Jung Han. Innovations in Food Packaging

Подождите немного. Документ загружается.

Lipid-based edible

films

and coatings

365

Common

name

Capric

Lauric

Myristic

Palmitic

Stearic

Oleic

Linoleic

Linolenic

Arachidonic

Behenic

Systematic name

Decanoic

Dodecanoic

Tetradecanoic

Hexadecanoic

Octadecanoic

9-Octadecanoic

9,12-Octadecadienoic

9,12,15-Octadecatrienoic

5,8,11,14-Eicosatetraenoic

Docosanoic

Carbon

atoms

Double

bonds

m.p.

Major

occuning

(OC)

natural oils

and

fats

31.3

Palmae seed

fit,

milk fat

43.9 Coconut oil

54.4 Butter, coconut oil, palm oil

62.9 Palm oil, butter, lard, tallow

69.6 Tallow, cocoa butter, lard, butter

16.3

Olive, peanut, lard, palm, tallow, corn, rapeseed, canola

-5

Soybean, safflower, sunflower, corn, cottonseed

-1 1 Soybean, canola

-49.5 Lard, tallow

Peanut, rapeseed

their chain lengths, the numbers of double bonds, and their melting points. Most fatty

acids derived from vegetable oils are considered GRAS substances and are commonly

used in the preparation of edible films and coatings (Hernandez, 1994; Baldwin

al.,

1997). The properties of fatty acids and of lipids derived from them are markedly

dependent on their physical state, chain length, and degree of saturation. Unsaturated

fatty acids have a significantly lower melting point than saturated fatty acids of the

same chain length. For example, the melting point of stearic acid is 69.6"C, whereas

those of oleic, linoleic, and linolenic acids are 16.3, -5, and

-1

1°C, respectively.

Chain length also affects the melting point of fatty acids, as shown in Table 21.3.

Generally, the melting points of fatty acids increase with chain length and decrease

with the number of double bonds. The water vapor permeability of fatty acid films is

dependent on the degree of saturation and the chain length of the fatty acids.

Resins are represented by shellac, wood rosin, and coumarone indene, and these are

the main coating components used to impart gloss to products (Hagenmaier and

Baker, 1994, 1995). Shellac resin, which is secreted by the insect Laccifer lacca, has

been used as a varnish and as edible coatings for pharmaceuticals, confectionery,

fruits, and vegetables. Shellac is composed of a complex mixture of aliphatic alicyclic

hydroxy acid polymers, such as aleuritic and shelloic acids (Griffin, 1979). It is solu-

ble in alcohols and in alkaline solutions, and it is also compatible with most waxes,

resulting in improved moisture-barrier properties and increased gloss for coated prod-

ucts. Its melting point ranges from 115 to 120°C (Martin, 1982). Shellac is not a

GRAS substance, and is therefore only permitted as an indirect food additive in food

coatings and adhesives.

Most natural waxes, such as beeswax, carnauba wax, and candelilla wax, also have

emulsifying properties, as they are long-chain alcohols and esters (E3aldwin

al., 1997).

Other compounds added to formulations of the coating materials include plasticizers,

emulsifiers, lubricants, binders, de-foaming agents, and formulation

aids.

The common

lipid compounds and additives permitted for use as components in the preparation of

edible films and coatings have been listed by Hernandez (1994) and Baldwin

al.

(1997). The choice of the material is mainly dependent on the target application.

366

Innovations in Food Packaging

When lipid-based edible films and coatings are a part of the food product and are

consumed with its contents, it is not only important that the films or coatings be com-

patible with the product, but also that the edible films or coatings are benign from a

sensory standpoint.

Two forces are driving the interest in natural lipid sources for coating foods; a con-

sumer interest in natural products, and regulatory restrictions regarding the use of

petroleum-based products. For instance, petroleum-based waxes, such as polyethyl-

ene and parailk, are restricted or banned in some countries, including Norway, the

United Kingdom, and Japan (Baldwin, 1994). Most likely, the naturally derived waxes,

such as beeswax, carnauba wax etc., are considered more acceptable in the food indus-

try. The same reasoning applies to oils, with mineral oil being replaced by vegetable-

based oils. Resins are also under inspection, and have been banned in some countries

(Hernandez, 1994).

