Jung Han. Innovations in Food Packaging

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Plasticizers in edible films and coatings

405

Common plasticizers for edible films and coatings are monosaccharides, oligosac-

charides, polyols, lipids, and derivatives (Guilbert, 1986; Baldwin

al., 1997). Water

is also an important plasticizer for edible

films

and coatings. Water content is dependent

on the polymer and external plasticizer selected.

Some plasticizer types and chemical structures are shown in Table 23.1. Most plas-

ticizers contain hydroxyl groups which will form hydrogen bonds with biopolymers,

and thus increase the free volume and flexibility of the film matrix. Various plasticizers

consist of different numbers of hydroxyl groups and have different physical states

(solid or liquid); thus they show differences in degree of softening film stiffness.

Water is the smallest molecular weight (M,

18), and it is well known as an excellent

plasticizer. However, most films still need an additional plasticizer to obtain desired

film flexibility. Moisture sorption of various plasticizers plays an important role in

affecting different film properties, as will be discussed later.

Other plasticizers besides those shown in Table 23.1 can be used; for example, a

variety of M, of polyethylene glycol (PEG). These include PEG 300 (Gioia

al.,

1998), PEG 600 (Lieberman, 1973; Guo, 1993; Gioia

al., 1998), PEG 1450

(Donhowe and Fennema, 1993; Turhan

al., 2001), PEG 1500 (Heinamaki

al.,

1994), PEG 4000 (Guo, 1993; Heinamaki

al., 1994; Turhan

al., 2001), PEG 8000

(Donhowe and Fennema, 1993; Guo, 1993; Turhan

al., 2001), and PEG 20000

(Donhowe and Fennema, 1993), depending on the biopolymer materials used.

Combinations of a variety of plasticizers have also been used to obtain averaged prop-

erties (Cherian

al., 1995; Cao and Chang, 2002).

Lipid plasticizers such as fatty acids and derivatives, lecithin, oils, and waxes are

also commonly used in edible coatings. Generally, the purpose of adding lipid is to

reduce film water vapor permeability, since lipids are non-polar nature or hydropho-

bic, and thus provide a good barrier against moisture migration. Moreover, lipids can

provide gloss and enhance the visual appearance of food products. However, lipids

exhibit poor mechanical properties because of their lack of cohesive structural integrity

(Gontard

al., 1995). Therefore, incorporating lipid in protein- or polysaccharide-

based films may produce a plasticizing effect, including reduction of film strength and

increase of film flexibility, as seen in milk protein (Shellhammer and Krochta, 1997)

and wheat gluten films (Gontard

al., 1994). Common fatty acids used are shown in

Table 23.2.

Oleic acid has been used as a plasticizer in zein films to provide modified atmos-

phere packaging for fiesh broccoli (Rakotonirainy

al., 2001). PaImitic acid-plasticized

methylcellulose films (Kester and Fennema, 1989; Rico-Pena and Torres, 199 1;

J. W. Park

al., 1994), lauric acid- and stearic-palmitic acid-plasticized laminated

methyl cellulose/corn zein films (J. W. Park

al., 1994) have been studied. In addi-

tion, fatty acids from CI4 to C22 (myristic, palmitic, stearic, arachidic, and behenic

acids) have been studied as plasticizers in whey protein isolate (WPI) films (Sherwin

al., 1998). Stearic acid, palmitic acid, myristic acid, lauric acid, stearyl alcohol,

hexadecanol, tetradecanol, and beeswax have been used as plasticizers in WPI-lipid

emulsion films (McHugh and Krochta, 1994a). Carnauba wax, candelilla wax, beeswax,

and a hard milkfat fraction have been found to plasticize WPI-glycerol films

(Shellhammer and Krochta, 1997). Generally, increasing the chain length of fatty

$06

Innovations in Food Packaging

rable

23.1

Plasticizer types, chemical structure and applications for edible films and coatings

Plasticizer

typ

zhemical structure

Reference*

Mannitol

PropyIene glycol

C3Hs02

Glycerol

Sorbitol

Polyethylene

H(OCH2-CH2),0H-200

glycol

200

Sucrose

Polyethylene

H(OCH2-CH2)80H-400 1,7,8,11,

glycol

400

14,15,16

Triethylene glycol

C6Hl4O4

150

Ethylene glycol

C~~602

(Continued)

Plasticizers

edible

films

and

coatings

407

OAc

AcO-CH2-CH-CH,-OAc

Urea

Diethanolamine C4HllN02

105

Dibutyl phthalate C16H2204

278

Glycerol C1sH2606

tributyrate 302

Tributyl citrate

C18H3207

Plasticizer

type

Chemical

structure*

1.4 butaned~ol

C4H~002

HO-(CH2)4-OH

1,6 hexaned~ol C8H1402-200

by+$:

@$2

r~k*etlti

"bfi,,?

