Jung Han. Innovations in Food Packaging

Подождите немного. Документ загружается.

Plasticizers

edible films and coatings

415

in most edible films and coatings (Donhowe and Fennema, 1993; Guo, 1993; Park

al., 1993; Gontard

al., 1994; Galietta

al., 1998; Anker

al., 1999; Sothornvit

and Krochta, 2000a; Sothornvit and Krochta, 2001). However, plasticizers also appear

to have the disadvantage of increasing film permeability, depending on the type of

plasticizer. For example, hydrophilic plasticizers are low barriers to moisture

and

tend

to have a more pronounced effect on increasing film WVP compared to OP. On the

other hand, hydrophobic plasticizers have a greater effect on increasing permeability

to oxygen, aroma, and oil compared to

WVP.

Other concerns are that plasticizers are

subject to aging, extraction, migration, volatility, and exudation (Daniels, 1989). These

phenomena limit effectiveness, and leave levels of plasticizer that can be detrimental

to film properties.

Migration of plasticizer from the film matrix to the surface can result from weak

interactions between polymer molecules and plasticizer, allowing excess plasticizer to

migrate easily through the film.

An indicator of plasticizer migration is the appearance

of greasiness and cloudiness on the

film

surface, as found with PEG 400-plasticized

P-Lg films (Sothornvit

al., 2002). To slow down the migration rate of plasticizers,

a mixture of different plasticizers such as Gly and PEG in zein films can be used

(H.

J. Park

al., 1994). This will help to maintain the mechanical properties of films

during storage.

Properties

edible films and

coatings

The main properties of edible films and coatings are their barrier properties (permeabil-

ity to moisture, oxygen, aroma, and oil), mechanical properties, and moisture sorption.

Barrier properties are important to separate food from the environment, which

causes food deterioration. The barrier properties of polymer films are generally related

to the physical and chemical nature of the polymers. The chemical structure of the

polymer backbone, the degree of crystallinity and orientation of molecular chains,

and the nature of the plasticizer added all affect barrier properties (Salame, 1986). No

polymer films, including edible films, are perfect barriers. However, each edible film

is a barrier to the degree that it limits the permeability of water, oxygen, aroma, and

oil. Therefore, the barrier is measured in terms of permeability. Briefly, the permeant

molecules (water, oxygen, etc.) collide with the polymer film, adsorb to the polymer

surface, diffuse through the polymer network, then desorb from the other side of the

film. The most common barriers of interest in edible films and coatings include:

Water barriers.

Water can enhance the rate of several reactions (browning, lipid

oxidation,

vitamin

degradation, enzyme activity), increase the rate of micro-organism

growth, and cause texture change, all of which are related to food shelf life and

quality. Thus, controlling water permeability is of substantial importance in the

development of an edible film and its application. Different edible polymer films

have their own strengths. While polysaccharide- and protein-based films provide

good barriers against oxygen, aromas, and oil (non-polar molecules), they are poor

barriers against water (polar molecules). Much effort has gone into decreasing film

416

Innovations in Food Packaging

permeability to water vapor, such as combining polysaccharide or protein with

lipids, cross-linking with transglwninase, irradiation, and heat curing. While

adding lipid decreases polysaccharide- and protein-film

WVP,

modification of

these polymers mainly affects their mechanical properties. The type and amount of

plasticizer also affects the WVP. The potential applications of edible films include

coatings on fruits (e.g. apples, citrus hits, and pears) to prevent moisture loss and

improve gloss (Grant and Burns, 1994).

2. Oxygen barriers. Oxygen transfer from the environment to food affects the deteri-

oration of food components (lipids, vitamins, colors, and flavors) and micro-organism

growth (especially molds), leading to sensory and nutrient changes. Depending on

the formation materials used, the properties of edible films and coatings can be

adjusted to achieve each food application (Cuq et al., 1998). Most protein-based

films are excellent oxygen barriers. Nuts provide an example of the use of protein

films to avoid oxygen contact. Fruits and vegetables need a balance of oxygen and

carbon dioxide flow to control respiration and ripening. Generally speaking, films

that are better water barriers are poorer oxygen barriers. Increasing the plasticizer

amount always increases oxygen permeability through edible films, because of the

higher free volume in the film network.