Preparation

Generally, edible films and coatings are made from a solution or a dispersion of the

film-forming agents, followed by a film-forming application method such as casting,

spraying, dipping, extrusion, falling film enrobing, etc. Edible films contain at least one

component with a high molecular weight polymer such as protein or polysaccharide,

particularly if a self-supporting film is desired. Long-chain polymeric structures are

required to yield film matrices with appropriate cohesive and tensile strength when

deposited from a suitable solvent (Banker, 1966). The solvent system used for film-

forming is also important in determining the properties of the finished films, and for

edible films and coatings it is limited primarily to water, ethanol, or a combination of

the two.

Environmental conditions during film formation can markedly influence the final

film properties. Application of

warm

film-forming solutions to a

warm

receiving surface

yields the most cohesive films. However, excessive temperatures resulting in an exces-

sive rate of solvent evaporation during

film

drylng may prematurely immobilize polymer

molecules, before they have an opportunity to coalesce into a continuous, coherent film,

possibly resulting in a brittle film (Banker, 1966). This may also result in film defects

such as pinholes or non-uniform film thickness, both of which increase water vapor or

gas permeability. Since solvents evaporate rapidly from atomized coating suspensions,

the potential for premature immobilization of polymer chains is greater in spray-

formed films than in films formed by casting or dipping (Kester and Fennema, 1986).

Another important factor is the relative humidity. Low relative humidity during film

drying may increase the rate of solvent removal, probably resulting in brittle films,

whereas high relative humidity may delay the drying time.

In contrast to the self-supporting films, edible coatings have been generally used as

protective coatings for fresh fruits and vegetables to prevent the decrease in turgor and

weight and to maintain high quality during commercialization (Avena-Bustillos

al.,

1994). The most well known and oldest coating method was the application of natural

waxes and lipid coatings on specific fruits and vegetables to reduce dehydration and

Lipid-based edible films and coatings

367

abrasion during processing and handling, and to improve appearance by adding gloss

(Baldwin,

1994;

Hagenmaier and Baker,

1994;

Baldwin

al.,

1997).

Limitations to

their use include their poor mechanical properties and oily appearance in some

products. Composite films and coatings have been developed to combine the advan-

tages of both lipid and hydrocolloid components (Baldwin

al.,

1997;

Krochta

and De Mulder-Johnston,

1997).

The lipid component in the formulation can serve as

a good barrier to water vapor, while the hydrocolloid component can provide a selec-

tive barrier to oxygen and carbon dioxide and the necessary supporting matrix

(Guilbert,

1986;

Kester and Fennema,

1986;

Baldwin,

1994;

Wong

al.,

1994;

Baldwin

al.,

1997).

Generally, two kinds of composite films are known, according to their preparation

method

lamination or emulsion. One is a bi-layer film in which a hydrophobic lipid

layer is laminated over a preformed hydrophilic

film,

resulting in the lipid being a distinct

layer within or atop the hydrophilic

film

(Park

al.,

1994;

Gontard

al.,

1995;

Weller

al.,

1998).

The other is an emulsion film in which the lipid material is uniformly dis-

persed throughout the hydrophilic film (McHugh and Krochta,

1994c;

Shellhammer and

Krochta,

1997;

Rhim

al.,

1999).

Figures

21.1

and

21.2

show the procedures for the

preparation of typical bi-layer and emulsion composite films, respectively.

Both bi-layer and emulsion films offer advantages. The laminate films are easier to

apply with regard to the temperature, due to the distinct nature of the support matrix

and lipid (Koelsch,

1994).

During the casting of the lipid onto protein or polysaccha-

ride film, the temperature of the film and lipid can easily be controlled separately.

Film forming solution (1 0% whey protein)

Protein denaturation

heating at 90°C

for

min

Add lipid materials (camauba, candelilla or beeswax)

Coarse homogenization (high-shear probe mixer)

1st

minat 15000rpm

2nd

min at

000 rpm

(while heating at 80 or 90°C)

Fine homogenization (microfluidizer)

passed at

MPa

(while heating at 80 or 90°C)

Cooling in an ice bath

Casting

Drying over

h at

4050°/.

RH,

22-25°C

Whey protein emulsion film

Fipm

21.1

Procedure for the preparation ofwhey protein emulsion films (Shellhammer and Krochta,

1997).

368

Innovations in Food Packaging

21@

Procedure for the preparation of bi-layer zein-wax films (Weller

a/.,

1998).