!?%a

21 8

Water H2O

Glucose CsHlzOs

n-BuO-C-CH,-C-CH,-C-OBu-n

$-OBu-n

EtO

/,&oEt

*Some obtalned from the SCI Flnder.

**Appllcatlon of plastlclzer used In ed~ble films and coatlngs as seen In these references.

1, Sothornvlt and Krochta, 2001; 2, Lln and Krochta, 2003; 3, Hong and Krochta, 2003; 4, Shaw

etal.,

2002;

Klm

etal.,

2002,

6, Klm and Ustunol, 2001a, 2001 b; 7, Turhan

etal.,

2001,8, Lee

etal.,

2002; 9, Cao and Chang, 2002, 10, Chlck and Ustunol, 1998;

11, Helnamak~

etal

1994; 12, heberman, 1973; 13, Park and Chlnnan, 1995, 14, Donhowe and Fennema, 1993, 15, Park

etal.,

1993;

16, Johnson

etal.,

1991

;

17, Glola

etal.,

1998, 18, Cherlan

etal.,

1995; 19, McHugh and Krochta, 1994b, 20, Ben~ta

a/.,

1986;

21, Beck and Tomka, 1996.

Diethyl tartrate C8H140s

206

408

Innovations

Food Packaging

Table

23.2

Fatty acids used in edible films and coatings

Lpeof

Chemical

structure

ttty

acid

auric

C12H2402

cid 200

Reference*

tearic

C18~36~2

COOH

1,2,6

cid 284

Ialmitic

C16H3202

COOH

1,3,4,6

cid 256

.inoleic

C18H3202

COOH

.cid 280

.inolenic

C18H3002

COOH

.cid 278

nyristic

C14ti2802

COOH

rcid 228

Neic

C~8H3402

COOH

~cid 282

Arachidic

CZOH4002

COOH

acid 312

*Application of fatty acids used in edible films and coatings as seen in these references:

Park

etal.,

1994; 2, Sherwin

etal.,

1998; 3, Rico-Pena and Torres, 1991; 4, Kester and Fennema, 1989;

Tanaka

etal.,

2001;

Rakotonirainy

etal.,

2001.

acids decreases the

WVP

of laminated MCIcorn zein-fatty acid films

(J.

Park

al.,

1994) and WPI-lipid emulsion films (McHugh and Krochta, 1994a).

Polymer molecular weight and plasticization

Another potential approach to increasing film flexibility is to reduce the polymer

molecular weight, thus reducing intermolecular forces along polymer chains and

increasing polymer chain end groups and polymer free volume (Sears and Darby,

1982). Therefore, polymer molecular weight (M,) can play a plasticization role for

edible films and coatings. Park

al. (1993) found that film oxygen permeability (OP),

water vapor permeability

(WVP),

and tensile strength (TS) increased as methylcellu-

lose (MC) and hydroxypropyl cellulose

(I-IPC)

M, increased. However, film

WVP

decreased with increasing hydroxypropyl methyl cellulose (HPMC) and

(Ayranci

al., 1997). The differences may be attributed to the more hydrophobic

Plasticizers in edible films and coatings

409

behavior of the methyl group in HPMC compared to the HPC (Ayranci

al., 1997).

Reducing whey protein isolate (WPI) Mw also showed no significant effect on film

WVP and OP (Sothornvit and Krochta, 2000), but the hydrolyzed WPI showed a

significant plasticization effect by reducing film EM and TS compared to unhy-

drolyzed WPI (Sothornvit and Krochta, 2000). Chitosan Mw effect showed the same

trend as film TS and

WVP

as hydrolyzed WPI (S. Y. Park

al., 2002). Generally, stud-

ies have found that lowering the polymer Mw increased film flexibility without chang-

ing film

WVP.

