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Plasticizers in edible films and coatings

425

WVP than PEG 400 in EA films (Gennadios

al., 1996) and WPI films (McHugh and

Krochta, 1994b). Furthermore, Gly gave higher OP than Sor in WPI films (McHugh

and Krochta, 1994b), LAC, and RC films (Chick and Ustunol, 1998). Thus, Gly gen-

erally appears to be the most effective plasticizer in terms of tensile properties, but

also gives the highest permeability. To reduce the effect on permeability increase, using

different plasticizer mixtures such as Gly-Suc or Gly-Sor in WG films (Cherian

al.,

1995) and Gly-PPG 400 and Gly-PEG 400 in zein films (Parris and Coffin, 1997)

helped to lower film

WVP.

Incorporation of fatty acids and other lipids as plasticizers

reduced the permeability in FMP films (Tanaka

al., 2001).

Moisture sorption

Similar to polysaccharide films, moisture sorption is important in protein films because

moisture acts as a plasticizer. Gly-plasticized WPI films at higher Gly content and higher

had greater flexibility, and the Guggenheim-Anderson4e Boer (GAB) model gave

the best fit of moisture sorption data (Coupland

al., 2000). Transglutarninase cross-

linked EA films plasticized with higher Gly content possessed higher film hydrophilic-

ity (Lim

al., 1998). Film moisture sorption isotherms were well defined by the GAB

equation. With increasing moisture content, plasticization and swelling of the polymer

matrix provided additional binding sites and resulting enhanced water sorption at ele-

vated

(Lim

al., 1998). The sorption isotherm and thickness of peanut protein films

also reflected film hydrophilicity (Jangchud and Chinnan, 1999). Gly content was found

to have greater effect on film tensile properties than film permeability. Data showing the

effect of various plasticizers (Gly, Sor and mixture of Gly-Sor) on the moisture sorption

isotherm of soy protein isolate (SPI) films were fitted to the GAB model (Cho and

Rhee, 2002). Gly-plasticized SPI films absorbed more water than Sor-plasticized films

at the same

due to the higher hygroscopicity of Gly. The high water affinity of Gly

contributes to the excellent plasticizing effect of Gly at even low Gly content and low RH.

RH, along with plasticizer type and content, affects hydrophilic film flexibility. The min-

imum

plasticizer content required decreases with increase in RH (Cho and Rhee, 2002).

Challenges and opportunities

Although plasticizers generally comprise a relatively small part of edible film formu-

lations, they are needed to enhance the ability of edible polymers to form usable films

and coatings. Understanding of plasticizers can enable tailoring of film permeability

and mechanical properties to make them more suitable for food applications. The

amount and type of plasticizer, and the effect of

RH,

are important issues to be con-

sidered for each polymer being investigated with regard to edible films and coatings.

While basic research on plasticizer effects on edible film properties is still needed

to improve understanding, enough information is available to pursue food applica-

tions. To achieve the implemention of plasticized edible films and coatings for com-

mercial use, it is important to maximize the desirable mechanical properties while

minimizing the effect on barrier properties for each polymer film

order to explore

426

Innovations in Food Packaging

and optimize food applications. Mathematical modeling of plasticizer efficiency

andfor plasticizer efficiency ratios needs to be continued to confirm the validity of

existing equations or adjust the equations for greater reliability. The incorporation of

antimicrobials or antioxidants into plasticized films or coatings is a recent trend wor-

thy of investigation. Furthermore, study of the possible migration of plasticizer out of

the polymer film matrix is essential for long-term shelf-life applications. Using a

smaller M, polymer is another approach to providing the desirable film flexibility

with lower plasticizer content, as shown with hydrolyzed WPI film (Sothornvit and

Krochta, 2000a; Sothornvit and Krochta, 2000~). Sensory evaluation of foods with

edible films and coatings as a new product line is also necessary.

Food

system applications

The objective of introducing edible films and coatings is to improve food quality and

shelf life while reducing the use of expensive and non-recyclable synthetic polymer

films. In addition to protecting foods from moisture change, oxidation, and aroma loss,

edible coatings can protect food from micro-organisms and mechanical damage, as well

as sometimes providing an additionally impressive appearance for customers. Many

food applications of edible films and coatings have been commercialized, and many

more explored, as summarized in several review articles (Kester and Fennema, 1986;

Conca and Yang, 1993; Grant and Burns, 1994; Nisperos-Carriedo, 1994; Guilbert

and Gontard, 1995; Krochta and De Mulder-Johnston, 1997; Debeaufort

et al., 1998;

Arvanitoyannis and Gorris, 1999; Petersen

et al., 1999; Krochta, 2002).

Selection of a suitable plasticizer for individual food applications will benefit food

stability. For example, Suc-plasticized WPI coating provides the highest and most stable

gloss on chocolate compared to Gly-, PEG 400-, and PG-WPI coatings (Lee

et al., 2002).

