Jung Han. Innovations in Food Packaging

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Edible films and coatings from animal-origin proteins

305

Figure

18.1

Cross-sections of WPI-caseinate films: (a) heated at 90°C for 30 minutes; (b) heated at 90°C for 30 minutes and irradiated

at 32 kGy (9 mm bar

3 pn). Reprinted with permission from Vachon

etal.

(2000). Copyright (2000) American Chemical Society.

WPI-caseinate ratios (5% (wlw) protein)

18.2

Puncture strength of un-irradiated and irradiated (32 Ky) whey-protein isolate (WPI)-caseinate

films. Reprinted with permission from Vachon

eta/.

(2000). Copyright (2000) American Chemical Society.

0-100 25-75 50-50 75-25 100-0

CWP-caseinate ratios (5% (wlw) protein)

Ffpm

18.3

Puncture strength of un-irradiated and irradiated (32 kGy) whey-protein concentrate

(WPC)-caseinate films. Reprinted with permission from Vachon

etal.

(2000). Copyright (2000) American

Chemical Society.

306

Innovations in Food Packaging

Whey proteins

Whey proteins are soluble proteins present in milk serum after caseinate is coagulated

during the cheese processing. Whey proteins represent around 20% of total

milk

proteins

(Brunner, 1977), and contain five main proteins: a-lactalbumin, P-lactoglobulin, bovine

serum albumin (BSA), immunoglobulin, and proteose-peptones. The P-lactoglobulin

monomer makes up approximately 57% of the protein in whey (Dybing and Smith,

1991). Monomeric P-lactoglobulin contains one free S-H group and two S-S bonds.

The a-lactalbumin is the second most abundant whey protein (20%), and contains

four S-S bonds. However, bound calcium and the four S-S bridges maintain the

globular structure of the protein and stabilize it against denaturation (Kinsella and

Whitehead, 1989).

BSA represents around

of the protein in whey. This protein contains fourteen

S-S bonds and one free thiol, which makes it highly structured (Morr and Ha, 1993).

The industrial processes used for whey-protein recovery are ultrafiltration, reverse

osmosis, gel filtration, electrodialysis, ion exchange chromatography, and diafiltration

(De Boer

al., 1977; Matthews, 1984; Glover, 1985; St-Gelais

al., 1992; Letendre

al., 2002b). These processes are generally used to produce whey-protein concen-

trate (WPC: 25-80% protein) or whey-protein isolate (WPI:

90% protein).

Formation of intact and insoluble whey-protein films can be produced by heat

denaturation of the proteins. Heating modifies the three-dimensional structure of the

protein, exposing internal S-H and hydrophobic groups (Shimada and Cheftel, 1998),

which promote intermolecular S-S bonds and hydrophobic interactions upon drying

(McHugh and Krochta, 1994). McHugh and Krochta (1994) produced WPI films by

heat treatment using a protein concentration from 8-12% and heating temperatures

between 75 and 100°C.

Le Tein

al. (2000) have observed that a film formulation solution containing a

protein concentration of

10% of native WPI (protein obtained by diafiltration with-

out any heat treatment) at pH 7 gelled during cross-linking treatment when heated at

90°C.

optimal temperature and concentration was obtained at 80°C for 30 minutes

with a protein concentration of 5%. Bane rjee and Chen (1995) reported that heating

WPC solutions at 75OC for 30 minutes and at pH 6.6 produced uniform films.

Contrary to casein films, insoluble whey-protein films can be produced by heat treat-

ment due to the formation of covalent disulfide bonds (Gennadios

al., 1994).

Transglutaminase has also been developed to cross-link whey proteins (Mahmoud

and Savello, 1993). Transglutaminase catalyzes the introduction of &-(y-glutamy1)-

lysine cross-links into proteins via an acyl transfer reaction between whey proteins, as

well as between peptides and primary amines (Mahmoud and Savello, 1993).

