5
Edible films and coatings as active layers
B. CUQ, N. GONTARD and S. GUILBERT
5.1
Introduction
Edible films and coatings are traditionally used to improve food appearance
and conservation. The most common examples are wax coatings for fruit
(used in China since the 12th century), chocolate coatings for confectionery,
lipid films to protect meat products, and soy milk-based lipoprotein films to
improve the appearance and preservation of certain foods in Asia.
Formulations for edible films or coatings must include at least one
component able to form a suitably cohesive and continuous matrix. The
basic materials can be classified in three categories: polysaccharides,
proteins and lipidic compounds. Polysaccharides (vegetable and microbial
gums, starches, celluloses and derivatives, etc.) have good film-forming
properties. Films formed from these hydrophilic compounds provide
efficient barriers against oils and lipids (Murray et al., 1972), but their
moisture barrier properties are poor. Although not as extensively studied,
protein-based films have highly interesting properties. Many protein materials have been tested: collagen, zein, wheat gluten, ovalbumin, soybean,
casein, etc. (Guilbert and Biquet, 1989). The mechanical and barrier
properties of these films are generally better than those of polysaccharidebased films; this is due to the fact that, contrary to polysaccharides which are
monotonous polymers, proteins have a specific structure which confers
larger potential functional properties (Guilbert and Graille, 1994). Many
lipidic compounds, such as animal and vegetable fats (natural waxes and
derivatives, acetoglycerides, surface-active agents, etc.), have been used to
make edible films and coatings (Guilbert and Biquet, 1989; Kester and
Fennema, 1986). They are generally used for their excellent moisture barrier
properties, but there can be problems concerning stability (particularly
oxidation), texture and organoleptic quality (opacity, waxy taste).
Edible films and coatings formed with several compounds (composite
films) have been developed to take advantage of the complementary
functional properties of these different constitutive materials and to
overcome their respective drawbacks. Most composite films studied to date
combine a lipidic compound and a hydrocolloid-based structural matrix
(Cole, 1969; Daniels, 1973; Gontard et al.9 1994a; Guilbert, 1986; Kamper
and Fennema, 1984a, b).
Coatings are formed directly on the food product using either liquid filmforming solutions (or dispersions) or molten compounds (e.g. lipids). They
can be applied by different methods: with a paint brush or by spraying,
dipping-dripping, fluidizing, etc. Films are preformed separately from the
food product. They can be produced, for instance, by drying a film-forming
solution on a drum-drier, by cooling a molten compound, or through
standard techniques used to form synthetic packagings, e.g. thermoforming
or extrusion techniques for thermoplastic materials (Guilbert and Biquet,
1989; Kester and Fennema, 1986).
The production process from a film-forming solution generally includes a
first step with macromolecule solubilization in a solvent medium (often
water-, ethanol- or acetic acid-based) which can contain several additives
(plasticizers, crosslinking agents, solutes, etc.). The film-forming solution is
spread in a thin layer, usually followed by a drying treatment. The functional
properties of the film are dependent on a number of parameters (Gontard,
1991; Gontard et al.9 1992): formulation (characteristics and concentration
of the basic and secondary components, pH, denaturing conditions, etc.),
film-forming conditions (type of surface upon which the film-forming
solution is spread, drying conditions) and conditions in which the film is
used (temperature, relative humidity).
The degree of cohesiveness of the matrix is a critical parameter affecting
the functional properties of edible films (Banker, 1966). It is sometimes
difficult to obtain adequate adhesion of the film to the food product, for
instance when a hydrophobic film-forming material is used to protect a
hydrophilic food product. In such cases, surface-active agents can be coated
on the food or added to the film-forming solution, or a material capable of
adhering to both components can be applied as an intermediate precoating
(e.g. precoating with cocoa before sugar-coating peanuts).
An edible film is an integral part of the food product it encloses and
therefore must have neutral sensorial properties (or compatible with product
nature) so as not to be detected during consumption. Application of an edible
barrier layer is an easy means to structurally strengthen certain foods, to
reduce particle clustering and to improve the visual and tactile features on
the surface of the product. For example, Allen et al. (1963b) used alginate
and cornstarch coatings to improve texture juiciness, general appearance,
surface texture and color of beef steaks and pork chops. Edible films can
also be used to package components or additives that are to be dissolved in
hot water or food mixes (Daniels, 1973; Kroger and Igoe, 1971).
Edible films or coatings act as an additional parameter for improving
overall food quality and stability. They represent one way to apply hurdle
technology to solid foods without affecting their structural integrity
(Guilbert, 1994; Guilbert et aL, 1995). Many functions of edible films are
the same as those of synthetic packaging (water, gas and solute barriers,
mechanical properties, opacity, etc.). However, they must be chosen
according to their specific application (i.e. type of food product and main
deterioration mechanisms).
Films with substantial gas and moisture barrier properties are required for
many applications: to control gas exchange for fresh foods and oxygen
exchange for oxidizable foods and to reduce moisture exchange with the
external atmosphere (Guilbert and Biquet, 1989). Retention of specific
additives in edible films can lead to a functional response generally confined
to the surface of the product (modification and control of surface
conditions), (De Savoye et ai9 1994; Guilbert, 1988; Torres et aL, 1985a, b;
Torres and Karel, 1985). Oil and solute penetration into foods during
processing can also be limited by edible coatings (Daniels, 1973; Guilbert
and Biquet, 1989).
In this chapter, we will expand on the use of edible films and coatings as
active layers, i.e when the edible film contributes by itself to the
preservation. Figure 5.1 gives a schematic representation of food preservation with edible films and coatings as active layers when the first mode of
deterioration results from respiration, from dehydration or moisture uptake,
or from surface microbial development or oxidation. The protective features
of edible films and coatings are dependent on gas and water vapor barrier
Food additives (e.g. antioxygen
and antifungic agents)
Diffusion
of food additives
Gas Transfers
Storage
Water Transfer
Food additives in
AOffiWOLM
SURFACE RETENTION
of food additives
CONTROL
of Gas Transfers
Storage
CONTROL
of Water Transfer
Figure 5.1 Schematic representation of food preservation with (top) or without (bottom) edible
films and coatings as active layers, when the first mode of deterioration results from respiration (a),
from dehydration or moisture uptake (b), or from microbial development or oxidation (c).
properties, on modification of surface conditions and on their own
antimicrobial properties.
