Jung Han. Innovations in Food Packaging
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Edible films and coatings from plant origin proteins
295
The protein isolate is combined with hydroxypropyl methylcellulose (HPMC) and
PEG as plasticizers to cast films. Film
WVP
increases with increasing amounts of pro-
tein isolate due to the high hydrophilic amino acid content.
In
addition, films swell by
absorbing water, showing a greater thickness with respect to the control HPMC-only
film (Ayranci and Cetin, 1995).
Grain sorghum protein
Grain sorghum ranks fifth among cereals produced in the world (Dendy, 1995). The
main protein fraction in the sorghum kernel is prolamin, known as kafirin. Kafirin is
similar to corn zein in molecular weight, solubility, structure, and amino acid compo-
sition (Shull
et
al., 1991). According to its prolamin nature, kafirin can be readily
extracted with alcohol and/or a reducing agent such as P-mercaptoethanol. The addi-
tion of P-mercaptoethanol greatly enhances extraction yield, but its toxicity makes it
useless if edibility is to be maintained. In sorghum wholegrain flour and sorghum
endosperm, the kafirin content is 68-73% and 77-82% of total protein, respectively
(Hamaker
et
al., 1995).
Films have been prepared from laboratory-extracted kafirin (89% protein) (Buffo,
1995) dissolved in ethanol, with glycerin and PEG added as plasticizers. The
dry
films
have similar TS, E and
WVP
to those of films prepared from commercial corn zein.
However, kafirin films are notably darker than zein films, presumably due to a higher
content of inherent pigments (Buffo
et
al., 1997).
Appendix
Table 17.1 lists the references relevant to each type of protein.
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Introduction
Edible coatings can potentially extend the shelf life and improve the quality of food
systems by controlling mass transfer, moisture and oil diffusion, gas permeability (02,
C02), and flavor and aroma losses. Coating formulations can be used to serve as adhe-
sive for seasonings or to improve the appearance of foods. For example, edible coat-
ings can be sprayed or dipped onto the surface of snack foods and crackers to serve as
a foundation or adhesive for flavorings (Druchat and De Mulder Johnston, 1997).
Candies are often coated with edible films to improve their texture by reducing the
stickiness (McHugh, 2000).
Active compounds (antimicrobials, antioxidants, nutrients) can be added to food
coatings in order to extend the shelf life, preserve the color, and improve the nutritional
value of foods. Edible coatings also have the potential for maintaining the quality of
foods after the packaging is opened by protecting it against moisture change, oxygen
uptake, and aroma losses (Krochta, 1997). The application of edible coatings to meat and
fish products may be produced by dipping, spraying, casting, rolling, brushing, and
foaming (Donhowe and Fennema, 1994). Coatings must have bamer properties with
regard to water vapor, oxygen, carbon dioxide, and lipid transfer, while maintaining good
color, appearance, mechanical, and rheological characteristics (Guilbert
et
al., 1996).
Composite coatings improve gas exchange, the adherence to coated products and
moisture vapor permeability properties (Baldwin
et
al., 1995). The addition of gly-
cerol, polyethylene glycol, and sorbitol can reduce film brittleness.
A
composite film
Innovations in Food Packaging
ISBN:
0-12-31 1632-5
Copyright
O
2005 Elsevier Ltd
All
rights of reproduction in any form resewed
302
Innovations in Food Packaging
containing antimicrobial or antioxidant compounds can prevent rancidity and improve
shelf life by controlling bacterial proliferation. Also, the addition of vegetable oils can
act as a moisture barrier. Sealing meats with a cross-linked sodium caseinate gel pro-
duced a juicier product by reducing drip loss; it also reduced the use of absorbent pads
and protected the color of the meat (Ben and Kurth, 1995).
Edible films as a solid sheet can be applied between the food components or on the
surface of the food system in order to inhibit moisture, oxygen, Cozy aroma, and lipid
migration. Edible films with adequate mechanical properties could conceivably also
serve as edible packaging for selected foods (Krochta and De Mulder-Johnston, 1997).
The benefits of edible films can be numerous, but some barriers to commercial
implementation have not been overcome. The raw material for much of the edible
films comes from underutilized sources, but the cost of purification can be financially
impractical. According to Krochta and De Mulder-Johnston (1997), the cost of edible
films can range from as low as $1.30/lb for soy protein isolate to $6-12Ab for whey
protein isolate. Collagen films were cited as $49-54Ab. In comparison, synthetic poly-
mers traditionally cost less than a $lAb.