Collagen and gelatin

Collagen is a constituent of skin, tendon, and connective tissues. It is a fibrous protein,

and represents about 30% of the total mass of the body (Gustavon, 1956). Collagen

fibrils are produced by self-assembly of collagen molecules in the extracellular

matrix, and provide tensile strength to animal tissues (Trotter

al., 2000). Collagen

Edible films and coatings from animal-origin proteins

307

can be dissolved in dilute acid or alkali solutions, and in neutral solutions. Two major

components are identified;

(MW 100 000 Da) and

(MW

200 OOODa), and consist

of two different types of covalent cross-linked chain pairs

al-al

and

a2-cwz

(Harrington,

1966; Piez

al., 1968).

Hydrolysis of collagen results in gelatin. The molecular weight of gelatin covers a

broad range, from 3000-200 000 Da, depending on the raw material employed during

gelatin production and handling conditions voung, 1967). Edible coatings made with

gelatin reduce the migration of moisture, oxygen and oil.

Collagen is the most commercially successful edible protein film. Film-forming

collagen has been traditionally used in the meat industry, for the production of edible

sausage casings. This protein has largely replaced natural gut casings for sausages.

Collagen films are not as strong and tough as cellophane, but have good mechanical

properties (Hood, 1987). Collagen films have an excellent oxygen barrier at 0% rela-

tive humidity, but the oxygen permeability increases rapidly with increasing relative

humidity in a manner similar to cellophane (Lieberman and Gilbert, 1973). Different

cross-linking chemical agents have been used to improve the mechanical properties, to

reduce the solubility, and to improve the thermal stability of these films. Carbodimide,

microbial transglutarninase, and glutaraldehyde are usually used as cross-linking

agents (Jones and Whitmore, 1972; Takahashi

al., 1999; Taylor

al., 2002).

Gelatin is known to form clear, flexible, strong, and oxygen-impermeable films

when cast from aqueous solutions in the presence of plasticizers (Gennadios

al.,

1994). Industrial applications of gelatin include capsule coating and microencapsulation.

Edible coatings with gelatin reduce oxygen, moisture and oil migration, or can carry

antioxidant or antimicrobial agents (Krochta and De Mulder-Johnson, 1997). They

can be good gas barriers but poor water barriers due to their hydrophilic nature.

According to Arvanitoyannis (2002), several researchers have studied the glass

transition temperature (Tg) and melting point temperature (Tm) of gelatin from vari-

ous sources. The Tg of gelatin from cow tendon was the lowest at 95°C and the high-

est was from limed ossein with a Tg of 220°C. The lowest Tm was from gelatin from

cow tendon (145°C) and the highest was from gelatin from pigskin (237°C).

The tensile strength (TS) and percentage elongation of a variety of gelatin-based

films was measured by Arvanitoyannis

al. (1997,1998a, 1998'0). A combination of

low and high temperature processes was used in the production of films containing

different types of gelatintcarbohydrate combinations with sorbitol and water ratios.

The film that produced the highest tensile strength (130 MPa) was made from a com-

bination of

gelatin/chitosan/sorbitol

and water using a low-temperature process. The

lowest tensile-strength (34.4 MPa) film was produced using a high-temperature process

and gelatin combined with soluble starch, sorbitol, and water; however, it also had the

highest elongation at 44.7%.

Myofibrillar proteins

Edible protein films have been formed from myofibrillar proteins from a variety of ani-

mal sources, but most of the work has been focused on fish as the main source of pro-

tein. The probability of forming intermolecular bonds mainly depends on the protein

308

Innovations in Food Packaging

shape (fibrous or globular), and on the physicochemical conditions during processing.

High molecular weight proteins (e.g. myosin) and fibrous proteins (e.g. myosin and

F-actin) can generally form

films

with good mechanical properties, while globular or

pseudo-globular proteins (e.g. G-actin) need to be unfolded first, before

film

formation.