5.2 Use of edible active layers to control water vapor transfer
Moisture transfers due to water vapor pressure or concentration gradients
have major effects on the organoleptic and microbiological qualities of food
products. For example, products such as raisins (Bolin, 1976; Kochnar and
Rossell, 1982; Lowe et a/., 1963; Marston, 1983; Watters and Brekke,
1961), candies and chocolate-coated products (Andres, 1984; Barron, 1977;
Cosier, 1957; Feuge, 1970; Jokay et al., 1967), dry crackers, biscuits, pizza
crust, and filled bakery crust (Dhale, 1983; Heiss, 1968; Katz and Labuza,
1981; Labuza, 1985) may become unsatisfactory as a result of moisture gain
or loss. Moisture exchanges are difficult to control in multicomponent foods
such as mixtures of dehydrated foods, jelly-filled cookies, pies, and pizzas.
Moisture transfer can be limited by reducing the vapor pressure gradient
between components, e.g. using aw lowering agents (salts, sugars, polyols,
etc.). However, this is not always possible and may result in drastic
modifications of the sensory and physiochemical characteristics of the
product (Guilbert, 1984, 1985; Karel, 1976). Another solution consists in
using edible films or coatings with good moisture barrier properties to
separate compartments and thus control further moisture transfer (Guilbert,
1986; Kamper and Fennema, 1985). Surface drying on some fresh and
frozen foods or, inversely, moisture uptake in dry or semi-moist foods, can
also be hindered by using films with a low water permeability (as
schematized in Figure 5.1).
Permeability is defined as a state which permits the transmission of
permeants through materials (Mannheim and Passy, 1985). When there are
no pores, faults or membrane punctures, permeability P, is equal to the
product of the diffusion coefficient D, representing the mobility of permeant
molecules in the polymer, and the solubility coefficient S, representing the
permeant concentration in the film in equilibrium with the external
pressure:
P = DxS
In practice, water, gas or solute permeability, P, through a membrane is
determined by steady-state measurements:
P =
AWx
AtAAp
where AW is the permeant weight that passes through a film of thickness x
and area A; where At is the time and Ap is the differential partial pressure
across the film. The diffusion coefficient can be obtained by taking
measurements before the steady state is reached. The solubility coefficient
can either be calculated from P and D, or measured in a separate experiment
(sorption isotherms).
The diffusion and solubility of permeants are affected by temperature and
by the size, shape and polarity of the diffused molecule. Moreover, these two
parameters depend on film characteristics, including the type of forces
influencing molecules of the film matrix, the degree of cross-linking between
molecules, the crystallinity, the presence of plasticizers or additives, etc. (De
Leiris, 1985; Gontard et ah, 1993; Kumins, 1965; Pascat, 1985; Schwartzberg, 1985).
Permeability is only a general feature of films or coatings when the
diffusion and solubility coefficients are not influenced by permeant content,
i.e. when Fick's and Henry's laws apply. In practice, for most edible films,
the permeant interacts with the film and the D and S coefficients are
dependent on the difference in partial pressure. For instance, in relation to
the water vapor permeability of hydrophilic polymer films, the water
solubility and diffusion coefficients increase when the water vapor differential partial pressure increases because of the moisture affinity of the film
(nonlinear sorption isotherm) and because of increased plasticization of the
film due to water absorption (Gontard et ai, 1993; Schwartzberg, 1985). The
film thickness can also influence permeability when using film-forming
materials that do not behave ideally. The permeability of an edible film is
thus defined as a property of the film-permeant complex, under specified
temperature and water activity conditions.
Water vapor permeabilities of some edible and synthetic films are given in
Table 5.1. Permeability is clearly high in edible films formed from
hydrophilic materials. These films can only be used as protective barrier
layers to limit moisture exchange for short-term applications or in lowmoisture foods such as dried fruits (Forkner, 1958; Swenson et aLy
1953).
Lipidic compounds are often used to make moisture barrier films and
coatings (Table 5.1). For instance, coating fresh fruit and vegetables with
wax reduces desiccation-induced weight loss during storage by 40-75%
(Kaplan, 1986). Water is not very soluble or mobile in lipid-based films
because of the low polarity and dense, well-structured molecular matrixes
that can be formed by these compounds.
Moisture resistance of lipid films is inversely related to polarity of the
lipids. Hydrophobic alkanes and waxes, such as paraffin wax and beeswax,
are the most effective barriers. More hydrophilic lipids, such as fatty acids,
are less resistant to water vapor transmission since their polar groups attract
migrating water molecules and thereby facilitate water transport. The
moisture barrier capacities of different films can be classified in increasing
order of efficiency, as follows: liquid oils < solid fats < waxes (Gontard et
ah, 1994a; Kamper and Fennema, 1984b). This efficiency order has been
Table 5.1 Water vapor permeability of various films
Film
Starch, cellulose acetate(1)
Wheat gluten(3)
Casein-gelatin(20)
Wheat gluten(l5)
Sodium caseinate(2)
Cornzein(33>
HPC and PEG(I8)
MC and PEG(I8)
MC and PEG(5)
Corn zein(18)
HPMC(13)
Glycerol monostearate(16)
MC (I6)
Wheat gluten and glycerol(9)
Wheat gluten and oleic acid(10)
Wheat gluten and carnauba wax(l0)
Wheat gluten(8)
HPC (I9)
Wheat gluten and soy protein(7)
Wheat gluten and mineral oil(8)
Corn zein and oleic acid(12)
HPMC and palmitic acid(l4)
Dark chocolate(4)
MC and beeswax bilayer<n)
LDPE(17)
HPMC/MC and beeswax bilayer(15)
Beeswax(6)
Wheat gluten-beeswax bilayer<9)
Carnauba wax(6)
HDPE(!2)
Beeswax(16)
Aluminium foil(17)
Water vapor
permeability
(X 10'2 mol m m~2 s"1 Pa"1)
T
(0C)
Thickness
(X 103m)
RH %
conditions
142
69.7
34.3
34.2
24.7
22.8
13.7
13.6
7.78
6.45
5.96
5.85
5.23
5.08
4.15
3.91
3.11
2.89
2.84
2.28
1.48
1.22
0.707
0.199
0.0482
0.0360
0.0320
0.0230
0.0185
0.0122
0.0122
0.000289
38
26
30
21
25
26
21
21
25
21
27
21
30
30
30
30
23
30
23
23
38
25
20
25
38
25
25
30
25
38
25
38
1.190
0.250
0.400
0.130
0.100
0.025
0.200
0.019
1.750
0.075
0.050
0.050
0.050
0.127
0.075
0.075
0.125
0.040
0.040
0.610
0.025
0.051
0.100
0.090
0.100
0.025
0.120
0.025
100-30
100-50
60-22
85-00
100-00
100-50
85-00
85-00
52-00
85-00
85-00
100-75
11-00
100-00
100-00
100-00
11-00
11-00
11-00
11-00
95-00
97-67
81-00
100-00
95-00
97-00
100-00
100-00
100-00
97-00
97-00
95-00
(According to Allen et al, 1963a(1); Avena-Bustillos and Krochta, 1993(2); Aydt et al, 1991(3);
Biquet and Labuza, 1988(4); Donhowe and Fennema, 1993a(5); Donhowe and Fennema, 1993b(6);
Gennadios et al, 199(F; Gennadios et al, 1993a(8); Gontard, 1991(9); Gontard et al, 1994a(10);
Greener and Fennema, 1989a(ll); Guilbert and Biquet, 1989(l2); Hagenmaier and Shaw, 1990(13);
Kamper and Fennema, 1984a(l4); Kester and Fennema, 1989a(15); Landman et al, 1960(16); Myers et
al, 1961(17); Park and Chinnan, 1990(l8); Park et al, 1991(19); Schultz et al, 1949(20).)