Another barrier is the concept of biodegradability. According to Hunt
et
al. (1990),
inert and non-biodegradable plastic materials represent 30% of municipal solid waste.
Although, the idea of biodegradable films continues to be of research interest, such
films are not practical under the current solid-waste handling conditions. Biodegradable
films could work in a compost environment, but it is well known that even biodegrad-
able materials do not degrade well
in
landfills, where the majority of all packaging waste
material is disposed.
To be accepted, an edible film should be generally recognized as safe (GRAS), and
used within any limitations specified by the Food and Drug Administration (FDA).
Many edible films can be produced from food-grade materials, but many require sol-
vents to become soluble. Ultimately, any material that is used for direct food contact
will face regulatory scrutiny.
In
particular, biopolymers that act as carriers of additives
intended to migrate to the food for preservative effects.
In addition, another challenge for the successful use of biodegradable films such as
protein films is the stabilization of the functional properties of the films during stor-
age, and the improvement of the insolubility and mechanical properties. Sensory and
quality characteristics of films also need further improvement for successful imple-
mentation for commercial applications.
Animal-origin proteins
Proteins are good film formers exhibiting excellent oxygen, carbon dioxide and lipid
barrier properties, particularly at low relative humidity. Edible films based on pro-
teins were found to possess satisfactory mechanical properties (Kester and Fennema,
1986; Peyron, 1991; Cuq
et
al., 1998a). However, their predominantly hydrophilic char-
acter results in poor water-barrier characteristics (Kester and Fennema, 1986; Peyron,
1991; McHugh, 2000). The increase of cohesion between protein polypeptide chains
was thought to be effective for the improvement of the barrier properties of the films.
Edible
films
and coatings
from
animal-origin proteins
303
For instance, the cross-linking of proteins by means of chemical, enzymatic (trans-
glutaminase) or physical treatments (heating, irradiation) was reported to improve film
water-vapor barriers as well as the mechanical properties and the resistance to prote-
olysis (Brault
et
al., 1997; Ressouany
et
al., 1998,2000; Sabato
et
al., 2001; Ouattara
et
al., 2002a).
When cross-linked whey proteins are entrapped in cellulose, this generates insoluble
films with good mechanical properties, high resistance to attack by proteolytic enzymes,
and with decreased water-vapor permeability properties (Le Tein
et
al., 2000). The addi-
tion of lipids in the film formulations can act as a good moisture barrier (McHugh,
2000). Although interesting, protein-lipid films are often difficult to obtain. For exam-
ple, bi-layer film formation requires the use of solvents or high temperatures, making
production more costly. Furthermore, the separation of the layers may occur with
time. For films cast from aqueous lipid emulsion solutions, the process is complex and
the incorporation of the lipids is limited. However, the addition of emulsifying agents
or surfactants can improve the emulsion stability (Everett, 1989) and decrease the
mean particle diameters of the emulsion, which results in a linear decrease in water-
vapor permeability values (McHugh and Krochta, 1994).
Protein films are brittle and susceptible to cracking due to the strong cohesive
energy density of the polymer (Lim
et
al., 2002). The addition of compatible plasti-
cizers improves the extensibility and viscoelasticity of the films (Brault
et
al., 1997).
According to Ressouany
et
al. (1998), sorbitol is a good plasticizer and significantly
increased puncture resistance. However, polyethylene glycol and mannitol had lower
plasticizer effects. The addition of glycerol in the film formulation produced an
important loss of protein-water interactions (Letendre
et
al., 2002a). The presence of
plasticizers can enhance the formation of cross-links. It was observed by Brault
et
al.
(1997), and by Mezgheni
et
al. (1998), that the presence of glycerol, propylene
glycol, and triethylene glycol in film formulations enhanced the formation of cross-
links during irradiation treatments.