When producing the emulsion films, the temperature of the emulsion must be above

the lipid-melt temperature but below the temperature for gelation and solvent volatiliza-

tion of the structural network. The main disadvantage of bi-layer films, however, is

that the preparation technique requires at least three steps during manufacture instead

of only one or two in the case of composite films prepared from emulsion. This is why

the laminated films are less popular in the food industry despite their good barrier

against water vapor (Debeaufort and Voilley, 1995). The preparation of the emulsion

films requires only one casting and one drying stage, but the finished films are still

rather poor barriers against water vapor, as the water molecules continue to permeate

through the non-lipid phase. Although emulsion films are not such effective barriers

as bi-layer films, they possess superior mechanical properties (Greener and Fennema,

1989a; Avena-Bustillos and Krochta, 1993; Fairley

al.,

1997).

The technique of lipid-based edible

film

preparation method should be adapted to the

application of the edible coatings. Generally, the use of an aqueous solution, colloidal

Lipid-based edible films and coatings

369

dispersion or emulsion of the film-coating materials with a relatively high concentra-

tion is preferred. The application and distribution of the film-coating material in a liq-

uid form can be achieved by (Guilbert, 1986):

Hand spreading with a paint brush

Spraying

Falling

film

enrobing

Dipping and subsequent dripping

Distribution in a revolving pan (pan coating)

Bed fluidizing or air brushing.

When considering possible edible coating applications, attention must be paid to the

requirement that edible coating formulations have to be wet when spread on the food

surface and upon drying form a film coating that has adequate adhesion, cohesion,

and durability to function properly. These properties are influenced by both edible film

formulation, and methods of coating and drying. Lack of attention to these facts has

resulted in inconsistent and unsatisfactory results in many studies. In addition, edible

coatings must provide a satisfactory appearance, aroma, flavor, and mouthfeel.

Selecting the best application for the appropriate foods and good control of environ-

mental conditions is necessary to ensure microbial stability.

Physical

properties

The intended use of edible films and coatings requires a clear understanding of their

moisture barrier and mechanical properties. These properties depend strongly on the

film composition, its formation, and the methods of application onto the food prod-

ucts (Cuq

al., 1995; Debeaufort

al., 1998).

Many researchers have examined the water vapor permeability

(WVP)

and the

mechanical properties of composite films made from proteins or polysaccharides with

added lipids. For example, composite protein-lipid films had lower WVP values than

control protein films from caseinates (Avena-Bustillos and Krochta, 1993), whey pro-

tein (McHugh and Krochta, 1994a, 1994b; Bannerjee and Chen, 1995; Pkrez-Gago and

Krochta, 1999), zein (Weller

al., 1998), and wheat gluten (Gennadios

al., 1993;

Gontard

al., 1994). Accordingly, the barrier against water vapor was improved with

composite polysaccharide-lipid films made from methylcellulose (MC) (Greener and

Fennema, 1989a, 1989b; Kester and Fennema, 1989a, 1989b; Koelsch and Labuza,

1992; Debeaufort and Voilley, 1999, and hydroxypropyl methylcellulose (HPMC)

(Kamper and Fennema, 1984a, 1984b; Hagenmaier and Shaw, 1990).

Morillon

al.

(2002) listed the WVP of lipid-based edible films as a function of

their composition and structure, and part of their list is shown in Table 2 1.4. While

some of the lipid and food-grade hydrophobic substances (principally waxes) have

WVP values close to those of synthetic plastic films, such as low-density polyethyl-

ene (LDPE) or polyvinyl chloride (PVC), they are nonetheless more permeable to

370

Innovations in Food Packaging

moisture transport

at least an order of magnitude. For the sake of comparison, the

WVP

of LDPE (25 pm thick, 38°C and 9010%

gradient) is 0.0722

10-l2 to

0.0972

10-l2 gdm2 s Pa and that of PVC (25 pm thick, 38°C and 9010%

gra-

dient) is 0.0007 to 0.0024

0-l2 g rn/m2 s Pa (Morillon

al.,

2002). Each

Films

T.bk

21.4

Water

vapor

pennclbility

(wvp)

lid-bad

sdi

fib

Monolayer

films

Paraffin

wax

Candelilla wax

Camauba wax

glycerol monostearate

Microcrystalline wax

Beeswax

Capric acid

Myriaic acid

Palmitic acid

Stearic acid

Shellac

Hydrogenated cottonseed oil

Hydrogenated palm oil

Hydrogenated peanut oil

Native peanut oil

Bilayar

films

MCIparaffin

wax

MC/paraffin oil

MC/ beeswax

MC/camauba wax

MC/candelilla wax

MC/triolein

MCIhydrogenated palm oil

HPMC/stearic acid

Emulsion

films

PEG400

behenic caid

triolein

hydrogenated palm oil

PEG400

myristic acid

Wheat gluten

oleic acid

Wheat gluten

soy lecithin

Wheat gluten

paraffin

wax

Wheat gluten

paraffin oil

Wheat gluten

triolein

Wheat gluten

hydrogenated palm oil

Na-caseinate

acetylated monoglyceride

Na-caseinate

lauric acid

Na-caseinate

beeswax

Whey protein isolate

palmitic acid

Whey protein isolate

steaiyl alcohol

Whey protein isolate

beeswax

From Morillon

eta/.

