Jung Han. Innovations in Food Packaging

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Emulsion and bi-layer edible films

395

films containing stearic acid exhibit more micro-dispersed acid globules and a more

complex interlocking network than do the films formulated with other fatty acids.

This complexity probably resulted in an increased tortuosity and path distance for a

water molecule to travel through the system, giving lower WVP (Krochta, 1997a).

The degree of saturation of fatty acids also affects the moisture-barrier properties

of emulsion films, since they are more polar than saturated lipids. Rhim

al. (1 999)

observed an increase in WVP for soy protein isolate (SP1)-fatty acid emulsion films

as the degree of saturation decreased. Emulsion films prepared with oleic acid (OA)

had higher WVP than saturated SA. Similarly, their results showed a decrease in WVP

as chain length and fatty acid content increased, with a reduction of about 260% for

films with 40% SA (Table 22.7). The mechanical properties of emulsified SPI-fatty

acid films were also affected by the amount of fatty acid incorporated into the film, the

chain length, and the degree of saturation. Tensile strength decreased when adding

fatty acids up to 20%, then decreased with the addition of fatty acids to more than 20%;

whereas elongation decreased as fatty acid concentration increased. Unsaturated OA

showed the lowest tensile strength and the highest elongation.

Shellhammer and Krochta (1997a) observed that WVPs of dispersed-lipid films

made with WPI and Gly plasticizer did not correlate with the WVPs of the pure lipid

components but with the viscoelastic properties of the lipids. Emulsion films were

prepared with candelilla wax (CanW), carnauba wax (CarW), beeswax

(BW),

and

hard anhydrous milk fat fraction (HAMFF). CanW and CarW, the materials with the

lowest WVPs, gave emulsion films with the highest WVPs; BW and HAMFF, the

materials with the highest WVPs, allowed formation of emulsion films with the high-

est lipid content and the lowest WVPs (Table 22.6). Interestingly, these results corre-

spond with the viscoelastic properties of the lipids and suggest that the BW and

HAMFF may have yielded more easily to the internal forces related to shrinkage of

the drying protein structure, thus preventing film breakage with a high lipid content.

The large drop in film WVP at 40-50% BW or HAMFF content could be explained

by formation of an interconnecting lipid network within the film, which could be

made possible by deformation of BW and HAMFF due to film internal stress as water

evaporates from the film-forming solution.

In spite of a continuous reduction in WVP in emulsion films as the lipid content

increases, some studies have found a critical volume fraction beyond which the bar-

rier properties of the emulsion films did not improve or even got worse. For instance,

PEG emulsion films showed a minimum WVP with about 14% SA content,

beyond which

WVP

increased (Sapru and Labuza, 1994). Wheat gluten (WG)-based

emulsion films also showed a decrease in WVP with increasing lipid content.

However, the effect depended on the lipid hydrophobicity, the melting temperature

and degree of unsaturation, and the interactions and organization of WG with the

lipids in the film (Gontard

al., 1994).

McHugh and Krochta (1994b) observed that, given a constant volume fraction of

beeswax, decreasing the lipid particle size significantly correlated with a linear decrease

in film

WVP

of WPI-based emulsion films (Table 22.8). This result could be explained

by the presence of a large number of lipid particles uniformly dispersed in the system,

which

(1)

increased the tortuous migration path length of water molecules diffusing

396

Innovations in Food Packaging

through the composite film, andlor (2) immobilized proteins at the lipid interfaces,

with a resulting more ordered and tightly cross-linked structure with lower perme-

ability. However, these conclusions were somewhat tentative due to the changing lipid

distribution within the film as particle size changed (i.e. large lipid particle sizes induced

unstable emulsions versus small lipid particle sizes that induced stable emulsions). In this

work, the mean lipid particle diameter ranged between approximately 0.9 and 2.0 Fm.

Measurement

WVP

revealed some lipid phase separation during drymg, because dif-

ferent

WVP

values were obtained depending on whether the film upper (lipid-enriched)

side was oriented toward the high or low

environment. In order to further eluci-

date the effect of particle size in determining the performance of whey protein-lipid

emulsion films, PCrez-Gago and Krochta (2001) studied the effect of lipid particle size

WVP

and mechanical properties for emulsion films

with

a low and

high

lipid content,

and at different drylng temperatures.

this work, it was shown that the effect of lipid par-

ticle size on film

WVP

and mechanical properties was influenced by lipid content and

film orientation during

WVP

measurements (Table 22.8). As lipid content increased,

a decrease in lipid particle size reduced the

WVP

of the WPI-BW emulsion films,

probably due to an increase in protein immobilization at the lipid-protein interface as

lipid content became more important in the film. This effect seemed to be supported by

the increase in film tensile strength. For those films that showed lipid phase separation

during drying,

WVP

was not affected by lipid particle size when the enriched lipid

phase was facing the high

side. This could be attributed to the greater barrier that

the water vapor experienced when the enriched lipid phase was exposed to the high

side during the

WVP

measurement overwhelming any effect due to lipid particle size.

