002). Other main agricultural sources for starch pro-

duction include wheat, potato, and cassava (Ellis

al., 1998; LMC International, 2002).

As major ingredients or additives, native or modified starches have been playing

important roles in the food industry. Nutritionally, starches supply more than 50% of

the calories consumed by the human population (Whistler, 1984).

addition,

starches and their derivatives have been used to modify physical properties of food

products such as soups, sauces, snacks, batters, and meat products (Thomas andAtwel1,

1997), contributing mainly to texture, viscosity, gel formation, adhesion, binding, mois-

ture retention, product homogeneity, and film formation (Thomas and Atwell, 1997).

Starch films are often transparent (Wolff

al., 1951; Lourdin

al., 1997a;

MyllZirinen

al., 2002a) or translucent (Rindlav

al., 1997), odorless, tasteless, and

colorless (Wolff

al., 195 1). They have been utilized in the packaging and coating of

food products, because of their edibility and low permeability to oxygen (Mark

al.,

1966; Roth and Mehltretter, 1970). Compared to non-starch edible films described in

this book, starch films are usually cost effective. The overall performance of starch

films and coatings is highly likely to be customizable, because of the availability of a

wide variety of starches and their capacity for physical andlor chemical modifications

(Ellis

al., 1998; Liu, 2002). In fact, edible starch films are perhaps the most

Innovations in Food Packaging

ISBN: 0-12-3 11632-5

2005 Elsevier Ltd

All

rights of reproduction in

any

form reserved

Edible films and coatings from starches

319

effective forms of soluble packaging in terms of performance, adaptability to prod-

ucts, and production operations and costs (Daniels, 1973). Edible starch films and

coatings are commonly used in bakery, confectionery, batters, and meat products

(Thomas and Atwell, 1997). A series of starch-based film formers has been commer-

cialized by National Starch and Chemical Company under the brand of CRYSTAL,

which can form thin, clear, and non-tacky films over bakery glazes, and reduce mois-

ture migration into icings (National Starch and Chemical Company, 2003). High-

amylose starch films are potential coatings for oxygen-sensitive foods under normal

and refrigerator storage (Senti and Dimler, 1959).

This chapter is dedicated to edible films and coatings from starches, with emphasis

on the film formation process and the relationship between the structure and proper-

ties of self-support starch films.

Starch

fundamentals

Starch is normally a mixture of amylose and amylopectin polymers. Amylose is a nearly

linear polymer of a-1,4 anhydroglucose units, with a molecular weight of lo5-lo6

(Galliard and Bowler, 1987; Durrani and Donald, 1995). Amylopectin is a highly

branched polymer consisting of short a-1,4 chains linked by a-1,6 glucosidic branching

points occurring every 25-30 glucose units, with a molecular weight of lo7-lo9

(Gaillard and Bowler, 1987; Durrani and Donald, 1995). The content of amylose in starch

varies from 0 to loo%, depending on the botanic origins (Shannon and Garwood, 1984;

Yoshimoto

al., 2000; Gerard

al., 2001). Most starches, such as those from wheat,

corn and potato, contain 2&30% amylose. However, the amylose content can be as

high

as 50-80% in high-amylose starches such as amylomaize and HylonVII, or as low as less

than

5% in waxy starches such as waxy corn, sorghum and rice.

addition, native cereal

starches often contain a small amount (<2%) of non-carbohydrate components such as

lipids, phosphates, and proteins (Galliard and Bowler, 1987; van Soest, 1996).

Native starches are in the form of granules, in which amylose and amylopectin are

structured by hydrogen-bonding in a manner of alternative semi-crystalline and amor-

phous rings (Cameron and Donald, 1992; Jenkins

al., 1993). Due to the orderly

arrangement of the crystalline areas, starch granules are birefringent (French, 1984;

Paris

al., 1999), showing an interference pattern of the Maltese cross under polar-

ized light. Since amylopectin accounts for the crystallinity in native starch granules

(Zobel, 1988; Jenkins

al., 1993), higher crystallinity is expected for starches con-

taining more amylopectin (Cheetharn and Tao, 1998). Basically, starch granules are

semicrystalline. The degree of crystallinity even for waxy corn starch (almost 100%

amylopectin) is lower than 50% (Table 19.1).