Grease/oil barriers. Greaseloil barrier packaging is essential for foods containing

fats or oils. Typically, synthetic polymers such as polyethylene

(PE)

are used to

provide grease resistance for coated paperboard. Biopolyrner-coated paperboard is an

alternative that can be used in fast-food packaging and for other forms of greaseloil

barrier packaging (Lin and Krochta, 2003).

Mechanical properties are important for edible films and film coatings, as they reflect

the durability of films and the ability of a coating to enhance the mechanical integrity of

foods. As discussed earlier, addition of plasticizers to biopolymers has a large effect on

film mechanical properties, such as increasing film flexibility and resilience.

Finally, moisture sorption is an important characteristic of hydrophilic films, since

the sorbed water acts as a plasticizer. Hydrophilic plasticizers increase the water-holding

capacity of the plasticized films, such as Gly-plasticized corn zein (Padua and Wang,

2002). The plasticizing effect of moisture sorption on edible films indicates that the

best applications for long-term food protection are for low water-activity foods.

Polysaccharide-based films and coatings

Polysaccharides used to form films and coatings are cellulose derivatives (e.g. methyl-

cellulose (MC), hydroxypropyl methylcellulose (HPMC), hydroxypropylcellulose

(HPC),

starches and starch derivatives (e.g. amylose, amylopectin, hydroxypropyl

amylose), and dextrins, pullulan, konjar, carrageenan, pectin, alginate, and other gums.

Polysaccharide-based films and coatings are hydrophilic, and thus exhibit poor water-

barrier properties. Nonetheless, polysaccharide films are good barriers against oxygen,

aromas, and oil at low to intermediate

RH.

They generally require plasticizers to achieve

the desirable mechanical properties.

Plasticizers

edible films and coatings

417

As polysaccharide-based films are generally water-based, the most effective plasti-

cizers are similar to the polysaccharide structure; therefore, hydrophilic plasticizers

containing hydroxyl groups are best suited to this use. The plasticizers commonly

used for polysaccharide-based films are glycerol, sorbitol, xylitol

(X),

mannitol

(M),

PEG (with M, from 400 to 8000), ethylene glycol (EG), and PG.

Properties

Polysaccharide-based films are poor barriers against water vapor and other polar

substances, especially at high

RH.

However, at low to intermediate

they are good

barriers against oxygen and other non-polar substances, such as aromas and oils.

Polysaccharide-based films are relatively stiff, and therefore plasticizers are needed to

facilitate handling. The effects of various plasticizers have been explored for films

made from MC, HPC (Donhowe and Fennema, 1993; Park

al., 1993; Ayranci and

Tunc, 2001), HPMC (Ayranci

al., 1997), locust bean

gum

(Aydinli and Tutas, 2000),

gellan (Yang and Paulson, 2000), highly carboxyl methylated starch

(Kim

al., 2002),

pullulan (Kim

al., 2002), and high-amylose cornstarch

(Ryu

al., 2002).

Permeability properties

Table 23.3 shows the water vapor permeability

(WVP)

and oxygen permeability (OP)

of different polysaccharide-based films with various plasticizers. Comparisons are

difficult because data were collected in different studies using different film composi-

tions, at different test conditions

(RH,

temperature), and with different methods of

measurement and numbers of replications. It can be concluded from Table 23.3 that