Sherwin

al.

(1998) showed an effect of fatty acid type on the film microstructure

of microfluidized whey protein-fatty acid emulsion films, which could explain some

of the functional property differences of WPI-fatty acid films. Emulsions of saturated

Table

22.8

Water vapor permeabilities of emulsion WPI-lipid edible films

. .

size

(pm)

McHugh and Krochta (1 994b) WPI

Sor (4: 1)

BW:WPI:Sor(2:4:1) 0.83

BW:WPl:Sor(2:4:1) 1.40

BW:WPl:Sor(2:4:1) 1.95

Perez-Gago and Krochta (2001)

BW: WPI

Cly (6: 3

1) 0.54

BW:WPl:Cly(6:3:1) 1.01

BW:WPl:Gly(6:3:1) 1.82

BW: WPl

Gly (6

1) 2.28

BW:WPl:Gly(6:3:1) 0.54

BW:WPl:Gly(6:3:1) 1.01

BW:WPl:Gly(6:3:1) 1.82

BW:WPl:Gly(6:3:1) 2.28

Test conditionsb

acornpositions rounded to nearest whole number.

RHs on top and bottom sides of film (top/bottom).

WPI, whey protein isolate; Sor, sorbitol; Gly, glycerol; BW, beeswax.

Emulsion and bi-layer edible films

397

fatty acids fiom CI4 to

C22

and WPI were prepared by microfluidization, and were

analyzed by polarized light microscopy coupled with digital image analysis. The

results showed that particle size increased with increasing fatty acid chain length, and

at least two populations of fatty acid particle size were observed in each film, which

could reflect differences in the manner in which crystals are formed during drying.

In addition to the effect of lipid particle size, Debeaufort and Voilley (1995) deter-

mined that the more homogeneous the distribution of the lipid particles, the better the

water vapor barrier of PW

PEG emulsion films. Films were prepared by adding

PW to the MC :PEG in water-ethanol solution at 75OC in the presence of different

emulsifiers, and then drying at several temperatures (2545"C), relative humidities

(1540%

RH),

and air speeds (0-21111s). Whatever the drylng conditions, a strong

destabilization of the emulsion was observed during drying, with resulting increased

mean globule diameter. In the slowest film-drying conditions, the average PW particle

diameter doubled in size from the original emulsion; with the fastest drymg, the parti-

cle diameter increased by a factor of 55. The slowest drying (low temperature, high

RH, stagnant air) gave the highest tensile strength and elongation, and the lowest

WVF?

The relatively low

WVP

for the slowest-drying films was attributed to the small

PW particle diameter, and equal PW particle distribution throughout the film. In spite of

the quite large average PW particle diameter, the fastest-drymg (high temperature, low

RH,

circulating air) film WVP was only

45%

greater than the WVP for the slowest-

drying

film. The effectiveness of films dried quickly was attributed to a high propor-

tion of large liquid paraffin globules, and enhanced creaming and coalescence due to

rapid solvent evaporation. The resulting proposed film structure was an imperfect bi-

layer, with a concentration of large coalesced PW at the film surface exposed to the

high RH conditions of the

WVP

test procedure. However, drying at a rapid rate pro-

duced some films with many bubbles and holes.

Pkroval

al.

(2002) studied the effect of lipid type on the barrier and mechanical prop-

erties of arabinoxylan-based films containing palrnitic acid, stearic acid, triolein, and

hydrogenated palm oil. In comparison with other hydrocolloid-based films, the

WVP

of emulsified films was suggested to depend mostly on the nature of the continuous

ara-

binoxylan phase, and secondarily on the lipid type.

this work, the relation between

lipid particle size and

WVP

could not be established because lipid particle size and dis-

tribution changed with lipid type. To better understand the influence of the structure of

arabinoxylan-hydrogenated

palm kernel oil (HPKO) films on their functional proper-

ties, Phan The

al.

(2002a) studied the effect of sucro-esters with different sterifica-

tion levels, used as emulsifiers, on the stabilization of the emulsified film structure.