(2002).

. .

gradient

(%I

Lipid-based edible films and coatings

371

hydrophobic substance has its own physicochemical properties, and thus edible films

based on lipids have variable behaviors against moisture transfer. Generally, the

WVP

decreases with increasing hydrophobicity of the added lipid materials. Waxes are the

most efficient substances to decrease the

WVP

because of their high hydrophobicity

due to their high content of long-chain fatty alcohols and alkanes. The hydrophilic

groups of lipid molecules normally promote water vapor sorption, which may bring

about water vapor migration through the film. In addition, many other factors inside

or outside the lipid-based edible films affect its barrier properties.

lnfluence of

film

preparation methods on

The structure and barrier properties of lipid-based films strongly depend on the film

preparation method (Kamper and Fennema, 1984a; Martin-Polo

al., 1992;

Debeaufort

al., 1993). The barrier efficiency of bi-layer films is in the same order

of magnitude as that of pure lipid or synthetic plastic films, and much lower than that

of emulsion films (Table 21.4). The structure and the state of the surface of the film

mainly depends on the film preparation technique. Debeaufort

al. (1993) used scan-

ning electron microscopy (SEM) to characterize the surfaces of both emulsified and

bi-layer films composed of MC and parafi wax. The emulsified films had an irregu-

lar surface and a heterogeneous structure. In this type of film, paraffin wax was dis-

persed in the form of globules in the MC matrix. In contrast, the lipid component of

the bi-layer or laminated film had a smooth surface and homogeneous structure. They

determined the

WVP

of both types of films, and found that the more homogeneously

the paraffin wax was distributed, the more the barrier efficiency increased. Nevertheless,

Kamper and Fennema (1984a) reported opposite results with both bi-layer and emul-

sion films composed of HPMC and stearic and palmitic acids. Their emulsion films

had 40 times lower permeability than the bi-layer films. These contradictory results

could be explained by considering film preparation conditions. Although the initial

emulsion prepared by Kamper and Fennema was homogeneous, a phase separation

was observed during drying that led to an apparent bi-layer structure in the emulsified

films. Such reorientation of lipid molecules during drylng and storage of the films

greatly influences the

WVP

of lipid-based composite films.

some cases, the

WVP

lipid-based edible films varies greatly with the orientation of the lipid molecules within a

film (Avena-Bustillos and Krochta, 1993; Kamper and Fennema, 1984a).

emulsion

films, water migrates preferentially through the continuous hydrophilic matrix and the

dispersed lipid phase only modifies the apparent tortuosity. McHugh and Krochta

(1994b) reported that lipid particle size and distribution significantly affected the

WVP

of whey proteinheeswax emulsion films. They found that as mean emulsion

particle diameters decreased, the

WVP

of the film decreased linearly.

lnfluence of fatty acids

Most fatty acids derived from vegetable oils are considered to be GRAS substances,

and they are commonly used

the preparation of lipid-based edible films and coatings.

372

Innovations in Food Packaging

The properties of fatty acids are markedly dependent on their chain length and on

their degree of saturation. As stated earlier, unsaturated fatty acids have a lower melt-

ing point than do saturated fatty acids of the same length (Table 21.3).

Similarly, the water vapor permeability of fatty acid films is dependent on the

degree of saturation and the chain length of the fatty acids. As chain length increases,

chain mobility decreases, making fatty acids with long chains good barriers to water

vapor transmission (McHugh and Krochta, 1994a;

Rhim

al.,

1999). On the contrary,

fatty acids with shorter chain lengths tend to have greater chain mobility within

the

structure of composite films, resulting

higher water vapor permeabilities (McHugh

and Krochta, 1994~). Although fatty acids such as stearic and palmitic acid lack the

structural integrity to form continuous coatings, they have been used successfully in

formulations for composite films. Furthermore, composite films containing unsatu-

rated fatty acids have been shown to be more permeable to water vapor than those

containing saturated fatty acids (Kamper and Fennema, 1984a). The poor barrier

properties of unsaturated fatty acids are a result of the expansion

the molecular vol-

ume that occurs with the introduction of a double bond, and of the lower melting point

with greater chain mobility (Greener and Fennema, 1992).