Gly is a commonly selected and effective plasticizer, as seen with MC films and high

carboxy methylated starch (HCMS) films. At similar levels

(wt/wt),

the smaller Mw

hydrophilic plasticizers such as PG and Gly produced higher WVP than larger Mw

plasticizer such as PEG 400 in MC films (Donhowe and Fennema, 1993; Park

al.,

1993) and HPC films (Park

al., 1993). Also, higher film OP was found with PEG

400 compared to higher Mw PEG 2000 (Donhowe and Fennema, 1993). However, the

same plasticizer content but with higher Mw gave slightly higher

WVP

locust bean

gum (LBG) films (Aydinli and Tutas, 2000). Therefore, the selection of plasticizer Mw

can tailor a plasticized film, including lowering its permeability. Increasing the plasti-

cizer content generally increases permeability. Besides the plasticizer type, the poly-

saccharide type also affects film properties. For example, pullulan films possess better

water barrier properties at mechanical properties similar to HCMS films. To improve

water barrier, lipids such as fatty acids are applied to films and can be very effective,

as seen with plasticized MC-fatty acid films (Ayranci and Tunc, 2001) and laminated

MC*orn zein-fatty acid films

(J.

Park

al., 1994).

Mechanical properties

Table 23.3 shows that, generally, polysaccharide film tensile strength (TS, stress at

maximum force before break), and elastic modulus (EM or Young's modulus, film

stiffness as calculated from the slope of force-deformation curve) decreases with

418

Innovations in Food Packaging

Polysaccharide/plasticizer

WVP

(gmm/ OP (cm3p,m/ ties Reference

(Wt/Wt)

m2d kPa)

vl~' :Gly

1.52: 1-5.88: 1 9.07-70.80 101.03

50-100 1

iPC2:~ly

1.52: 1-5.88: 1 7.00 201.46

100-1 25 1

vlC1: PEG 400

1.52

1-5.88:

7.78 242 67

78-1 00 1

+PC':

PEG

400

1.52: 1-5.88:

4.75 242 67

100-1 98 1

vlC7:pG

1.52

1-5.88: 1 8.64-30.24

242-1 51 2.5

4U-50

25-50 1

+P@:PG

1.52: 1-5.88:

12.96-20.74 605-1 028.5 20-24

63-67 1

.BG :PEG 200

0.58: 1-2.3: 1 1.51-1.83

.BG: PEG 400

0.58: 1-2.3: 1 1.51-2.19

- -

.BG:PEG 600

0.58: 1-2.3:

1.67-2.78

.BG:PEG 1000

0.58: 1-2.3: 1 1.89-2.76

iellan

Gly

0.5

1 36

30 25 30 3

;ellan

PEG 400

0.5

27 44 8 3

4CMS:Sor

3.3:l-10:l 103-1 29

4.7-10.2

2.0-2.8

4CMS:M

3.3:l-10:l 155-1 81

4.0-6.7

2.0-2.4 4

-ICMS:X= 3.3:l-10:l 11 2-1 55

6.8-13.0

2.2-5.6 4

-ICMS:Gly

3.3: 1-10:l 103-1 90

9.7-1 5.3

2.6-7.7 4

)ullulan

Sor

3.3

1 2.88

29.2

2.6 4

'ullulan

3.3

1 3.6

15.7

9.5 4

vIC: PEG 400

2.3

1 12.1 623 41.3

33.0 5

4C:PEG 1450

2.3:l 11.6 472 50.3

41.2 5

vIC: PEG 8000

2.3

1 11.2 460 43.7

17.8 5

vIC

PEG 2000

2.3

1 10.3 351 45.0

13.3 5

vIC:Gly

2.3:l 13.8 242 48.6

36.7 5

vIC

2.3

1 8.62 200 70.9

11.6

V~C~-PEG

400

2.5

1-20

1 2.1-2.9

YIC~-PEG 400: PA

2.5

1-20: 1 1.3-2.8

VIC~-PEG

400:SA

2.5: 1-20:? 1.1-1.8

- -

vlC:PEG400

2.5:l 3

- -

aminated film4 35.30

33.00

28.41 7

aminated film4:

1-1.5: 1 6.0-32.7

16.5-20.96

57.40-68.38 7

aminated film4: PA

1-1.5: 1 1.6-12.3

17.76-24.92

37.35-65.21 7

aminated film4:sp

1-1.5: 1 1.9-8.64

18.55-19.81

60.89-69.03 7

~PMC': PEG 200 or PEG 1

3-1.63

10000

1.67: 1

-IACS~:GI~

1 :I-$:I 1011-1270

2-32

22 9

-IACS~:SO~

1 :I-5:l 1011-1270

7-47

vl~',

methylcellulose (M,

20 000); HPC', hydroxypropyl cellulose (M,

1 000000); LBG, loc

m; HCMS, highly

:arboxy rnethylated starch;

MC~,

methylcellulose (M,

41 000); laminated4, laminated MC/corn zein plasticized with PEG 400 and

lly; HPMC', hydroxypropyl methylcellulose;

HACS~,

high-amylose cornstarch; Gly, glycerol; PC, propylene glycol; Sor, sorbitol; Suc,

;ucrose; PEG, polyethylene glycol; PPC, polypropylene glycol; M, mannitol;

xylitol;

LA,

lauric acid; PA, palmitic acid; SA, stearic

icid; SP, stearic-palmitic acid mixture; OA, oleic acid.

Lferences:

Park

eta/.,

1993; 2, Aydinli and Tutas, 2000; 3, Yang and Paulson, 2000; 4, Kim

etal.,

2002; 5, Donhowe and Fennema, 1993;

Ayranci and Tunc, 2001

;

Park

a/.,

1994; 8, Ayranci

etal.,

1997; 9,

Ryu

etal.,

2002.

Plasticizers

edible films

and

coatings

419

increasing plasticizer content. Film elongation

(%E,

maximum percentage change in

length of film before breaking) normally increases with increase of certain plasticizer

content. Table

23.3

indicates that flexible and high-permeability films sometimes pos-

sess good film strength, as seen with MC and HPC films (Park

al., 1993). Thus,

flexible films are not necessarily weak films. Different plasticizer types also result in

similar film tensile properties. Incorporation of fatty acid has been seen to lower film

TS in laminated MC+orn zein-fatty acid films

(J.

Park

al., 1994).

Moisture sorption

Moisture sorption must be considered for hydrophilic films that are sensitive to mois-

ture change. Furthermore, plasticizers with a greater degree of hygroscopicity cause

higher moisture sorption, with a resulting increase in plasticization. The equilibrium

moisture contents for both amylopectin and amylose films were lower at low humidity

(>50% equilibrium relative humidity, ERH), with or without Gly added (Myllarinen

al., 2002). Films with a large amount of Gly had high water content above 50%

ERH. Above 70% ERH, amylose sorbed more water than amylopectin; therefore,

amylose films were much more flexible than amylopectin films. Polymer crystalliza-

tion in hydrophilic films during storage is known to decrease film moisture sorption.

Protein-based

films

and coatings

Proteins used to form films can be placed into one of two categories; animal or plant

proteins. Animal proteins include whey protein, casein, gelatin, collagen, fish myofib-

rillar protein, egg-white protein, and keratin. Plant proteins include wheat gluten, corn

zein, soy protein, peanut protein, and cottonseed protein. Similar to polysaccharide-

based films, protein-based films have high affinity with polar substances such as

water, but low affinity with non-polar substances such as oxygen, aromas, and oils.

Nevertheless, both polar and non-polar permeability depend on plasticizer concentration

and relative humidity (RH).

The plasticizers commonly used for protein-based films include Gly, PG, Sor, PEG,

Suc, and

(X).

Plasticizers used for corn zein films have included liquid organic com-

pounds such as polyols, and solids compounds such as monosaccharides, oligosaccha-

rides, lipids, and lipid derivatives (Padua and Wang, 2002). Plasticizers used for wheat

gluten films have included Sor, mannitol

(M),

diglycerol, PG, triethylene glycol,

polyvinyl alcohol, and polyethylene glycol; however, Gly and ethanolamine are widely

used (Guilbert

al.,

2002). Plasticizers that have been used with soy protein include

Gly, Sor, PEG, sugars, and lipids (S.

Park

al.,

2002). Other protein sources, such

as cottonseed protein, whey protein, casein, fish myofibrillar protein, egg-white protein,

collagen, and gelatin, have been most commonly plasticized with Gly, Sor, and PEG.