The results showed that the addition of emulsifier had a great effect on emulsion sta-

bility, and the effectiveness of the sucro-esters depended not only on the esterification

degree, but also, and mainly, on its concentration. From their results the

WVP

and lipid

mean particle of the different films could not be correlated, because both the

hydrophobicity of emulsifiers and the film structure (lipid distribution across the film)

changed. However, emulsified films with small-sized HPKO globules homogeneously

distributed within the matrix and the less stable systems corresponding to the film

prepared without emulsifier both produced low permeability values. Therefore, the

authors concluded that an improvement in the moisture-barrier property of composite

films could be achieved by either great stability of the emulsion during

drying

and the

398

Innovations in

Food

Packaging

homogeneous distribution of very small lipid globules within the film cross section,

or destabilization of the emulsion by creaming, aggregation, andlor coalescence of the

HPKO at the evaporation surface

which leads to an apparent bi-layer film structure.

PCrez-Gago and Krochta (1999) studied the role of emulsion stability, as affected

by pH, on the final morphology and

WVP

of whey protein-lipid emulsion films. The

films were cast from aqueous solutions of WPI (5% wlw), BW, and glycerol. Phase

separation was observed at pH values different from the isoelectric point (pI

4-5)

due to large lipid particle size distributions, in spite of the electrostatic repulsion

between lipid particles charged by the adsorbed protein layer. However, no significant

differences among WVPs of emulsion films prepared at values of pH

pI were

observed. Furthermore, rather than enhanced-phase separation due to lipid particle

coalescence at the PI, phase separation was inhibited, and the film WVP was signifi-

cantly higher that at pH different from the PI. This was attributed to an increase in

emulsion viscosity at the PI, and the formation of a weak gel due to protein-protein

aggregation, which lowered lipid mobility and inhibited any phase separation.

Since lipid distribution in the matrix has been shown to affect the moisture-barrier

properties of emulsion films, some works have focused on the effect of the principal

processes that can affect emulsion stability and, as a consequence, the morphology of

the resulting film. Phan The

al.

(2002b) studied the influence of the structure of

films obtained from emulsions based on arabinoxylans, hydrogenated palm kernel oil,

and emulsifiers on their functional properties during drying. Films were prepared with

sucrose esters (emulsifier) having different HLB values, and films were dried at 30,40

or 80°C. It was found that the emulsifier had a great effect on the stabilization of the

emulsified film structure. In addition, an increase in the drying temperature affected

emulsion film stability (higher creaming and coalescence), which gave films a bi-

layer-like structure and thus reduced film WVl?

PCrez and Krochta (2001) studied the effect of the drymg temperature on the proper-

ties of whey protein-lipid emulsion films. Emulsion films were prepared by

emulsifying

Gly-plasticized WPI films with CanW, BW or AMFF. The results confirmed that a

decrease

WVP

of emulsion

films

occurred as drylng temperature increased from 40°C to

80°C.

large drop was observed in

WVP

of WPI-BW emulsion films at 20% BW con-

tent, reaching values as low as those found by Shellhammer and Krochta (1997a), even

though the plasticizer level was five times greater

than

that used by these authors. In addi-

tion, the large drop in

WVP

of the WPI-BW emulsion films shifted to lower BW contents,

compared to the 40% BW content effect shown by Shellhammer and Krochta (1997a).

This could be an indication of a change in lipid distribution in the emulsion films, cre-

ating regions of higher BW content and, as a consequence, lower film permeability.

Conclusions

Incorporation of lipid materials into polysaccharide and protein films to form edible

composite films has the potential to improve film moisture-barrier properties. The

hydrophobic lipid component can form a continuous layer over the hydrophilic

Emulsion and bi-layer edible films

399

polysaccharide and protein phase, or it can be dispersed in the hydrophilic matrix to

form dispersed-lipid films.

both bi-layer and emulsion films, the

WVP

depends

mainly on the polarity and the degree of saturation of the lipid type. This way, high melt-

ing-point fatty acids, monoglycerides, hydrogenated fats, and waxes are useful edible

lipid barriers.

Bi-layer films can be formed in one or two steps. Bi-layer films that are formed

two

steps provided the best moisture barriers. This technique requires two different cast-

ing and drying stages, and the application of the hydrophobic layer requires the use of

high temperatures or solvents. However, bi-layer films tend to delaminate, and exhibit

poor mechanical properties compared to emulsion films. In the one-step method, the

bi-layer film is formed due to the destabilization of the emulsion. This has only been

achieved using a blend of PA-SA in an aqueous-ethanolic

HPMC

solution, and dry-

ing at temperatures above the fatty acid melting point.