(HPC = hydroxypropylcellulose; HDPE = high-density polyethylene; HPMC = hydroxypropyl
methylcellulose; LDPE = low-density polyethylene; MC = methylcellulose; PEG = polyethylene
glycol; RH = relative humidity; T = temperature).
confirmed by Kester and Fennema (1989a) in a study on the resistance of
various lipids, heated and adsorbed into filter papers, to water vapor
transmission. Composition, fusion and solidification ranges, lipid crystalline
structure, in addition to the interaction with water, oxygen and other
components of the food product influence the physico-chemical, functional
and organoleptic properties of lipid-based films (Fennema et al, 1994;
Kamper and Fennema, 1984a, b; Kester and Fennema, 1989a).
In general, the rate of transmission of water through a lipid film increases
as the length of the lipid hydrocarbon chain is decreased and the degree of
unsaturation or branching of acyl chains is increased (Archer and Lamer,
1955; Fettiplace, 1978; Fettiplace and Haydon, 1980; Kamper and Fennema,
1984b and 1985). This is a consequence of enhanced mobility of
hydrocarbon chains and less efficient lateral packing of acyl chains caused
by a reduction of interchain van der Walls' interaction (Jain, 1972; Taylor et
al., 1975).
However, specific information on the water vapor barrier properties of
films of more hydrophobic lipids is lacking, and almost all the information
available has been gathered using a 100-0% RH gradient which is not
commonly encountered during storage of foods under commercial conditions.
The water vapor barrier properties of a lipid-hydrocolloid composite film
is generally determined by the potentials of its component parts (Table 5.1).
The coating operation used, i.e. emulsion (suspension or dispersion of nonmiscible compounds), successive layers (multilayered films) or solutions
with a common solvent, affects the water vapor barrier properties of these
films (Table 5.2).
According to Schultz et al (1949), Martin-Polo et al (1992), Debeaufort
et al (1993) and Gontard et al (1994a), who investigated the moisture
permeability of multicomponent films composed of methylcellulose, pectinate or gluten and various lipids (waxes, fatty acids, etc.), it is better to form
two successive layers than to apply a dispersion in solvent. Kamper and
Fennema (1984a, b) carried out detailed studies of films, composed of
soluble cellulose esters and a mixture of palmitic and stearic acids, and
Table 5.2 Effect of coating operation on water vapor permeance of edible multicomponent films
composed of MC and paraffin wax (weight ratio 1:1); gluten and beeswax (at 2.4 mg/cm2); and
HPMC and stearic-palmitic acids (at 9 mg/cm2)
Water vapor
permeance
(X 108 mol m~2 s'1 Pa'1)
_
T
(0C)
_
MC and paraffin wax (emulsion)(1)
MC and paraffin wax (bilayer)(1)
7.39
0.251
25
25
0.090
0.130
84-22
84-22
Glutten(2)
Gluten and beeswax (emulsion)(2)
Gluten and beeswax (bilayer)(2)
10.1
0.727
0.0257
30
30
30
0.050
0.120
0.090
100-00
100-00
100-00
25
25
25
0.040
0.040
0.125
85-00
85-00
85-00
Film
_ _
HPMC(3)
HPMC and fatty acids (emulsion)(3)
HPMC and fatty acids (bilayer)(3)
8.02
0.00643
1£7
Thickness
RH %
(X 103 m) conditions
_ _
84_22
(According to Debeaufort et al., 1993(l); Gontard et aLy 1994a(2); Kamper and Fennema,
1984b(3).)
(HPMC = hydroxypropylmethylcellulose; MC = methylcellulose; RH = relative humidity; T =
temperature.)
demonstrated that application of emulsions resulted in reducing moisture
permeability by 10-fold relative to bilayer systems. Variations in homogeneity and/or structure (size, form and orientation of the crystals) of the lipid
layer related to the film-forming process and to coating operation, could
explain this discrepancy.
Permeability of composite films decreases substantially when the proportion of lipids increases. For instance, an increase of glyceride sucro ester
(beeswax, stearic acid or monoglycerides) contents in gluten- (caseinate- or
hydroxypropyl methylcellulose-) based composite films, causes a substantial
reduction in water vapor permeability (Figure 5.2), (Avena-Bustillos and
Krochta, 1993; Gontard et al, 1994a; Hagenmaier and Shaw, 1990). The
number of hydrophobic residues (from lipid derivatives) in the matrix affects
the water interaction potential, and consequently the barrier properties.
Solidification of lipids (especially saturated) in a densely organized
crystalline structure results in a very significant reduction in moisture
permeability (Kamper and Fennema, 1984b; Kester and Fennema, 1989b;
Landman et ai, 1960; Watters and Brekke, 1961).
Water vapor permeability variations relative to aw reveal the non-Fickian
behaviour of biological materials. At low aw, water diffusion, and especially
the water solubility coefficient, remains relatively low: film permeability is
minimal at this point. As expected in a hydrophilic film (Barrie, 1968;
Biquet and Labuza, 1988; Kamper and Fennema, 1984a; Crank, 1975; De
Leiris, 1985; Pascat, 1986; Schwartzberg, 1985), increasing aw leads to an
Water Vapor Permeability (i ltf2 mol. m . m*. s l . Pa *)
Lipid Materials Content (%w/w)
Figure 5.10 Effect of reduced surface pH on the microbiological quality of an intermediate moisture
cheese analog coated with a carrageenan and agarose film; challenged with Staphylococcus aureus
S-6 (aw = 0.88 and 35°C) (after Torres and Karel, 1985).
increase in film moisture content (rise in the sorption isotherm) and so
induces an increase in water vapor permeability. At high aw, extensive
swelling of the protein network with water probably enhances water
molecule diffusion and such films would clearly not be efficient water vapor
barriers (Figure 5.3).