Properties

Table

23.4

shows the effect of plasticizer on

WVP,

OP, and mechanical properties of

protein films. All these properties are important to food applications and food stability.

420

Innovations in Food Packaging

rable

23.4

Water vapor permeability (WVP), oxygen permeability (OP), and mechanical properties (tensile strength

(TS),

elastic

nodulus (EM) and

elongation (%E)) from various plasticizers used in protein-based films

Mechanical properties

(MPa) EM (MPa)

Reference

SC:Gly

0.89 11-1.67: 1

5.4-1 1.4

PEG 400

0.81

1-1.32

1 14.8-22.6

SC:Gly

2: 1

SC:PEG400

1.9:l

$-Lg: Gly

1.70

1-3.20

$-Lg: PC

2.06

1-3.87: 1

$-Lg: Sor

0.86

1-1.62

$-Lg:Suc

0.46: 1-0.86: 1

$-Lg: PEG 200

0.78

1-1.47: 1

$-Lg: PEG 400

0.39

1-0.73

WPI:Gly

1:l-2:l 116-144

WPI:X= 1:l-2:l 84-89

WPI:Sor= 1 :1-2:l 84-1 12

WPI:Gly

1:l-1.6:l 11 9.8-1 54.56

WPI:Sor

1 :1-1.6:l 61.92-84.72

WPI

PEG 200

134.64

WPI

PEG 400

1 129.6

WPI

Gly

2.3

WPI:Sor

EA:Gly

2.0

1-3.3: 1 8.77-1 0.68

EA:Sor

1.7: 1-2.0: 1 4.90-5.69

PEG 400

1.7: 1-2.0

1 6.21-6.22

WG:Gly:Suc

15:6:0 121.55

WG:Gly:Suc

15:4:2 99.1 4

WG:Gly:Suc

15:3:3 81.90

WG:Gly:Suc

15:0:6 15.52

WG:Gly:Sor

15:3:3 105.1 7

LAC:Gly

0.6: 1-1.4: 1

54.7-59.3

LAC:Sor

0.6: 1-1.4: 1

34.0-45.0

RC:Gly

0.6: 1-1.4: 1 45.2-58.2

RC:Sor

0.6:l-1.4:l 39.6-49.6

Zein

Gly

2.3

1 35.52

Zein

Gly

5.67: 1 24.24

Zein

PEG 400

2.3

Zein

PPG 400

2.3

1 16.8

Zein

Gly: PPG 400

9.3

3 25.68

Zein

Gly: PEG 400

9.3

Zein

Gly: PPG 400

9.3

1 34.8

Zein: Gly: PPG 400

4.67: 1

31.2

WGI:EC

2.7:l

WGI: DEG

2.7: 1

WGI :TEG

2.7: 1

WGI :TEEG

2.7: 1

WGI

DEGMET

2.7: 1

FMP:Gly

2.5: 1 6.53

FMP:Sor

2.5: 1 5.1 5

FMP:Suc

2.7: 1 3.9

CMP':W

1.5:1-4:1 3.80-4.75

Plasticizers in edible films and coatings

421

rble

23.4

(Continued)

Proteinlplasticizer

WVP

mml OP (cm3 pml Mechanical properties Reference

(wt/w)

m2 d kPa) m2d

kPa)