In general, native starch granules exhibit three X-ray diffraction

(XRD)

patterns: A,

B and C (Figure 19.1). In the A-type crystal, the double helices pack in an anti-parallel

manner into an orthorhombic unit-cell (Figure 19.2a), resulting in nearly hexagonal

close-packing (Wu and Sarko, 1978a). In the B-type crystal, the double helices pack

also in an anti-parallel manner but into a hexagonal unit cell with two helices per cell

320

Innovations in Food Packaging

Starch

Polymorph

(%)

Com/maize

Waxy (Amioca)'

Regulara

Amylomaize

Amylomaize Vlia

Pea (r~~4)~

~a~ioca~

potatob

~hea$'

X,,

degree of crystallinity;

TQI,

gelatinization temperature; 'Mani and Bhattacharya (2001); b~llis

eta/.

(1 998); 'Bogracheva

eta1

(1 999); d~igh amylose starches are not completely gelatinized in boiling

water (Murphy, 2000).

Diffraction angle

Figure

19.1

X-ray diffraction spectrum for A-type (corn), B-type (potato) and C-type (pea) starch.

Reprinted from Bogracheva

et a/.

999), with permission from Elsevier.

(Figure 19.2b), leaving an open channel that is filled with water molecules (Wu

and

Sarko, 1978b). Although the B-type crystal has less denser packing than the A-type

(Figure 19.2), the similarity in their packing modes suggest that A- and B-type crys-

tals are inter-convertible

(Wu

and Sarko, 1978a). Regular cereal (e.g. wheat, corn)

starches show the A-type polymorph, whereas root (e.g. potato) and high-amylose

(>50%) starches show the B-type polymorph (Table 19.1). The C-type polymorph

(Figure 19. l), which is actually a mixture of A- and B-type polyrnorphs (Bogracheva

al., 1998,2002), is usually found with low-amylose pea starches (Table 19.1). Some

starch granules show a trace of V-type polymorph, attributed to complexes of amylose

with lipids (Fukui and Nikuni, 1969). The V-type crystal resembles a single amylose

helix with small lipid molecules aligned with the helical center (Bulkon

al., 1998).

Edible films and coatings from starches

321

19.2

Packing ofstarch double helices. (a) A-type and (b) &type polymorphs. Each circle

represents

view down the z-axis of

double helix. Reprinted from Bulkon

etal.

998), with permission

from Elswier.

Film

formation

Starch

solution

Preparing a clear starch solution is normally the first step when making edible starch

films. While it is important to minimize or avoid hydrolysis and oxidation of starch, a

solution-making process must ensure that starch polymers are completely gelatinized,

disintegrated, and solubilized (Protrman

al., 1967; Bader and Goritz, 1994a; Lourdin

al., 1995, 1997b).

It is important to prepare starch solutions with a proper solid concentration,

order to insure both the continuity of films and the ease of casting. When the solid

concentration is very low (e.g. <4%), the solution of completely solubilized starch

polymers hardly gels, causing problems in forming a continuous

film

of sufficient thick-

ness (Protzman

al., 1967). However, when the concentration is very high (e.g. >30%),

the solution is too viscous to be cast evenly (Protzman

al., 1967).

solid concen-

tration of 10-15% is therefore suggested for casting conventional starch films.

To completely dissolve high-amylose (>50%) starches in water it requires elevated

temperatures

120°C) and super-atmospheric pressures, which can be fulfilled by

autoclaving. Bader and Goritz (1994a) found that 155°C is the lowest possible tem-

perature at which high-amylose (55%) starch granules will have completely disinte-

grated so as to obtain a homogeneous solution. Even higher temperatures are needed

for fractionated or retrograded amylose (Muetgeert

al., 1962; Sarko

al., 1963).

Although low-amylose (<40%) starches, including waxy starches and fractionated

amylopectin, are literally soluble in hot water (>50°C) (Daniels, 1973; Ritzl

al.,

1998), high temperatures and super-atmospheric pressures are still necessary to obtain

nnovations roo+ D-ickaginy

a clear starch solution (Lourdin

al., 1997a, 1997b). To minimize or avoid thermal

and oxidative degradation of starch in water at high temperatures, it is often necessary

to heat up the solution rapidly (e.g. 2.5"CImin) (Bader and Goritz, 1994a) and keep

the solubilization process under the protection of nitrogen atmosphere (Lourdin

al.,

1997a, 1997b).