Reportedly, fatty acids showed a pronounced effect on the

WVP

of emulsified com-

posite films with fatty acids and

support matrix (Figure 21.3). Stearic acid was

the most effective in decreasing the

WVP

of the film. The low permeability of the com-

posite films containing stearic acid was explained by the fact that stearic acid forms

an interlocking network; the effect of this network is significant enough to retard

moisture transfer more effectively than those based on, say, myristic acid, which has

a shorter chain length, or behenic acid, which has such long chains that they hinder

Capric Myristic Palmitic PalIStea Stearic SteaIArach Arach Behenic

Fatty acid within methyl cellulose matrix

Figure

213

Water vapor permeability as a function of the fatty acid used (at a concentration of

30%)

in emulsion barriers with a methyl cellulose support matrix (from Koelsch,

1994).

Lipid-based edible

films

and coatings

373

the formation of a tightly interlocking network. In essence, stearic acid provides the

optimum chain length without hindering the formation of an interlocking network.

However, this is not always the case. Tanaka

al.

(2000) reported that among the lipid

materials tested for preparation of composite films with fish water-soluble proteins,

the incorporation of oleic acid was the most effective from the standpoint of the over-

all properties (tensile strength, elongation at break, and WVP) of the films.

Rhim

al.

(1999) showed that palrnitic acid was more effective in decreasing the WVP of emulsi-

fied soy protein isolate films with fatty acids. The effect of fatty acids on the perme-

ability properties of fatty acid-based composite films depended on the fatty acid

characteristics, and on the interactions between the fatty acid and the hydrocolloid

structural matrix.

Influence of the concentration of lipids

Lipid concentration is important in controlling the WVP of lipid-based edible films.

Hagenmaier and Shaw (1990) tested the effect of stearic acid concentration on the

WVP of HPMC composite films (Figure 21.4). The WVP of the composite films

decreased about 300 times with the addition of 40-50% of stearic acid. With less

stearic acid, the permeability was higher. The permeability was sharply dependent on

the stearic acid content and decreased slowly with stearic acid concentration increases.

The phenomenon of a

WVP

decrease in non-linear fashion was also observed with

whey protein-lipid emulsion films (Shellhammer and Krochta, 1997). These results

indicate that a critical concentration exists beyond which a sharp decrease in film

WVP

occurs. Excessive levels of lipid materials result in the film becoming brittle.

Stearic acid concentration

(%)

Figure

21.4

Effect of fatty acid concentration on

WVP

of hydroxypropyl methylcellulose composite films

(from Hagenmaier and Shaw,

1990).

374

Innovations in Food Packaging

Therefore, the optimum concentration of lipid material in the preparation of lipid-

based edible films should be determined by considering both the effect of decreasing

the

WVP

and the physical strength of the films. As shown in Figure 21.4, stearic acid

concentration of about 20% seems to be the optimum for modifying the water vapor

barrier properties.

Rhim

al. (1999) also found that the incorporation of 20% fatty acids

was most effective from the standpoint of the overall properties, including the

WVl?

Influence

film

thickness

In contrast to hydrophobic, synthetic, polymeric films, the

WVP

of films prepared

from biopolymer materials depends on the thickness of the film (McHugh

al.,

1993). The same is true with lipid-based edible films, but to a much lesser extent. The

"thickness effect" is explained by the hydrophilic nature of most biopolymers. The

difisivity of moisture in these materials is moisture dependent; therefore at high rel-

ative humidities the material has a much higher moisture content and hence greater

difisivity. Thus as the film thickness increases, the equilibrium relative humidity of

the film on the moisture side increases and the

WVP

increases.

With multicomponent films, such as lipid-biopolymer emulsions, the same issues

exist. In addition, the state of the lipid phase will affect the

WVl?

In emulsion films

the lipid phase is rarely completely stable, and thus during drying of the film there

exists some degree of phase separation. The phase-separated layer contributes sub-

stantially to the decrease in

WVP.

Therefore, as the film thickness increases the

amount of potential phase-separated material increases, thereby improving the resist-

ance of the overall film. Hagenmaier and Shaw (1990) observed this phenomenon

with

HPMC

composite films containing 42

3% of stearic acid (Figure 21.5). The

0 5 10 15 20 25 30 35 40

Film

thickness

(pm)

Figure

21.5

Effect of film thickness on the

WVP

of hydroxypropyl methylcellulose composite films

containing

42%

stearic acid (from Hagenmaier and Shaw,

1990).