Many authors have studied the effect of temperature on water vapor
transfer using synthetic, simple edible films (protein or cellulose-based) or
composite edible films (cellulose derivatives and lipids). A rise in temperature causes an increase in water vapor permeability (Figure 5.4). This is
often characterized by Arrhenius-type representations (Donhowe and
Fennema, 1993b; Higuchi and Aguiar, 1959; Kester and Fennema, 1989a, c).
In films formed with hydrophilic materials, temperature-dependent variations in barrier properties are affected by the moisture level (see Figure 5.3
for gluten films). However, it was difficult to interpret the temperature
dependence of the effect of hydration on water vapor permeability of gluten
film in terms of disruptive water-polymer hydrogen bonding in a polymer
hydrogen-bonded network. The critical role of water as plasticizer of gluten
film appeared to be highly temperature dependent. According to Levine and
Slade (1987) and Slade et al (1989), the structure/property relationships of
hydrated proteins, and particularly of gluten, could be better understood
through the theories of glass transition used in polymer science, in terms of
critical variables of time, temperature and moisture content. Glass transition
Water Vapor Transmission Rate (WVT), (x 108 mol. m . iff2. r 1 )
Water activity
Figure 5.3 Effect of average water activity on water vapor transmission rate of edible films
composed of methylcellulose and palmitic-stearic acids (at 0.76 mg/cm2) at 25°C and 32% RH
gradient (according to Kamper and Fennema, 1984a), and of edible wheat gluten films at 5, 30 and
500C, and 10% RH gradient (according to Gontard et al., 1993). •, MC and C 1 8 -C 1 6 at 25°C
(WVT x 0.1); D, gluten at 500C; D, gluten at 300C; D, gluten at 5°C
hydrated proteins, and particularly of gluten, could be better understood
through the theories of glass transition used in polymer science, in terms of
critical variables of time, temperature and moisture content. Glass transition
is typically described as a transition from a brittle glass to a highly viscous
or rubbery solid. It is well established that plasticization by water affects the
glass transition temperature of amorphous or partially crystalline proteins
such as gluten (Gontard et al., 1993; Hoseney et al., 1986), gelatin (Marshall
and Petrie, 1980; Yannas, 1972), collagen (Batzer and Kreibich, 1981) and
elastin (Kakivaya and Hoeve, 1975; Scandola et al., 1981), thus resulting in
a drop in the glass transition temperature. The anomalous diffusion behavior
of glassy polymers may be directly related to the influence of the changing
polymer structure on solubility of the penetrant and diffusional mobility of
the penetrant (Crank, 1975).
The water vapor barrier properties of edible active films and coatings have
been used in several food systems. For example, meat coatings composed of
corn starch and alginate were used by Allen et al. (1963a) to reduce moisture
loss by 40-48%. Avena-Bustillos et al. (1993) found that sodium caseinate
and stearic acid emulsion coatings improved the storage stability and
reduced water loss of peeled carrots. The use of sucrose esters of fatty acids,
mono- and diglycerides, and the sodium salt of carboxymethylcellulose
coatings allowed the increase of water vapor resistance (up to 75%) in
zucchini fruit (Avena-Bustillos et al., 1994). Wax is commercially applied to
Water Vapor Permeance (WVP), (x 10* mol.m . m 2 . s l )
Temperature (0C)
Figure 5.4 Effect of temperature on water vapor permeance of edible wheat gluten films, at RH
conditions 90-80% (according to Gontard et al., 1993); of edible glycerol monostearate films, at RH
conditions 100-0% (according to Higuchi and Aguiar, 1959); and of edible films composed of
methylcellulose and beeswax, at RH conditions 97-0% (according to Kester and Fennema, 1989b).
a, Gluten (WVP X 0.1); A, MC and beeswax; •, monostearate.
many fruits and vegetables to reduce dehydration and improve consumer
appeal (Hall, 1981), e.g. to oranges (Albrigo and Brown, 1970; Bursewitz
and Singh, 1985; Eaks and Ludi, 1970), to apples (Hall, 1966; Trout et al,
1953), to prune plums (Bain and McBean, 1967), and to sweet cherries
(Drake et aL, 1988; Lidster, 1981). Ukai et al (1976) used hydrophobic
emulsions to coat fruits and vegetables such as orange, peas, apples, green
beans, tomatoes, pears, and peaches. Lidster (1981) suggested the use of
xanthan gum applied to cherries as a postharvest dip to prevent water loss.
El Ghaouth et al. (1991) used edible chitosan coating to reduce water loss of
cucumber and bell pepper fruits.
The use of edible films to lessen internal migration of moisture in foods
has been studied by several investigators, and some of these films appear
promising for commercial use (Fennema et al, 1993; Greener and Fennema,
1989b; Kamper and Fennema, 1985; Kester and Fennema, 1989d; Rico-Pena
and Torres, 1990). One standard example of moisture transfer control in
heterogeneous food is the wafer-ice cream system of ice cream cones. In
standard conditions, the wafer loses its crispness after three months' storage
at -23°C. This qualitative degradation is due to moisture transfer between
the ice cream and the wafer. Rico-Pena and Torres (1990) tested composite
films (methylcellulose and chocolate and methylcellulose, chocolate and
palmitic acid) to separate the two components. Films can limit moisture
transfer during ice cream cone storage at -23 0 C or -12°C and preserve wafer
crispness. Films based on methylcellulose, chocolate and palmitic acid are
the most efficient. In addition, several studies have confirmed the efficiency
of chocolate and cocoa butter coatings as moisture barriers (Biquet and
Labuza, 1988; Kempf, 1967; Landman et al, 1960; Neidiek, 1981; Soboleva
and Chizhikova, 1978; Tiemstra and Tiemstra, 1974).
These examples show that water vapor transfer in food products can be
controlled using edible films and coatings. The knowledge of the influence
of parameters such as structure and composition of film-forming materials,
allows modulation of the water barrier properties of these edible active
layers either on the food surface or in multicomponent foods.
5.3 Use of edible active layers to control gas exchange
The gas barrier properties of edible films and coatings are potentially of
great interest. For instance, edible oxygen barrier films can be used to
protect foods that are susceptible to oxidation (rancidity, loss of oxidizable
vitamins, etc.). In contrast, a relatively high gas permeability is necessary for
fresh fruit and vegetable coatings (especially carbon dioxide permeability).