External plasticizer requirements

Besides cost, the selection of a plasticizer requires consideration of three basic criteria:

compatibility, efficiency, and permanence.

Compatibility

It is necessary to use a plasticizer that is compatible with the intended polymer.

Compatibility depends on polarity, structural configuration (shape), and size (M,) of

plasticizer. Good compatibility results from the plasticizer and polymer having a similar

chemical structure (Immergut and Mark, 1965; Banker, 1966; Wilson, 1995). Therefore,

different polymers require different plasticizers (Sears and Darby, 1982). In addition,

plasticizers should have low volatility, as well as being non-toxic and aroma free.

Efficiency

Generally, good plasticizers provide

high

plasticization at low concentration and exhibit

rapid polymer diffusion and interaction. The "plasticizer efficiency" is defined as the

quantity of plasticizer required to produce the desired film mechanical properties. One

method to define the efficiency is the lowering of T, at a given amount or volume frac-

tion of plasticizer. There is no exact number to indicate the efficiency of each plasti-

cizer, because it depends on the polymer properties. Not only the size (M,) but also

the rate of plasticizer diffusion into the polymer matrix is another important factor in

defining plasticizer efficiency (Immergut and Mark, 1965). A higher diffusion rate

results in higher plasticizer efficiency. Small molecules have high diffusion rates, but

possess higher volatility and therefore it is easier to lose the plasticizer in the process.

Permanence

The permanence of plasticizers in polymers depends on the size of the plasticizer mol-

ecule and on the rate of diffusion in polymers. Larger plasticizer molecules possess

lower volatility, resulting in greater permanence. Moreover, polarity and hydrogen

bond capability will influence the volatility of plasticizers. Greater plasticizer effi-

ciency, defined by rapid diffusion into the polymer matrix, may also result in lower

plasticizer permanence due to diffusion out of the polymer matrix. Thus it is often nec-

essary to compromise in selecting which type of plasticizer is more suitable for each

polymer material (Immergut and Mark, 1965; Banker, 1966).

410

Innovations in Food

Packaging

The most effective plasticizers show the following properties (Sears and Darby, 1982):

smaller plasticizer (lower

M,)

is more effective in lowering film T,

2. Plasticizer efficiency is proportional to the

of plasticized films; this means that

small amounts of plasticizer are the most effective and larger amounts have less effect

Lesser affinity between plasticizer and polymer compared to the affinity between

polymer and polymer gives more efficient plasticization

in other words, a good

plasticizer is a bad solvent

Lesser affinity between plasticizer and plasticizer compared to the affinity between

polymer and polymer gives more efficient plasticization, thus low-viscosity plasti-

cizers are more efficient.

Theories

plasticization

There are four theories to explain plasticizing effects. The earliest are the free volume

theory, which involves the intermolecular spaces in polymer, and the coiled spring theory,

which deals with tangled macromolecules (Sears and Darby, 1982). The gel theory

and the lubricity theory were later theories introduced to explain plasticization. The

gel theory explores the rigidity in an unplasticized gel formed by weak polymer-

polymer interactions along the polymer chains (Doolittle, 1965; Sears and Darby,

1982). The lubricity theory considers the plasticizer to act as a lubricant to facilitate the

movement of polymer chains over each other, consequently lowering resistance to

deformation (Doolittle, 1965; Sears and Darby, 1982). These two theories account for

different portions of the total phenomenon of plasticization and the deformation of

plasticized polymers. For edible films, the most useful concepts are the gel theory and

the free volume theory. Thus, these two theories will be considered in greater detail.

Gel theory

The gel theory in food hydrocolloids pertains to an internal, three-dimensional rigid

structure of polymer. Plasticizer molecules attach along the polymer chains, replacing

polymer-polymer attachments at places and hindering the forces holding polymer

chains together (Van der Waals, London, Debye, hydrogen bonding, crystal, or primary

valence forces). This reduces the rigidity of the gel structure, resulting in increased

gel flexibility. In addition, plasticizer molecules that are not attached to polymer form

aggregated plasticizer domains that facilitate the movement of polymer molecules.

This also enhances the gel flexibility.