Myofibrillar proteins are found in the muscle, and are mainly composed of myosin and

actin, regardless of whether the source of protein is mammalian or fish (Table 18.1).

Proteins obtained from mammal or fish sources must be washed and purified to

obtain the myofibrillar portion. Components such as blood, lipids, myoglobin, and

collagen must be removed through a series of washing treatments. According to Cuq

(2002),

a suitable method for obtaining myofibrillar protein from fish is the process

known as surimi. It produces a product that is odorless, colorless, forms a gel upon

heating, and is stable under freezing conditions. Beef, pork, and chicken proteins have

also been processed using the surimi process, but it is not commercially commonly

performed as it is with fish (Stanley

al., 1994; Sousa

al., 1997).

order to form a film, the proteins must be soluble in solution. According to

Steffansson and Hultin (1994), cod myofibrillar protein was soluble in water at neu-

tral pH with 0.0003 ionic strength or less. There was a strong relationship between pH

and salt concentration. Increasing the sodium chloride concentration or decreasing the

pH to 5.5 reduced the solubility, possibly due to the denaturation of the protein.

Aggregation and precipitation of proteins can occur when there is a loss of stable

hydrophilic surface charges, thus exposing the hydrophobic areas of the protein to

interact. This phenomenon is commonly referred to as "salting outyy. Steffansson and

Hultin's (1994) research established important factors for the separation and solubi-

lization of fish myofibrillar proteins to be used in gel or

film

formations.

Orban (1992) stated that the isoelectric point of myofibrillar proteins is pH

and

that they are not soluble at that pH but are soluble at pH

4.5 and

6.5. Below pH

4, myofibrillar proteins gelate in water-acetic acid solutions at room temperatures.

According to Cuq (2002), there are three stages of film formation:

1. Destabilization of intermolecular protein bonds through the use of solvents or heat

2. Polymerization

Tobk

18.1

Protein

composition

mptibrillu

proteins

(*ted

fmm

Cuq,

2002)

Myosin

Actin

Tropomyosin

Troponins

M-proteins

C-proteins

F-proteins

a-Actinin

P-Actinin

Nebulin

Titin

Mammal

source

(dl

g protein)

0.1

Fish

source

&I00

protein)

Edible films and coatings ti-om animal-origin proteins

309

3. Formation and stabilization of the three-dimensional protein network, upon the

removal of the initial destabilization component (removal of solvent or removal of

heat).

As stated above, a method for destabilizing intermolecular protein bonds can be

obtained by using solvents such as acetic acid, lactic acid or ammonium hydroxide. A

low pH (2-3) provides optimal film-forming conditions (Cuq

al., 1995, 1998b;

Cuq, 2002). The viscosity of the film is also affected by the protein content and the

pH. As the protein content decreased, the film solution viscosity decreased (Cuq

al.,

1995). Plasticizers such as glycerol, sorbitol, and sucrose increase the flexibility of

myofibrillar protein films. Lactic acid also has a plasticizing effect on films, in addi-

tion to lowering the pH (Sousa

al., 1997).

The second method of destabilizing proteins for film formation involves heat.

According to Cuq (2002), the glass transition temperature (Tg) of fish myofibrillar pro-

tein is between 215°C and 250°C, which is similar to that of collagen, gelatin, and

starch. Above 250°C thermal degradation occurs, and this is too low for most polymer-

processing methods, such as extrusion (Cuq

al., 1997a). The addition of sucrose and

sorbitol as plasticizers decreased the Tg, as did water. Cuq

al. (199%) found that high

thermal degradation occurred with low moisture content, and even caused the material

turn

brown in color due to cararnelization. However, glassy translucent materials were

produced with moisture content of 2.2% and at temperatures less

than

200°C.

Myofibrillar proteins are not considered to be soluble

water (Cuq

al., 1997b,

1998~). Films immersed in water for 24 hours did not lose their integrity. Reasons for

their water insolubility have been studied, but it is yet not fully understood. However,

it is known that the addition of plasticizers increases the water solubility of most

polymers.