The development of edible films with selective gas permeability (oxygen,
carbon dioxide, ethylene) allows the control of respiration exchange and
microbial development and seems very promising for achieving a * modified
atmosphere' effect in fresh fruit and for improving the storage potential of
these products (Smith et a/., 1987; Trout et al., 1953), as schematized in
Figure 5.1.
Gas permeability can be measured using air porosimeters and specific
permeability cells. As in the case with moisture barrier properties, the
formulation, manufacturing and environmental parameters have an impact
on a film's response to gases. The oxygen and carbon dioxide permeability
values of various edible films and synthetic films are given in Table 5.3.
Films formed with hydrocolloids (proteins, polysaccharides) generally
have good oxygen barrier properties, particularly under low-moisture
conditions. The oxygen permeability of hydrocolloid-based films (at 0%
relative humidity) is often lower than that of common synthetic films such as
polyethylene and non-plasticized PVCs. For example, the oxygen permeability of wheat gluten film was 800 times lower than that of low-density
polyethylene and twice as low as that of polyamide 6, a well-known high
oxygen barrier polymer.
Films formed with lipid derivatives have suitable oxygen barrier properties. For example, the oxygen permeability value for beeswax film lies
between those of low-density and high-density polyethylene (Table 5.3).
According to Blank (1962 and 1972), lipids with the best oxygen barrier
properties are those formed with straight-chain and saturated fatty acids.
Increased unsaturation (or branching) and a reduction in the length of the
carbon chain result in decreased oxygen permeability. The following barrier
efficiency order was observed by Kester and Fennema (1989c): stearic
alcohol > tristearine > beeswax > acetylated monoglycerides > stearic
acid > alkanes. These differences can be explained by the presence of pores
or cracks, and by the homogeneity of the composition density of the network
(Kester and Fennema, 1989b, c). The network density is dependent on the
polymorphic shape and orientation of the chains and morphological
differences in the lipid layers.
As previously mentioned for water vapor permeability, formulation of
composite films allows advantage to be taken of the complementary barrier
properties of each component. At high aw, where hydrophilic materials are
not effective as gas barriers (see below), the addition of lipidic compounds
results in a decrease in the gas permeability of the film. For example, at aw
= 0.91, the oxygen permeability is reduced by about 30% for a composite
gluten and beeswax film (Table 5.3).
The effect of temperature on gas permeability is similar to that reported
for water vapor permeability (Donhowe and Fennema, 1993b; Gennadios et
al.9 1993). These variations can be characterized by Arrhenius-type
representations. But, as far as gas solubility decreases with temperature
increase, the increase of gas permeability with temperature is lower than for
water vapor permeability (Gontard et a/., 1994b).
High aw conditions cause an increase in gas permeability in hydrophilic
Table 5.3 Oxygen and carbon dioxide permeabilities of various films
Film
_ _ _
O2 Permeability
(X 1O18HIoImIn-2S-1Pa-1)
__
HDPE (3)
Rigid PVC (II)
PET<3)
Polyamide 6(3)
PVDC (3)
Cellophane(3)
EVOH (H)
Polybutadiene<3)
Polypropylene(3)
LDPE (l3)
Flexible PVC
film(3)
Uncoated cellulose 0}
HDPE (3)
HDPE (I)
Cellophane (H)
Polyvinyl alcohol(1)
PET (3)
PET™
Nylon 6 (I)
Cellophane (l4)
EVOH (I3)
MCandPEG ( I O )
MCandPEG ( 1 0 )
MC and beeswax<9)
Beeswax<5)
HPC and PEG(IO)
HPC and PEG(IO)
MC and palmitic
acid (l2)
Carnauba wax(5)
Corn zein(2)
Cornzein (2)
Wheat gluten and
glycerin(6)
Cornzein ( l 0 )
Wheat gluten
protein (l0)
Wheat gluten
protein (l0)
Corn zein(10)
Wheat gluten protein(2)
Soy protein(6)
Wheat gluten and
mineral oil(7)
Wheat gluten(8)
Wheat gluten and soy
protein(4)
Wheat gluten protein(2)
Pectin<8)
Wheat gluten(8)
Wheat gluten(8)
T
(0C)
__
0 0
972
37.6
19.5
7.65
-
23
23
23
23
23
23
23
0.0
0.0
0.0
0.0
0.0
0.0
0.0
703
-
23
23
23
23
23
23
23
23
23
23
23
23
23
23
0.50
0.50
0.50
0.50
i.00
0.50
1.00
0.95
1.00
0.50
1.00
1.00
0.50
0.95
29900
28900
-
30
21
25
25
30
21
24
0.0
0.0
0.0
0.0
0.0
0.0
0.0
81.1
34.8
19.6
216
-
25
38
23
23
0.0
0.0
0.0
0.0
16.1
8.92
-
30
30
0.0
0.0
119
21
0.0
285
16.0
11.9
11.6
1.86
1.34
0.155
19400
1170
961
682
406
227
224
130
50.8
24.3
12.2
10.2
8.27
6.20
522
499
480
470
190
-
CO2 Permeability
(X 1018 mol m m 2 s 1 Pa 1 )
_ _
aw
3.50
2.30
1.70
95.0
-
21
38
23
23
0.0
0.0
0.0
0.0
1.24
1.19
-
25
25
0.0
0.0
1.20
21300
36700
24500
23
25
25
25
0.0
0.96
0.95
0.91
1340
1290
982
Table 5.3 Continued
Film
Wheat gluten and
beeswax (8)
Chitosan(8)
MC and palmitic
acid(12>
O 2 Permeability
CO 2 Permeability
T
( X 10 l 8 molm HT2S-1 Pa"1) ( X 1018 mol m m"2 s"1 Pa"1) (0C)
aw
687
6614
25
0.91
472
407
8010
-
25
24
0.93
0.79
(According to Ashley, 1985 (l) ; Aydt et al., 1991 (2) ; Bakker, 1986(3); Brandeburg et al, 1993(4);
Donhowe and Fennema, 1993b(5); Gennadios et al, 1990(6); Gennadios et al, 1993a(7); Gontard et
al., 1994b(8); Greener and Fennema, 1989a(9); Park et Chinnan, 1990(l0); Poyet, 1993 (n) ; Rico-Pena
and Torres, 1990 (l2) ; Salame, 1986 (l3) ; Taylor, 1986(l4).)