Free volume theory

The free volume of amorphous materials or polymers is the volume unoccupied by the

material's molecules (Sears and Darby, 1982). Another definition is the volume dif-

ference between the temperature at absolute zero and the temperature of interest. Free

volume can be expressed as Vf

V,, where Vf is the free volume, V, is the

Plasticizers in edible films and coatings

411

specific volume at temperature T, and

is the specific volume at reference tempera-

ture (e.g. absolute zero).

Approaches to increasing the free volume of polymer system could include:

1. Using low

polymer, or reducing the

of polymer, to increase the number of

polymer end groups

Increasing the length of side chains, thus increasing steric hindrance and lowering

chain intermolecular forces to increase polymer chain motion, related to internal

plasticization

Using low

plasticizers that are compatible with the polymer molecules to

increase the motion of chain ends, side chains, and main chain, related to external

plasticization

Increasing temperature.

Free volume theory is used to describe many things, such as plasticizing action, T,,

viscosity, cross-linking, diffusion,

film

drylng and

film

properties (Wicks, 1986; Duda

and Zielinski, 1996).

Plasticization models

Plasticization models are used to predict the effect of components on the T, of food

systems, in order to control or estimate the properties of food materials (Roos, 1995).

Gordon and Taylor equation

The Gordon and Taylor equation can be applied to predict plasticization of food com-

ponents such as carbohydrates and proteins, as well as of pharmaceutical materials.

This equation requires an empirical constant

(k)

from experimental data for T, at dif-

ferent plasticizer contents. For water acting as a plasticizer, the equation covers the

range of

&50%

water content.

Thus, the constant k can be obtained from the following equation:

where wl and w2 are the weight fractions of solid component and plasticizer, respec-

tively. Tg, Tgl and TgZ are the glass transition temperatures of the system, solid and

plasticizer, respectively.

If the k and Tg of the system are known, the weight fraction of solid (wl) can be

obtained with the following equation:

412

Innovations in Food Packaging

Application of the Gordon and Taylor equation is only possible for a binary system.

A related empirical equation is the Kwei equation, which has another term, qwlw2:

where q is another constant.

Couchrnan and Karasz equation

The Couchman and Karasz equation is based on the heat capacity changes at the T, of

a binary system. This equation is equivalent to the Gordon and Taylor equation if

ACp2/ACpl, where Cpl is the heat capacity of polymer and Cp2 is the heat capacity

of the plasticizer.

This equation requires knowing ACpl and ACp2 at Tgl and Tp, respectively, from DSC.

In biological material such as proteins and starch, it is difficult to determine ACp of

the anhydrous polymer, due to thermal decomposition. Furthermore, the wide range

of T, is another problem

determining ACp. AC,

trends

to decrease when water content

decreases. The usefulness of this equation depends upon obtaining the T, for many

biopolymers.

Couchrnan and Karasz equation

exact

form

The equation below is the exact form of the Couchman and Karasz equation:

This equation is the same as Couchman and Karasz equation when In (1

It is important to be aware that sources of error come from the determination of

ACp at T,. The main error in using the simplified form of the Couchman and Karasz

equation is the assumption that

Other equations

Other equations that are related to T, but do not apply for water plasticization are:

The Fox equation, which is used in predicting Tg of binary blends of certain

materials:

Plasticizers

edible films and coatings

413

2. The Pochan-Beatty-Hinrnan equation, which is used in predicting the composition

dependence of T,:

The linear equation, which is the simplification of the Pochan-Beatty-Hinman

equation:

4. The Huang equation, which is based on the Couchman and Karasz equation and

gives somewhat higher prediction of T, than the Gordon and Taylor equation at

intermediate water content:

Plasticization and

The effect of plasticizer on biopolymer film Tg is difficult to detect, because biopoly-

mer chain mobility is much less than that of synthetic polymers (Mitchell, 1998).

Furthermore, Tg determination is complicated by crystallization and enthalpic relaxation

(Sichina, 2000), as well as chain-to-chain interactions such as hydrogen bonding,

hydrophobic interactions, ionic interactions, and disulfide bonds (Mitchell, 1998).

Nevertheless, it is suggested that T, be determined from a distinct transition in the

motion of the backbone, rather than smaller transitions (Seyler, 1994).

Water is an excellent plasticizer for biopolymers, shown by its effective lowering of

T, with increasing water content. Plasticization occurs in the higher molecular-mobility

amorphous region, and the ability of plasticizers to interrupt hydrogen bonding along

the protein chains depends on the plasticizer type and amount. Generally, increasing

the amount of plasticizer decreases the film T,.