Suc-WPI films also have the lowest OP, and highest EM and TS. Suc-plasticized WPI

coating on polypropylene films also gives the best oxygen barrier compared to Gly-,

Sor-, PG- and PEG 200-WPI coatings, although OP increased exponentially with

(Hong and Krochta, 2003). Furthermore, Suc-whey protein concentrate (WPC)-

plasticized coating on paperboard have been found to provide a good grease barrier

and minimum plasticizer migration (Lin and Krochta, 2003). Suc appears to be a

more compatible and effective plasticizer in certain food coatings in certain aspects

over Gly, because it has a high

kMo.

To use edible films for food pouches, it is necessary to know the heat-sealing tem-

perature that will give an effective heat seal with good integrity. The optimum heat-

sealing temperature for Sor-plasticized WPI-lipid emulsion edible films appears to be

130°C, while the temperature for Gly-plasticized films is 110°C

(Kim

and Ustunol,

2001). Different plasticizers revealed different thermal transition onset temperatures.

This is also an important plasticizer issue, essential to continuous film formation by

the extrusion process.

Finally, as mentioned earlier, the sensory attributes of edible films and coatings must

be investigated to ensure consumer satisfaction. WPI coatings on chocolate compared

favorably with shellac coatings in a consumer test (Lee

et al, 2002). WPI-emulsion lilms

plasticized with either Sor or Gly had no characteristic milk aroma, but they were slightly

Plasticizers in edible films and rnatings

427

sweet (Kim and Ustunol, 2001). In addition, Sor-plasticized films were more adhesive

than Gly-plasticized films, based on results from trained sensory panelists. This might be

the advantage of using polyhydric alcohols plasticizers such as Sor, Gly, M,

and Suc.

Conclusions

Most polysaccharide- and protein-based films and coatings are brittle in nature. To

solve this problem, plasticizers are incorporated to enhance the film flexibility and

resilience. However, film permeability always increases with increasing plasticizer

content. Proper selection of a plasticizer for a given polymer will allow optimization

of the film mechanical properties with a minimum increase in film permeability.

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

charides, polyols, lipids, and derivatives. Different moisture sorption by various plas-

ticizers contributes to differences in film permeability and mechanical properties.

Selection of plasticizers needs consideration of the issues of plasticizer compatibility,

efficiency, permanence, and economics.

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 amount and type of plasticizer. Generally, increasing the amount of plasticizer

decreases the film Tg.

Plasticizers generally act to produce plasticization, but can also provide antiplasti-

cization. Therefore, plasticizers should only be used at the minimum amount required

to obtain the advantage of enhancing the film and coating properties.

Another potential approach to increase film flexibility is reducing polymer M,,

thus increasing the polymer-chain end groups, reducing the intermolecular forces

along polymer chains and increasing the polymer free volume.

Polysaccharide- and protein-based films are poor barriers against water and polar

substances, especially at high RH, but good barriers against oxygen and other non-

polar substances. Nevertheless, both polar and non-polar compound permeabilities

depend on the plasticizer concentration and the relative humidity (RH). Incorporating

fatty acids or other lipids can help improve water-barrier properties, whilst providing

a certain degree of plasticization. Combining plasticizer types can utilize the advan-

tages of each plasticizer and reduce the disadvantages. Addition of a polysaccharide

substance to a protein film can also improve film tensile properties. Thus, film perme-

ability and tensile properties depend on both polymer and plasticizer. Migration of

plasticizer during film formation and storage must be studied to guarantee the feasi-

bility of applying edible films and coatings to food products.

The effect of the amount and type of plasticizer on achieving desirable mechanical

properties with optimal permeability is of continuing interest to researchers. The

mathematical models of plasticizer efficiencies should be explored further to verify

the equations and allow application to food systems. Selection of the appropriate plas-

ticizer for an edible film or coating applied to a food system should, ideally, be based

on basic understanding. Thus, study of the interesting topic of plasticizer effects on

edible film and coating properties will continue as an adventure in food science, tech-

nology, and engineering.

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Sensory

quality

foods associated

with

edible

film

and

coating

svstems

and

extension

Yanyun Zhao and Mina McDaniel-

Introduction

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

434

Sensory quality attributes associated with edible films and coatings

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

436

Edible coatings to improve the quality and extend the shelf life of foods

case studies

...

440

Sensory evaluation of edible films, coatings and coated products

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

444

Edible films and coatings incorporating functional ingredients

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

446

Futureresearch

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

448

References

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

449

Introduction

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shelf life of foods, especially fresh and minimally processed produce. Generally, an

edible coating is defined as a

thin

layer of edible material formed on a food, whereas an

edible film is a preformed thin layer of edible material placed on or between food com-

ponents (Krochta and De Mulder-Johnston, 1997). In other words, edible coatings are

applied in liquid form on food by dipping, spraying, or panning. Edible films, on the

other hand, are preformed into solid sheets and then applied on or between food com-

ponents. Edible films and coatings are natural polymers obtained from agricultural

products, such as animal and vegetable proteins, celluloses, gums, and lipids, and are

biodegradable (Debeaufort

al.,

1998).

By regulating the transfer of moisture, oxygen, carbon dioxide, lipids, aroma, and fla-

vor compounds in food systems, edible films and coatings can increase food-product

Innovations in Food Packaging

ISBN:

0-12-3 11632-5

2005

Elsevier

Ltd

All rights of reproduction

any

form reserved