(aw = water activity; EVOH = ethylene-vinyl alcohol; HDPE = high-density polyethylene;
LDPE = low-density polyethylene; MC = methylcellulose; PEG = polyethylene glycol; PET =
polyester; PVDC = poly(vinylidene chloride); PVC = poly(vinyl chloride); T = temperature.)
films but generally not in synthetic hydrophobic films which are not water
sensitive (Figure 5.5). In these hydrophilic films, increased aw promotes both
gas diffusivity (due to the increased mobility of hydrophobic macromolecule
chains) and gas solubility (due to the water swelling of the matrix), leading
to a sharp increase in gas permeability (Kumins, 1965). The effects of aw on
gas barrier properties of composite films (methylcellulose and palmitic acid)
and of gluten protein-based films was studied by Rico-Pena and Torres
(1990) and by Gontard et al. (1994b) respectively, and are compared in
Figure 5.5 with oxygen permeabilities of synthetic hydrophilic films.
Carbon dioxide permeability in hydrocolloid-based films is often much
Oxygen Permeability (x 10* mol. m. m 2 . s 1 . P a ! )
Water activity
Figure 5.5 Effect of water activity on oxygen permeability of edible and synthetic films at 25°C
(according to Gontard et al, 1994b (o, gluten); Poyet, 1993 (A, EVOH; •, Nylon 6); Rico-Pena and
Torres, 1990 (D, MC and palmitic acids, at weight ratio 3:1); Rigg, 1979 (•, cellophane)). (MC is
methylcellulose; EVOH is ethylene-vinyl alcohol).
higher than oxygen permeability (Table 5.3). The effect of film aw on carbon
dioxide permeability is similar to that on oxygen permeability, but the sharp
increase of permeability is more important. This could be explained by the
differences in water solubility of these gases (Schwartzberg, 1985), i.e.
carbon dioxide is very soluble (carbon dioxide solubility in water =
34.5mmol/l at 25°C and 105 Pa; oxygen solubility = 1.25mmol/l at 25°C
and 105 Pa).
At high aw, the addition of lipidic components to gluten film results in a
high decrease of carbon dioxide permeability. For example, at aw = 0.91,
the carbon dioxide permeability is reduced by about 75% for a composite
gluten and beeswax film (Table 5.3). This could be related to the
hydrophobic characteristics of these components which for the same aw
reduce the amount of water available for solubilisation of carbon dioxide.
The selectivity coefficient between carbon dioxide and oxygen is defined
as the ratio of the respective permeabilities of both gases. In hydrophilic
materials, the effect of an aw increase on permeability is greater for carbon
dioxide than for oxygen. The selectivity of these materials is thus sensitive
to moisture variations (for example, the selective coefficient of edible gluten
films varies from 4.0 at aw = 0.30, to 25 at aw = 0.95), (Gontard et aL,
1994b), whereas the selectivity coefficient for synthetic polymers remains
relatively constant, at 4 to 5 (Table 5.4).
Edible films with selective gas permeability can be applied to reduce
degradation of some fresh fruits and vegetables. In fact, the diffusion of
Table 5.4 Gas selectivity coefficient of various films (CO2/O2)
Gas selectivity
coefficient (CO2/O2)
T
(0C)
aw
PP(2)
PA(2)
PVC(2)
Wheat gluten(3)
Corn zein(3)
HPC and fatty acid(3)
MC and PEG(3)
HPC and PEG(3)
4.0
4.2
5.8
7.5
9.5
23.7
31.6
40.6
23
23
23
25
25
25
25
25
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
Gluten(I)
Gluten(1)
MC and beeswax(l)
Gluten and beeswax 0}
Pectin(l)
Chitosan(1)
Gluten(l)
Gluten*'>
4.0
4.5
6.7
9.6
16.0
17.0
25.0
2$A
25
25
25
25
25
25
25
25
0.35
0.62
0.56
0.91
0.96
0.93
0.91
0.95
Film
(According to Gontard et aL, 1994b(1); Lefaux, 1972(2); Park and Chinnan,
1990(3).)
(aw = water activity; HPC = hydroxypropylcellulose; MC = methylcellulose; PA =
polyamide; PEG = polyethylene glycol; PP = polypropylene; PVC = poly(vinyl
chloride).)
oxygen and carbon dioxide between fruit or vegetables and the environment
is a basic element in the post-harvest physiology of fruit (Burton, 1974,
1978; Cameron and Reid, 1982). The type of coating material applied alters
the relative effects on the skin permeability to oxygen and carbon dioxide
(Trout et a/., 1953). For example, waxing increased the carbon dioxide
content and decreased the oxygen content of the orange's internal atmosphere (Eaks and Ludi, 1960). The coating of peach surfaces with beeswaxcoconut oil emulsion decreased oxygen gas transfer (Erbil and Muftugil,
1986). Chitosan coatings retarded ripening and prolonged storage life of
tomatoes, cucumber and bell pepper fruit without affecting their ripening
characteristics (El Ghaouth et a/., 1990, 1991). Lowings and Cutts (1982)
reported that an edible composite coating (carboxymethylcellulose, sucrose
fatty acid esters and mono- and diglycerides) would produce a semipermeable modified atmosphere within fresh fruit after application. The
post-storage uses of these coatings were investigated on apples (Chu, 1985;
Drake et a/., 1987; Elson et al., 1985; Smith and Stow, 1984), on pears
(Elson et al.y 1985), on bananas (Banks, 1983, 1984; Lowings and Cutts,
1982), on limes (Motlagh and Quantick, 1988) and on mangoes (Dhalla and
Hanson, 1988). For example, the selective gas permeability properties of
these films applied on bananas can reduce oxygenfluxfivefold,while carbon
dioxide flux is only decreased by about half (Banks, 1984). Other examples
of gas permeability control by edible active layers are gelatin films used to
protect frozen meats from rancidity (Klose et ah, 1952), to coat candies
and dried products (Grouber, 1983) and to microencapsulate flavors
(Anandaraman and Reineccius, 1980).
The development of edible active films with selective gas permeability
seems have considerable potential applications for achieving a modified
atmosphere effect for fruits and vegetables.
5.4 Modification of surface conditions with edible active layers
Edible active films and coatings can be applied on foods to modify and
control surface conditions. They can be used as surface retention agents to
limit food additive diffusion in the food core. The improvement of food
microbial stability can also be obtained by using edible active layers which
have specific antimicrobial and pH lowering properties.
Surface microbial growth is a main cause of spoilage for many food
products (Gill, 1979; Maxci, 1981; Olson et al, 1981; Vitkov, 1973, 1974).