Composite films may be of heterogenic nature and be formed via a mixture of a
proteinlpolysaccharide/plasticizer
and a lipid. This approach allows better exploitation
of the functional properties of each of a film's components. The addition of a polysac-
charide in a film formulation could improve the moisture barrier, the resistance, and the
mechanical properties of the films (Ressouany
et
al., 1998; Letendre
et
al., 2002b). It is
believed that some polysaccharides, such as carboxymethylcellulose, alginate, and
pectin, form charge-charge electrostatic complexes with proteins (Imeson
et
al., 1977;
Shih, 1994;
Thakur
et
al., 1997; Sabato
et
al., 2001; Letendre
et
al., 2002b). Under cer-
tain
conditions, polysaccharides like pectin may form cross-links with proteins (Thakur
et
al., 1997). Heat treatments may enhance protein-polysaccharide interactions, result-
ing in a three-dimensional network with improved mechanical properties. Untreated
polysaccharides keep their orderly structure, thus preventing any favorable interactions
between their functional groups and those of the proteins (Letendre
et
al., 2002b). The
addition of starch in film formulations can improve oxygen and oil barriers (Kroger and
Igoe, 1971; Morgan, 1971; Sacharow, 1972). Alginate can reduce dehydration and
retard oxidative off-flavours in meats (Wanstedt
et
al., 1981). Pectin and chitin can
reduce bacterial growth in foods (Baldwin
et
al., 1995; Chen
et
al., 1998).
9n
A
\nnovations in Food Packaging
Milk proteins, such as whey and casein proteins, were extensively studied due to
their excellent nutritional value and their numerous functional properties, which are
important for the formation of edible films (McHugh and Krochta, 1994; Chen,
1995). Caseinates can easily form films from aqueous solutions due to their random-
coil nature and their ability to form extensive intermolecular hydrogen, electrostatic,
and hydrophobic bonds, resulting in an increase in the inter-chain cohesion (McHugh
and Krochta, 1994). Moreover, edible films based on milk proteins were reported to
be flavorless, tasteless, and flexible, and, depending on the formulation, they varied
from transparent to translucent (Chen, 1995). Whey and caseins are the main milk-
protein fractions. Caseins represent 80% of the total composition of milk proteins,
with a mean concentration of 3%
in
milk (Brunner, 1977; Dalgleish, 1989).
Caseins
Commercial caseinates are produced by adjusting acid-coagulated casein to pH 6.7
using calcium or sodium hydroxide. Caseins are predominantly phosphoproteins that
precipitate at pH 4.6 and at 20°C (Gennadios
et
al., 1994). Four principal compo-
nents, a,,, aS2, p-, and K-caseins are identified. The amino acid composition in casein
is characterized by low levels of cysteine. Consequently, disulfide cross-linkages can-
not form water-insoluble films (Gennadios
et
al., 1994; Chen, 2002). Among the pro-
tein fractions of casein, p-casein is the most interesting fraction to produce films of
weak permeability to water vapor. The a-casein fraction contains more charged
residues and fewer hydrophobic residues than the p-casein fraction (Dalgleish, 1989).
In the presence of calcium, K-casein associates with
aSl
casein and p-casein to form
thermodynamically stable micelles (Brunner, 1977). Calcium acts by reducing the
electrostatic repulsions of casein phosphate groups, consequently facilitating hydropho-
bic interactions and the formation of micelles (Kinsella, 1984).
Caseins form films from aqueous solutions without further treatment due to their
random-coil nature and their ability to hydrogen bond extensively. It is believed that
electrostatic interactions also play an important role in the formation of casein-based
edible films (Gennadios
et
al., 1994). According to Vachon
et
al. (2000), pure
caseinate/glycerollCMC films are highly soluble in water (88%). Film solubility in
water decreased with buffer treatments at the isoelectric point of the films (Chen, 2002),
with cross-linking of the protein using irradiation (Vachon
et
al., 2000), and with the
transglutarninase enzyme (Nielson, 1995). A direct correlation between mechanical
properties and film porosity based on caseinatewhey proteins was also observed by
Vachon
et
al. (2000). This study demonstrated that the structure of cross-linked films
and caseinate-based films containing whey-protein isolated (WPI) were generally more
dense and homogenous, and comparable to caseinate films without WPI (Figure 18.1).
A protein ratio up to 5&50 (2.5% WPI and 2.5% caseinate) did not significantly affect
the puncture strength of the films. At higher WPI concentrations, the puncture strength of
the films was significantly reduced (Figure 18.2). The
film
based on whey-protein con-
centrate (WPC)-caseinate showed a granular structure and a large content of impurities
such as fats, salts and lactose.
An
increase of WPC concentration in the WPCsaseinate
based films significantly decreased the puncture strength of the films (Figure 18.3).