(MPa) EM (MPa) %E

FMP':PA= 1.5:1-4:l

5.27-7.17

4.1

5-1 6 13

FMP1:SA

1.5:l-4:l

4.49-5.75

2.9-3.7

3-13 13

~M~':~eewax= 1.5:l-4:l

3.80-7.52

3.2-4.6

82-120 13

FMP':OA

1.5:l-4:l

2.25-3.46

5.7

145-200 13

FMP1:LL

1.5:l-4:l

3.80-4.41

3.8-4.1

115-135 13

FMP':LN

1.5:l-4:l

3.80-5.01

4.1-4.4

98-130 13

FMP':PO

1.5:l-4:l

8.38-10.37

90-130 13

FMP':CO

1.5:l-4:l

5.44-6.13

3.5-4.2

100-165 13

FMP':SO

1.5:l-4:l

6.13-7.34

4.1-4.4

130-145 13

FMP':CL= 1.5:l-4:l

7.1 7-8.64

4.1-5

130-145 13

EA:PEG400:MMM

6:4:1-3:2:1

196.8-204

4.36-4.75

47.2-54.9 14

EA:PEG400:HMM =6:4:1-3:2:1

194.4-213.6

4.06-4.54

43.1-44.1 14

EA:PEG400:OA

6:4:1

199.2

5.77

89.2 14

EA:PEC400:LPL= 6:4:1-3:2:1

21 1.2-220.8

4.56-4.80

77.7-84.5 14

Soybean

Gly

0.25

1-0.67: 1

1.55-2.08

250-275 15

Soybean :TEC

0.25

1-0.67: 1

1.98-2.38

240-267 15

Soybean:Cly:TEG

0.5: 1

1-1.33: 1

1.75-2.3

240-255 15

SC:S:Gly:water

9.5:9.5:0:1 0.22

35.4

SC:S:Cly:water

9.0:9.0:1

1.81

80.4 38.2

8.2 16

SC:S:Cly:water= 8:8:3:1

4.50

46.6 32.0

18.5 16

Cly

water

6.5

13.05

30.2 22.5

26.8 16

SC:S:Sor:water= 9:9:1 :1

2.85

12.1 36.3

9.5 16

SC:S:Sor:water

8:8:3:1

5.53

82.9 29.4

20.9 16

SC: S :Sor: water

6.5: 6.5: 6: 1 18.40

63.1 20.2

29.7 16

SC:S:xylose:water

9:9:1 :1

3.37

4.3 33.1

2.4 16

SC:S:xylose:water

8:8:3:1

6.57

67.4 27.2

15.2 16

xylose :water

6.5

1 21.25

45.8 19.0

18.0 16

Zein: PEC+Cly

4: 1-10: 1

345.6-985.0

19.5-22.8

1.3-4.3 17

Zein:OA= 4:l-10:l

259.2-388.8

17.5-21.7

1.8-5 17

SC, sodium caseinate, P-Lg, P-lactoglobulin, WPI; whey protein isolate; EA, egg albumen; WC, wheat gluten; LAC, lactic acid casein;

RC, rennet casein; WCI, wheat gliadin; FMP, fish myofibrillar protein;

FMP',

fish myofibrilla plasticized with Cly at 1: 1; S, soluble

starch; Cly, glycerol; PC, propylene glycol; Sor, sorbitol; Suc, sucrose; PEG, polyethylene glycol;

xylitol; PPG, polypropylene glycol;

EG, ethylene glycol; DEG, diethylene glycol; TEC, triethylene glycol; TEEC, tetraethylene glycol; DECMET, diethylene glycol

monomethyl ether; LL, linoleic acid; LN, linolenic acid; OA, oleic acid; PO, peanut oil; CO, corn oil; SO, salad oil; CL, cod liver;

MMM, middle-melting milkfat; HMM, high-melting milkfat; LPL, lysophospholipid.

1, Siew

et a/.,

1999; 2, Sothornvit and Krochta, 2000; 3, Sothornvit and Krochta, 2001

;

4, Shaw

a/.,

2002; 5, McHugh

eta/.,

1994;

6, McHugh and Krochta, 1994a; 7, Cennadios

etal.,

1996; 8, Cherian

etal.,

1995; 9, Chick and Ustunol, 1998; 10, Parris and Coffin,

1997; 1 1, Sanchez

etal.,

1998; 12, Cuq

etal.,

1997; 13, Tanaka

etal.,

2001

;

14, Handa

etal.,

1999; 15, Cao and Chang, 2002;

Mechanical properties

Similar to polysaccharide-based films, protein-based films are brittle without plasti-

cizer. Comparing Tables 23.3 and 23.4, it can be seen that protein-based films gener-

ally have lower TS than polysaccharide-based films.