However, the plasticizer type, but not the plasticizer amount, significantly affected

the film Tg in both P-lactoglobulin (Sothornvit

al., 2002) and methylcellulose

(Debeaufort and Voilley, 1997). Sothornvit and Krochta (2001) explained the lack of

change in film Tg with increasing plasticizer content or water content by a mechanism

of external plasticization involving

excess of plasticizer that produced domains of

plasticizer aggregates. Another explanation is that these plasticizers acted as a lubri-

cant between molecules in these systems (Debeaufort and Voilley, 1997).

effective plasticizer needs to be compatible with and remain permanent when com-

bined with the polymer. Thermal transitions for PEG 400-plasticized P-lactoglobulin

(P-Lg) films could not be observed, because crystallizing and melting peaks of water

occurred during heating (Sothornvit

al., 2002), similar to the PEG 400-plasticized

methylcellulose (MC) films, which showed two water melting peaks (Debeaufort and

Voilley, 1997). Using a combination of immiscible plasticizers showed two transitions

in wheat gluten films (Cherian

al., 1995).

414

Innovations

Food Packaging

The physical state of the plasticizer is another important factor that affects film T,.

For example, sorbitol (Sor)- and sucrose (Suc)-plasticized P-Lg films possessed higher

film T, than glycerol (G1y)-, PEG 200- and propylene glycol (PG)-plasticized films,

presumably because the solid state of Sor and Suc reduced their efficiency in disrupting

hydrogen bonding along protein chains compared to the liquid state plasticizers (Gly,

PEG 200 and PG) (Sothornvit

al., 2002). Small molecular size

(M,)

plasticizers

also appear to be more effective in reducing film T,. Furthermore, the diverse hygro-

scopicity of plasticizers affects film Tg differently. Sor is less hygroscopic and exhibits

less efficient plasticization compared to Gly. The last factor is plasticizer shape, such

as straight vs ring structure. Presumably, the bulky ring structure of Suc resulted in

higher T, than PEG 200 in P-Lg films (Sothornvit

al., 2002).

In summary, the effect of various plasticizers on film T, is related to film flexibility

and other properties. Thus, film T, is quite important for applications of edible films

and coatings.

Advantages and disadvantages

edible

films

and coatings

Most edible films and coatings are quite brittle due to extensive intermolecular forces

such as hydrogen bonding, electrostatic forces, hydrophobic bonding, and disulfide

bonding. Plasticizers are required to interrupt intermolecular forces and lower the T,,

resulting in greater flexibility and toughness. Unfortunately, plasticizers increase film

permeability to moisture, oxygen, aroma, and oils. Therefore, there are advantages

and disadvantages of using plasticizers in edible films and coatings.

Plasticizers do not only produce plasticization. They can also produce antiplasti-

cization. Plasticization is known to involve the reduction of intermolecular forces

between polymer chains, thus improving film and coating flexibility. On the other

hand, antiplasticization can occur at lower plasticizer concentrations and tempera-

tures. Antiplasticization in films has been attributed to several mechanisms, such as

reduction of polymer free volume, interaction between the polymer and plasticizer,

and film stiffness due to the presence of rigid plasticizer molecules adjacent to the

polar groups of the polymer (Guo, 1993; Seow

al.,

1999). In addition, adding a low

concentration of plasticizer can lead to an increase in polymer crystallinity, due to

lowering the energy barrier for a change of polymer state and thus enhancing the forma-

tion of ordered structures within amorphous regions (Wilson, 1995). Antiplasticization

decreases edible film and coating

WVP,

and increases the elastic modulus and brittle-

ness, which is the opposite behavior to plasticization. Water has been shown to have both

plasticizer and antiplasticizer effects on tapioca starch film properties over the low to

intermediate moisture range (Chang

al.,

2000). Moreover, triacetin, PEG 600, PEG

4000, and PEG 8000 have shown plasticization and antiplasticization effects on

WVP

of cellulose acetate films (Guo, 1993). Therefore, plasticizers must be used in the cor-

rect amount to obtain the advantage of enhancing the

film

and coating properties.

The main advantage of using plasticizers is to increase film toughness or resiliency

by increasing flexibility, decreasing tensile strength, and increasing elongation, as seen