Emulsion composite films require only a single casting and drylng stage for film

formation. However, they give poorer moisture-barrier properties than bi-layer films.

Issues of lipid type, location, volume fraction, and drylng conditions in emulsion

composite films have been studied, as affecting the barrier properties of protein- and

polysaccharide-based emulsion films. The results show that the moisture-barrier prop-

erties of emulsion films can be improved by using viscoelastic lipids, increasing lipid

content, reducing lipid particle size, and improving film-drying conditions.

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Plasticizers in edible

films

and coatings

Rungsinee Sothornvit andJohn

Krochta

Introduction

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

Definition and purpose of plasticizers

...

Types of plasticizing

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

Theories of plasticization

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

Advantages and disadvantages of edible films and coatings

. .

Properties of edible films and coatings

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

Polysaccharide-based films and coatings

....

Protein-based films and coatings

..........

Challenges and opportunities

.........

Conclusions

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

References

lntrodu

-ion

Edible films and coatings are intended to help maintain the quality and shelf life of

food products by controlling the transfer of moisture, oxygen, carbon dioxide, lipids,

aromas, flavors, and food additives (Krochta, 1997a, 199%; Krochta and De Mulder-

Johnston, 1997). Film formation as a coating is dependent on two types of interaction;

cohesion (attractive forces between film polymer molecules) and adhesion (attractive

forces between film and substrate). Polymer properties such as molecular weight,

polarity, and chain structure are relevant to cohesion and adhesion. Cohesive forces in

polymer films can result in the undesirable property of brittleness. To overcome this

limitation, food-grade plasticizers are added to the film formulation to decrease inter-

molecular forces resulting from chain-to-chain interaction. By reducing intermolecu-

lar forces and thus increasing the mobility of the polymer chains, plasticizers lower

the glass transition temperature of films and improve film flexibility, elongation and

toughness (Banker, 1966).

Commonly used plasticizers in film systems are monosaccharides, disaccharides or

oligosaccharides (e.g. glucose, fructose-glucose syrups, sucrose, and honey), polyols

(e.g. glycerol, sorbitol, glyceryl derivatives, and polyethylene glycols), and lipids and

derivatives (e.g. phospholipids, fatty acids, and surfactants). Plasticizers are generally

Innovations

Food

Packaging

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Innovations in Food Packaging

required at approximately 1040% on

dry

basis, depending on the stiffness of the

polymer (Guilbert, 1986; Daniels, 1989). The main drawback of plasticizers is the

increase in gas, water vapor, and solute permeability that results from decreased film

cohesion (Gontard

al.,

1993).

The objectives of this chapter are:

1. To define the term "plasticizer", classify plasticizer types, and present plasticizer

theories

To summarize the advantages and disadvantages of plasticizers, plasticizers used,

and plasticized-film properties of both polysaccharide- and protein-based films

and coatings

3. To discuss the challenges and opportunities in food application and research.

Definition and purpose

plasticizers

Generally, plasticizers are defined by two purposes, which are to aid processing and

modify the properties of the final product. As a processing aid, plasticizers lower the

processing temperature, reduce sticking in molds, and enhance wetting. As a modified

final product, plasticizers increase the temperature range of usage; increase flexibility,

elongation, and toughness; and lower the glass transition temperature (Sears and

Darby, 1982). For films or coatings, there are different definitions of plasticizers

depending on the purpose of the polymer-plasticizer system. Plasticizers might be

defined as small low molecular weight, non-volatile compounds added to polymers to

reduce brittleness, impart flexibility, and enhance toughness for films. As a specific

definition for coatings, plasticizers reduce flaking and cracking by improving coating

flexibility and toughness. Generally speaking, plasticizers reduce intermolecular

forces along the polymer chains, thus increasing free volume and chain movements

(Irnrnergut and Mark, 1965; Daniels, 1989).

Types of plasticizing

Internal and external plasticization

polymer science, two types of plasticizers are defined: internal and external

(Irnmergut and Mark, 1965). Internal plasticizers are part of the polymer molecules,

which are either co-polymerized into the polymer structure, or reacted

with

the original

polymer. Internal plasticizers generally have bulky structures that provide polymers

with more space to move around and prevent polymers from coming close together.

Therefore, they soften polymers by lowering the glass transition temperature (Tg) and

thus reducing the elastic modulus. On the other hand, external plasticizers are low-

volatility substances that are added to polymers. They do not chemically react, but

they interact with polymers and produce swelling. The benefit of using external plas-

ticizers over internal plasticizers is the opportunity to select from a variety of plasticizers,

depending on the film properties desired (Banker, 1966; Wilson, 1995).