with polysaccharide films,

increasing the plasticizer content generally has a large impact on film mechanical

,nnovations in

Food

Packaging

properties by reducing TS and EM and increasing %E. However, PG-plasticized P-Lg

films were very brittle and did not experience any change in mechanical properties

with increasing PG content (Sothornvit and Krochta, 2001). This may have been a

result of less interaction with P-Lg due to the lower polarity of PG compared to the

other plasticizers in the study. This was in agreement with PG-plasticized MC films

(Sothornvit and Krochta, 2001). The overall effect of plasticizers on mechanical prop-

erties of P-Lg films can be ranged, from the greatest to the least effect, as Gly, Sor,

Suc, PEG 200, and PEG 400, respectively (Sothornvit and Krochta, 2001). TS and

EM with each of the five plasticizers exhibited negative exponential dependence on

plasticizer content (EM

1500 eCkEMX or TS

37.28 eekTSX, X

plasticizer con-

tent). Plasticizer efficiency was defined by Sothornvit and Krochta (2001) for the first

time in terms of kEM, kTs and kE determined from the fitted EM, TS and %E data,

respectively. Moreover, the effect of plasticizer related to the size (M,), shape, and

number of plasticizer oxygen atoms was elucidated. Smaller Mw plasticizers and

straight-chain plasticizers were most effective

i.e., Gly and PEG 200 were the most

efficient plasticizers. Related work has shown an exponential relationship between the

mechanical properties and OP of P-Lg films that included a ratio of the mechanical

and OP plasticizer efficiencies defined as kMo (Sothornvit and Krochta, 2000a). Gly

has also been found to be the most efficient plasticizer in WPI films (McHugh and

Krochta, 1994b; Shaw

al., 2002) and peanut protein films (Jangchud and Chinnan,

1999). PEG 400-plasticized egg-white (EA) films possess greater TS and %E than

Sor- and Gly-plasticized films (Gennadios

al., 1996). Sor is required in higher

amounts than Gly to achieve similar mechanical properties, due to the larger size (i.e.

lower efficiency) of Sor (Gennadios

al., 1996), which is in agreement with plasti-

cized-P-Lg films (Sothornvit and Krochta, 2001).

Other research has studied Gly-Suc and Gly-Sor mixtures of plasticizers to improve

wheat gluten (WG) film properties (Cherian

al., 1995). Here, Gly alone with WG

gave the largest film

WVP

and %E, but combining Suc or Sor with Gly decreased

WVP

with little effect on film TS, as shown in Table 23.4. Addition of Sor had less

effect on reducing

WVP

than Suc, but gave less reduction in %E. Results with lactic

acid casein (LAC) and rennet casein (RC) confirmed that Sor-plasticized

films

had

better water-barrier properties, while Gly-plasticized films had better elongation

(Chick and Ustunol, 1998). PEG 400 plasticized-zein films had greater flexibility than

Gly-plasticized films, because Gly tended to migrate to the zein film surface within a

few hours of preparation (Parris and Coffin, 1997). If plasticized with the more

hydrophobic plasticizer, polypropylene glycol (PPG), zein films remained brittle.

However, a mixture of PPG-Gly produced flexible zein films (Parris and Coffin, 1997).

A synergy between PPG and Gly, perhaps related to the methyl side-chain of PPG that

reduced chain-to-chain interaction to enhance film elongation, did not occur with

PEG-Gly. Wheat gliadin (WGI) film plasticized with the same level of glycol of

increasing Mw showed increase of film elongation (Sanchez

al., 1998). The most

effective plasticizers for WGI films in terms of increase in %E were Gly and tetraeth-

ylene glycol. However, the study concluded that the plasticizer effect depends on the

nature of the plasticizer and the polymer, and the same trend in mechanical properties

might not be found for different polymer-plasticizer pairs.