Complete solubilization of high-amylose starches in water at relatively low tem-

peratures can be facilitated by using amylose-complexing agents, which have been

utilized to assist isolating amylose from native starches (Wolff

al., 1952). Normally,

amylose is readily dissolvable in hot water (e.g. at 95OC) saturated with a complexing

agent (Wolff

al., 195 1,1952; Davis

al., 1953; Lourdin

al., 1995), which can be

low molecular weight aliphatic alcohols (e.g. ethanol, propanol, butanol, pentanol),

glycol ethers (e.g. diethyl Cellosolve), or organic bases (e.g. pyridine) (Davis

al.,

1953). Among those agents, butanol is the most commonly used in making amylose

films. It is recommended that the complexing agent should be driven off prior to film

casting (Wolff

al., 1952; Lourdin

al., 1995), otherwise the film may appear spot-

ted (Wolff

al., 195 1).

Aqueous starch solutions are normally unstable. In particular, those with a high

solid concentration andlor high-amylose starch tend to gel immediately upon cooling

(Young, 1984), which makes casting more difficult (Daniels, 1973). It is therefore

necessary to keep the starch solutions at a temperature above their gelation tempera-

tures prior to casting (Davis

al., 1953). For amylose solutions, the gelation temper-

ature is basically a linear function of the concentration of amylose (Muetgeert

al.,

1962). When the amylose solution has a solid concentration of 1&15%, the gelation

temperature is 6&74"C (Muetgeert

al., 1962). To be on the safe side, the amylose

solutions should be kept at about 80°C before casting.

Film

formation

Self-supporting starch films are commonly produced by casting the starch solution

upon a support with a smooth surface, on which it is dried until the film can be taken

off the support (Wolff

al., 195 1

;

Daniels, 1973). Instead of a static support, a rotat-

ing

drum

or a moving belt can be used for the continuous production of starch films.

Sometimes, applying a release agent (e.g. lecithin) to the surfaces is necessary to

make the film-stripping process easier (Protzman

al., 1967).

Having been cast onto a surface at room temperature, the aqueous solution of high-

amylose starch or amylose starts gelling upon cooling, due to the association of glu-

cose chains (Protzman

al., 1967; Wu and Sarko, 1978a, 1978b; Leloup

al., 1992).

The concurrent evaporation of water leads to the concentration of the solution and

accelerates the gelling process. When all possible chain associations have been com-

pleted, further evaporation of water reduces the bulk of the gel until the majority of the

free water is removed. Often, the surface upon which the solution is cast serves as a

sufficient restraint to prevent the gel or film from shrinking (Protzman

al., 1967).

Structurally, starch films resemble condensed starch gels. For example, amylose films

show an open network composed of nanoscale rod-like strands (Unbehend and Sarko,

Edible

Glms

and coatings

fro^

=parches

323

1974; Rindlav-Westling

al., 1998), which presumably consist of contiguously asso-

ciated blocks of amylose double helices (Leloup

al., 1992).

Drying is profoundly crucial to the appearance and performance of starch films

(Davis

al., 1953; Protzman

al., 1967). Rapid drying is normally undesirable,

because it can induce cracks, warping, and concentric

drying

marks on the films (Davis

al., 1953), or even granulate the films (Sarko

al., 1963; Germino

al., 1964;

Unbehend and Sarko, 1974). Over-drying should also be avoided to prevent the films

from becoming too brittle (Protzman

al., 1967). Film thickness is roughly controlled

by the solid concentration and the amount of solution cast on the support (Wolff

al.,

1951). For example, amylose films with thickness ranging from 11 to 170 pm can be

prepared by varying the concentration of the casting solution from 7 to 15%, and the

clearance of the casting blade from 380 to 1270 pm to the support (Wolff

al., 1951).

Structures in starch films

Similar to starch granules, starch films are often semicrystalline, containing both

amorphous and crystalline phases. The amorphous and crystalline phases are charac-

terized by the glass transition temperature and the degree of crystallinity, respectively,

both of which will affect the major properties of starch films such as mechanical and

gas barrier properties.