Myofibrillar proteins have a higher tensile strength

than

other protein-based films

shown in Table 18.2. The only synthetic film that compares to fish myofibrillar protein

Tensile

strength

Elongation

WVP*

4P**

(MW

(96)

'olyester

178 85 0.02 12

'olyvinyl chloride

93 30

1.2 23

xiw-density polyethylene

13 500 0.04-0.05 1003

iigh-density polyethylene

26 300

0.014 224

iydroxypropyl cellulose

15 33

6.2 300

Nheat gluten

3.3 192 5.1 1290

ioy protein

3.6 160 194 14

Zorn eein

3.9 213 6.5 35

=ish myofibrillar protein

17 23 3.9-3.8 1-873"'"

WVP

(water-mpor permeability) (X

0-l2

mol m ./m2. s. Pa)

02P (oxygen permeability) (X

~-'~rnol. m ./m2.s. Pa)

***

measured under dry conditions,

873

measured under high relatively humidity.

310

'nncwations in Food Packagin-

films in tensile strength is low-density polyethylene, while polyester is approxi-

mately ten times stronger. Fish myofibrillar protein film has a relatively low percent-

age elongation, but comparison with other films is difficult without knowing the rela-

tive thickness and whether other films have plasticizers. However, it can be generally

concluded that myofibrillar protein films are stronger than most biopolymer films, but

less elastic.

Cuq

al. (1996) developed a rheological model to predict the mechanical proper-

ties of fish myofibrillar proteins. The model included factors such as elastic modulus,

viscosity, force-deformation, relation, and creep. They determined that the model

could be used successfully to characterize the molecular interactions in the films.

Furthermore, their research compared the elastic modulus of fish myofibrillar protein

films to that of cellulose and of low-density polyethylene films. The fish myofibrillar

protein films were found to have more relaxation than the cellulose-based films, but

less relaxation than the polyethylene-based films.

Permeability rates of synthetic and edible films are also shown in Table 18.2. The

water-vapor permeability (WVP) of fish myofibrillar protein is relatively high due to

its hydrophilic nature (Cuq, 2002). However, it has a lower WVP compared to most

other edible films. The oxygen permeability of the myofibrillar protein is highly

dependent upon the relative humidity and the water activity. Under

dry

conditions the

film is an excellent oxygen barrier, but when it is exposed to high moisture conditions

it is not as good. This is an important consideration, since edible films are usually used

under moist conditions.

The use of plasticizers has a significant effect on the properties of the films. For

example, the addition of glycerol, sorbitol or sucrose decreased film strength and elas-

tic modulus; however, it increased the percentage elongation and the water-vapor per-

meability (Cuq

al., 1997a). Strain at failure increased 100-fold when the ambient

relative humidity increased from 11% to 95% (20°C), and 12-fold when the glycerol

content increased (Cuq, 2002). The role of water as a plasticizer of amorphous mate-

rials in relation to the T, was studied by Cuq

al. (1997d). Using differential scan-

ning calorimetry (DSC), myofibrillar films exhibited a non-linear decrease in Tg with

increase in water content.

Very little work has been done on the sensory properties of myofibrillar protein

films; however, Cuq

al. (1996) included measurements of mechanical resistance of

sliced fish containing a fish myoliibrillar protein coating. Films were applied to fish

specimens and tested using a strength-deformation rheogram. The coating was found

to affect the texture of the meat slightly by increasing the mechanical resistance when

sliced. It was determined that the film should be reduced in strength for it to have

acceptable sensory properties as an edible coating on fish surfaces. No studies have

reported the effects of protein films on taste; however, Cuq (2002) indicated that

the films had a slight fishy odor.

The majority of the research on myofibrillar proteins has focused its attention on

coating fish to reduce the oxidative effects.

driving force for further development of

these films appears to be the underutilization of fish and fish by-products. In addition,

other sources of myofibrillar protein films (beef, pork and chicken) are too well utilized

to find a cost-effective purpose for them to be used to produce edible films.