Plasticizers in

edible

films

and

coatings

423

Incorporating fatty acids and other lipids was found to improve the tensile strength

of fish myofibrillar protein (FMP) films (Tanaka

al., 2001), egg albumin (EA) films

(Handa

al., 1999), and soybean films (Cao and Chang, 2002). Unsaturated fatty

acids (linoleic acid, LL; linolenic acid, LN), except for oleic acid (OA), decreased the

TS of FMP films; however, fatty acid content did not exhibit any change in film TS

(Tanaka

al., 2001). Beeswax and saturated fatty acids (palmitic acid, PA; stearic

acid, SA), except lauric acid (LA), decreased the TS of FMP films (Tanaka

al., 2001).

It was observed that increasing the M, of saturated fatty acids dramatically reduced

the film flexibility. Edible oils (peanut oil, PO; corn oil, CO; salad oil, SO; and cod

liver oil, CL) decreased TS as well. It can be concluded from this study that fatty acids

and other lipids produce flexible films with a certain level of lipids. Generally, the

main purpose of using fatty acids and other lipids is for a water barrier.

Permeability properties

Protein-based films have polar hydrophilic characteristics; consequently, they have

poor water vapor barrier properties but good oxygen-, aroma-, and oil-barrier proper-

ties. Increasing the plasticizer content of protein films increases permeability. At sim-

ilar plasticizer contents, sodium caseinate (SC) films plasticized with PEG 400 had

higher

than films plasticized with Gly (Siew

al., 1999). The effects of various

plasticizers on the OP of P-Lg films using different bases of comparison have been

studied. The bases of comparison used were mole of plasticizer multiplied by the

number of oxygen atoms per mole of P-Lg (Figure 23.la); mole of plasticizer per

mole of P-Lg (Figure 23.lb); and mass of plasticizer per mass of P-Lg (Figure 23.1~)

(Sothornvit and Krochta, 2000b). Plasticizer size, shape, and number of plasticizer

oxygen atoms were shown to have a significant effect on plasticizer efficiency (kop).

The relationship between OP

data

and plasticizer amount was linear on all three bases

(Figure 23.1). Plasticizer efficiency for OP (kop) of P-Lg films, from low to high effi-

ciency values, is in the following order for all bases: Suc

Sor

PG, Gly

PEG

200

PEG 400. PG and Gly did not give any significant difference in OP, likely due

to their similarity of plasticizer structure and hygroscopicity. Sor possesses a larger

size and lower hygroscopicity than Gly, resulting in lower OP values and correspond-

ingly higher tensile properties (TS and EM). PEG 400-plasticized P-Lg films exhibited

higher OP than PEG 200-plasticized films, reflecting their larger and less efficient

size. The relationship between OP and the mechanical properties of P-Lg films was

expressed in terms of plasticizer efficiency ratios (kMo

kEdkoP or kTSJkoP), which

were used in a negative exponential correlation (EM

1500 e-(k~&~p)~~ or

37.28 e-(kz@op)~~). Suc- and Sor-plasticized f3-Lg films were the best oxygen

barriers. High plasticizer efficiency ratios are desirable because they reflect a plasti-

cizer's ability to improve film flexibility while having little effect on OP (Sothornvit

and Krochta, 2000b).

In other studies comparing plasticizers at similar content in films, Gly gave higher

in WPI films (McHugh

al., 1994; Shaw

al., 2002), LAC and RC films

(Chick and Ustunol, 1998), and FMP films (Cuq

al., 1997). Gly also gave higher

424

Innovations

Food Packaging

0 200 400 600 800 1000 1200

(a) Mole plast-0-atomlmole P-Lg

0 20 40 60 80 100 120 140

(b)

Mole plastlmole P-Lg

0 0.5

1.5 2 2.5

(c)

Mass plastlmass P-Lg

Figure

23.1

Effect of plasticizer on oxygen permeability of P-Lg films based on (a) mole plasticizer-

oxygen-atom/mole P-Lg, (b) mole plasticizer/mole P-Lg, and (c) mass plasticizer/mass P-Lg. The symbols

represent the data sets and the lines represent the empirical fitting results with zero intercept. Reprinted