Edible films and coatings can be used in combination with treatments such
as refrigeration and controlled atmosphere to improve the microbiological
quality of certain foods. For example, calcium alginate-based films were
tested to limit microorganism contamination on the surface of beef pieces
(Williams et al., 1978). These films were found to have a significant effect
on microbial growth (natural microflora and coliform inocula). The specific
antimicrobial activity of calcium alginate coatings has not yet been
explained, but could be partially due to the presence of calcium chloride. El
Ghaouth et aL (1990, 1991) used chitosan-based coatings to protect fresh
vegetables (cucumbers, peppers). These coatings improved fruit appearance
and reduced microbial degradation. This effect, which has also been
obtained on strawberries, can be attributed to the specific antifungal
properties of chitosan molecules (Allan and Hadwiger, 1979; Hirano and
Nagao, 1989; Stossel and Leuba, 1984). Zein-based films have been found to
successfully reduce microorganism penetration into chicken egg shells after
surface inoculation (Tryhnew et aL, 1973).
Development of processes that specifically enhance surface microbial
stability is required; food processors have used preservative dips and sprays.
Potassium sorbate (or sorbic acid), which has a wide range of bacteriostatic
and mycostatic properties, can be used by dipping to reduce the total number
of viable bacteria at both refrigeration and elevated temperatures
(Cunningham, 1979; D'Aubert et aL, 1980; Lueck, 1984; Robach, 1979,
Robach and Ivey, 1978; Robach and Sofos, 1982; To and Robach, 1980;
Torres et aL, 1985b; Zamora and Zaritzky, 1987a, b). However, the shelf-life
extension achieved by these surface treatments is limited by problems
related to potassium sorbate (or sorbic acid) stability and diffusion. The
stability of sorbic acid (in its active non-dissociated form) is dependent on
application conditions; e.g. a lowered pH improves stability (Eklund, 1983).
The diffusion into the core of the food results in a reduction in preservative
concentration on the surface which allows microorganisms to overcome the
sorbate-induced bacteriostasis (Greer, 1981; Torres, 1987).
It is important to be able to predict and control surface preservative
migration between phases during food treatments (e.g. absorption of sorbic
acid during the processing of dried prunes, or its loss during cooking of
fabricated foods), storage of composite foods (e.g. dairy products or cakes
containing pretreated fruits), storage of foods in contact with wrapping
materials or films containing sorbic acid (absorption by dairy products
covered with paper saturated with sorbic acid) and storage of foods coated
with an external edible layer highly concentrated in sorbic acid (Guilbert,
1988).
Edible films and coatings can be used as food preservative media
(particularly as antioxygen and antifungal agents) and as surface retention
agents to limit preservative diffusion in the food core (Guilbert, 1986, 1988;
Kester and Fennema, 1986; Torres et aL, 1985b; Vojdani and Torres, 1989a,
b, 1990). Maintaining a local high and effective concentration of preservative may allow, to a considerable extent, a reduction of its total amount
in the food for the same effect (as schematized in Figure 5.1), i.e. at the
surface of the food to reduce aerobic contamination and/or oxygen
influence.
% Retention
Time (days)
Figure 5.6 Tocopherol retention at 25°C in gelatin layers and in gelatin layers treated with tannic
acid in contact with aqueous model food (aw = 0.95) or in contact with margarine (according to
Guilbert, 1988). D, Gelatin layer - aqueous model food; •, gelatin layer treated - aqueous model
food; o, gelatin layer - margarine; •, gelatin layer treated - margarine.
Guilbert (1988) determined the retention rate of a-tocopherol in gelatinbased films formed on intermediate moisture foods and margarine (Figure
5.6). The rapid decrease of tocopherol content in the gelatin layer in contact
with margarine is due to the high apparent solubility of tocopherol in fatty
material. The low solubility of tocopherol in water, and so its low diffusivity
value, explains the retention effect exerted by the gelatin layer in contact
with aqueous model food. Cross-linking treatment with tannic acid increases
tocopherol retention by the gelatin layers in contact with margarine. These
active layers enriched with tocopherol could be used to reduce the oxidative
deterioration in some food products.
Guilbert (1988) also investigated the retention of sorbic acid in gelatin and
casein films treated respectively with tannic and lactic acid, and placed over
an aqueous model food system (aw = 0.95). After 35 days at 25°C, a
retention of 30% was observed with the treated casein film. Swelling and
poor retention were observed with the gelatin film (30% retention after 10
days).
Sorbic acid retention has been studied in zein-based (Torres et aL, 1985b)
and casein-based films (Guilbert, 1986, 1988), gluten-based and pectinbased films (De Savoye et aL, 1994), and in composite polysaccharide-lipid
derivative films (Vojdani and Torres, 1990). The sorbic acid permeability
values determined for these films can also be compared with published
apparent diffusion values for sorbic acid in food systems. In an intermediate
Table 5.5 Apparent coating diffusion coefficients for sorbic acid in intermediate moisture model food (agar model or cheese analog) at 24°C and aw =
0.88, in agar gel (with sucrose or glycerol) at 25°C and aw = 0.88, and in
cheese analog coated with edible zein films
Diffusion coefficient
(m2 s~')
Experiment
Agar gel and sucrose(1)
Cheese analog(2)
Agar model(2)
Agar gel and glycerol(1)
Cheese analog and zein
Cheese analog and two zein
0.5
1.0
2.0
3.5
film(2)
films(2)
X
X
X
X
IO"10
IO"10
10"10
10"10
3.3 X 10~13
6.8 X 10~13
(According to Giannakopoulos and Guilbert, 1986a(l); Torres et al.,
1985b(2).)
moisture agar model system, Guilbert et al. (1985) reported a value of 2.0 x
10~10m2/sec. Torres et al. (1985b) found with an intermediate moisture
cheese analog a value of 1.0 x 10"10m2/sec (Table 5.5). The sorbic acid
diffusivity values in edible films were found to be 150- to 300-fold lower
than those determined for model intermediate moisture foods (Table 5.5).
These few examples indicate that edible films could be used for additive
retention on the surface of food products.
Preservative diffusion through edible films is influenced by various
parameters: film characteristics (type, manufacturing procedure), food
characteristics (pH, aw), storage conditions (temperature, duration, etc.) and
solute characteristics (hydrophilic properties, molar mass).
The effect of the film-forming material on sorbic acid permeability has
been studied for various edible films (Table 5.6). It appears that film
composition (type of film forming agent, presence of lipids) affects even the
sorbic acid permeability. For example, a 65% reduction of the sorbic acid
permeability of a methylcellulose edible film is observed when palmitic acid
is added to the hydrocolloid matrix, there is a 75% reduction for a
hydroxypropyl methylcellulose edible film.