Edible films and coatings fiom animal-origin proteins

311

Egg

white

The surplus of egg-white products has created a reason to produce more edible films

and coatings, and has led to development of new applications in the food indus-

try.

Egg white is a mixture of eight globular proteins; ovalbumine, ovotransferrin,

ovomucoid, ovomucin, lysozyme, G2 globulin, G3 globulin, and avidin. Ovalbumin,

ovotransferin, and ovomucoid constitutes 54%, 12% and 11% of the protein weight,

respectively. In addition, ovalbumin is the only protein containing free S-H groups.

Ovotransferrin, ovomucoid, and lysozyme contain S-S bonds (Mine, 1995). The S-S

bonds are considered important in film formation (Gennadios

al., 1994). The use of

egg white in film and coating production represents a nutritional interest, since it is an

effective antioxidant (Negbenebor and Chen, 1985). Edible packaging made from egg

white is clear and transparent, and has properties similar to other proteins (Gennadios

al., 1996). It has been reported that a pH range from 10 to 12 is necessary to obtain

homogeneous films (Okarnoto, 1978; Gennadios

al., 1996; Handa, 1999).

pro-

teins, at alkaline pH, the S-S bonds are reduced to S-H groups. The S-H groups are

converted to inter- and intramolecular S-S covalent cross-links during the heating

treatment (Gennadios

al., 1996). The formation of cross-linked films was also

observed when proteins were treated under ultraviolet light, resulting in lower total

soluble matter and better mechanical properties (Rhim

al., 1999).

Egg albumen has been studied for its potential to retain moisture inside raisins in

cerealhaisin mixtures (Bolin, 1976) and inside meat products (Reutimann

al.,

1996). Egg-white coatings have also been evaluated for their effectiveness

making

pizza components impermeable (Berberat and Wissgott, 1993).

Biodve

protein-basd

coatings

and

films

Active compounds such as antioxidants and antimicrobials can be incorporated into

edible films and coatings. The resulting bioactive films or coatings provide more

inhibitory effects against spoilage and pathogenic bacteria by lowering the bacterias'

diffusion through the food product and maintaining high concentrations of the active

compounds on the food surfaces (Gennadios and

Kurth,

1997). Organic acids, essential

oils, salts, lipids, spices, and bacteriocins (nisin) have been largely studied for their effi-

ciency in controlling the growth of spoilage and pathogenic bacteria in foods (Siragusa

and Dickson, 1992; Ming

al., 1997; Padgett

al., 1998; Ouattara

al., 2001).

Ouattara

al. (2001) demonstrated that the use of an edible coating based on

milk

proteins containing natural antimicrobial compounds (e.g. thyme oil and trans-

cinnarnaldehyde) that was applied by immersion on shrimp was able to extend the shelf

life by 12 days. It

was

also demonstrated that a synergistic inhibitory effect was obtained

by using a combination of a coating with irradiation (Lacroix and Ouattara, 2000). The

incorporation of antimicrobial compounds extracted from spices and incorporated in

milk-protein

film

reduced lipid oxidation and -S-H radical production during the post-

irradiation storage of ground beef (Ouattara

al., 2002b). Whey proteins are effective

antioxidants (Le Tein

al., 2001), and are effective in protecting salmon against oil

312

Innovations

Food Packaging

oxidation. The application of active compounds in whey-protein coatings can also

reduce the oxidation of cut hits and sliced mushrooms (Nisperos-Carriedo et al., 1991;

Le Tein et al., 2001). Cysteine and aromatic amino acids are potent free-radical targets

(Berlett and Stadtman, 1997), and can reduce the polyphenol oxidase activity in vegeta-

bles and thus reduce browning reactions. Coatings based on proteins rich in this amino

acid can have good antioxidant properties (Le Tein et al., 2001).

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