Amorphous structure and glass transition temperature

Upon complete gelatinization and solubilization, native starch loses its crystallinity,

which can rarely be restored upon immediate

drying

(Ritzl

al., 1998) because the ret-

rogradation or recrystallization of starch is often much slower

than

the

rate

of drylng

(Kalichevsky

al., 1992). Therefore, freshly prepared starch films are basically amor-

phous (Rindlav

al., 1997; Rindlav-Westling

al., 1998; Garcia

al., 2000; Myllkinen

al., 2002a), even though some fresh amylose films may contain a little crystallinity

10%) (Mylliirinen

al., 2002a). Because of its high molecular weight and large num-

ber of branches, amylopectin retrogrades or recrystallizes more slowly

than

amylose

(Goodfellow and Wilson, 1990), and its major presence in starch could interfere with the

recrystallization of amylose. Even though aged starch films are semicrystalline, the

majority (>70%) is still amorphous (Rindlav

al., 1997; Rindlav-Westling

al., 1998).

Glass transition is by far the most important transition among the many transitions

observed in amorphous and semicrystalline polymers, and has a drastic effect on the

performance of such polymers (Bicerano, 2003). Glass transition temperature

(T&

defined as the temperature at which the forces holding the principle components (e.g.

amylose and/or amylopectin) of an amorphous solid together are overcome, so that

these components are able to undergo large-scale molecular motions (Bicerano, 2003).

The glass transition temperature of starch films is commonly measured by differential

scanning calorimetry (DSC) (van Soest, 1996; Bizot

al., 1997;

Lourdin

al., 1997a,

1997b) and dynamic thermal mechanical analysis (DMTA) (Lourdin

al., 1997a;

Gaudin

al., 1999,2000; Moates

al., 2001). Even though amylose and amylopectin

324

Innovations

Food Packaging

Table

192

Glass

transition

~anp~~ac

and

coefkht

A5,

tha

Cauchmuc-Kuuz

squdon

Pea arnylose (cast 100°C)a 605

Potato starch (cast 90°C)' 589

Waxy maize starch (cast 90°C)' 558

~l~cerol~ 187

watep 134

Sodium lactateb 246

Sorbitolb 271

Bizot

a/.

(1 997);

our din

a/.

(1 997c).

are incompatible in aqueous solutions (Miles

al., 1985; Kalichevsky and Ring,

1987), their blends apparently behave compatibly, with a single calorimetric glass

transition spanning a similar range on their DSC curves (Bizot

al., 1997).

The

of completely anhydrous starch cannot be directly measured by DSC, because

the material tends to degrade before the occurrence of the glass transition. However,

it can be obtained by extrapolation on a plot of

versus

water

content for a starch-water

system (Bizot

al., 1997). In general, low molecular weight polymers show low

Tg.

For example, the

(410 K) of lintnerized potato starch (M,

2.3

lo3) is lower

than

that (605 K) of pea amylose (M, 5.9

1 05) (Table 19.2).

The amorphous phase of starch, because of its less ordered structure, is more prone

to attack by small molecules such as water and other plasticizers like glycerol. These

small molecules tend to interact through hydrogen bonding with glucose chains,

thereby loosening up the very strong inter- and intra-molecular interactions (also through

hydrogen bonding) of starch polymers, and apparently facilitating the large-scale motion

of glucose chains. In other words, the presence of plasticizers depresses the

of starch

(Biliaderis, 1991). As the content of plasticizer increases, the

of starch decreases.

As a function of water content, the

for a starch-water binary system can be esti-

mated by using the Gordon and Taylor (1952) equation

where m, is the water content,

and

Tg,

are the glass transition temperatures of

water and starch, respectively, and

is a constant. The glass transition temperature for

a multi-component system (e.g. starch-water-glycerol) can be predicted using the

Couchman-Karasz equation (Couchman, 1978,1987):

where mi,

Tgi,

and ACpi are the mass fraction, the glass transition temperature, and the

heat capacity change at

Tgi

of the ith component, respectively. For a starch-water sys-

tem, equation (19.2) is equivalent to equation (19.1) with

ACp JACp,. Table 19.2

lists the values of

and ACp for some starches and plasticizers. The glass transition