Table 5.6 Sorbic acid permeability of various edible films at 24°C and aw =
0.77
Film
Chitosan
MC
HPMC
MC and palmitic acid (weight ratio 3:1)
HPMC and palmitic acid (weight ratio 3:1)
Zein
Sorbic acid permeability
(x 108 g mm m"2 s'1 (g/1)"1)
0.865
0.334
0.830
0.120
0.205
Similar values
(According to Torres et al, 1985b; Vojdani and Torres, 1989a, b.)
(HPMC = hydroxypropylmethylcellulose; MC = methylcellulose.)
According to Vojdani and Torres (1990), sorbic acid permeability
decreases in composite films (hydroxypropyl methylcellulose or methylcellulose and fatty acids) as the lipid derivative concentration increases; it also
decreases as the length of the carbon chain in fatty acids increases and with
the presence of double bonds. This is consistent with published data on the
solute permeability of synthetic lecithin liposomes. De Gier, et al. (1968)
found that increasing the lecithin fatty acid chain length decreased the
permeability of glycerol and erythritol through these artificial membranes.
McElhaney et al. (1970) showed that the permeability of liposomes was
altered by the geometrical configuration and the number of double bonds in
the fatty acid component.
The composite film-forming technique used to form composite films or
coatings (emulsions or multilayers) also affects the sorbic acid barrier
properties. According to Vojdani and Torres (1989a) the lowest permeability
values are found with bilayer composite films. For example, the sorbic acid
permeability for an emulsion composite film of methylcellulose and palmitic
acid was 3 times lower than for bilayer composite film.
As noted for water vapor and gas permeability, temperature and relative
humidity affect the permeability of edible film to sorbic acid (Figure 5.7). At
constant temperature, the sorbate permeation rate decreased as aw decreased
(Vojdani and Torres, 1989a; Rico-Pena and Torres, 1991). This is consistent
with studies in food model systems (Giannakopoulos and Guilbert, 1986a)
where sorbic acid diffusivity rises at high aw.
Sorbic Add Permeability
(xlO f g.m.nr 2 .* 1 .**/!)- 1 )
Sorbic Add Diffusivity
(xlO lf m ' . s 1 )
Water activity
Figure 5.7 Effect of water activity on sorbic acid permeability of edible multicomponent films
composed of methylcellulose and palmitic acid (weight ratio 3:1), at 24°C (•), (according to RicoPena and Torres, 1991); and on sorbic acid diffusivity in agar gels with sucrose (•) or glycerol (o)
at 25°C (according to Giannakopoulos and Guilbert, 1986b).
Sorbic Add Permeability (SAP), (x 108 g . m . nr*. s ! . (g /1)')
Temperature (0C)
Figure 5.8 Effect of temperature on sorbic acid permeability of edible multicomponent films
composed of methylcellulose (MC) or hydroxypropylmethylcellulose (HPMC) and palmitic acid
(weight ratio 3:1); of edible chitosan films, at aw = 0.77 (according to Vojdani and Torres, 1989b
and 1990); and of edible pectin films, at aw = 1.0 (according to De Savoye et al., 1994). o, MC; •,
MC and palmitic acid; a, HPMC; •, HPMC and palmitic acid; A, chitosan; A, pectin (SAP X
0.005).
An increase in temperature causes a decrease in film sorbic acid barrier
properties (Figure 5.8). These variations can easily be analysed through
Arrhenius-type representations. Vojdani and Torres (1989b) noted that
activation energy is affected by the solvent embedding the film, which
suggests that the diffusion process in the film occurs mainly through the
aqueous phase. Consequently, the performance of edible coatings controlling
surface preservative concentration will depend strongly on the composition
of the aqueous phase of the coated food.
The sorbic acid permeability of edible films varies according to pH
(Figure 5.9). At constant aw and temperature, sorbic acid permeability
decreases when pH increases. An increase in pH lowers permeability,
possibly due to a change in the sorbic acid formed around the pKa (equal to
4.8), thus modifying the diffusion properties. Longer surface retentions of
sorbic acid are possible at higher pH (and also at lower aw and lower
temperature). It should be noted that the higher retention at higher pH would
help balance the lowering of sorbic acid effectiveness as pH is increased
(Eklund, 1983).
The microbiological analyses carried out have confirmed the sorbic acid
retention efficiency of zein-based edible films, formed on the surface of
intermediate moisture cheese analogs, relative to microbial stability of the
food product (Torres et al., 1985b; Torres and Karel, 1985). These edible
Sorbic Add Permeability (SAP), (x 10f g. m. m2 . s '. (g / W )
pH
Figure 5.9 Effect of pH on sorbic acid permeability of edible multicomponent films (•) composed
of methylcellulose and palmitic acid (weight ratio 3:1), at aw = 0.77 and 24°C (according to RicoPena and Torres, 1991) and of edible pectin (Psa X 0.01) films (D) at aw = 1.0 and 200C (according
to De Savoye et ah, 1994).
barrier layers double the shelf-life of the food product before the appearance
of microorganisms. Torres et al. (1985b) have assessed the potential sorbic
acid retention efficiencies of edible films according to utilization conditions.
For example, a composite film (methylcellulose-palmitic acid) applied to a
food item (at pH = 5.0 and aw = 0.8) stored at 24°C reduces the amount of
sorbic acid diffused in the food mass by more than 50%. The effect that
edible active film (according to sorbic acid permeability) has on increased
surface microbial stability has been estimated by Torres (1987). For
example, with the use of a methylcellulose and palmitic acid (weight ratio =
3:1) edible film on food stored at 24°C, then the permeability = 0.042 g mm
m"2 24 h"1 (g/1)"1; the surface protection can be predicted to last 30 and 120
days for a 0.1 mm film and 0.2 mm film respectively. If the item is stored
under refrigeration (at 5°C), these values increase to 82 and 328 days
respectively (Torres, 1987; Vojdani and Torres, 1989a). According to these
authors, these estimations need to be confirmed using a specific food system
and challenging microorganism.
Guilbert (1988) conducted microbiological tests on intermediate moisture
fruit pieces (Table 5.7). Significant improvement of the microbial stability
was observed with coatings containing sorbic acid and no spoilage could be
detected after 40 days' storage in the case of papaya cubes coated with
carnauba wax containing sorbic acid. The coating efficiencies were in the
following order: carnauba wax + sorbic acid > carnauba wax > casein +
sorbic acid > casein > no coating. The fact that casein film with sorbic acid
was less effective than the carnauba wax with sorbic acid may be explained
Table 5.7 Stability of coated intermediate moisture papaya cubes (as a function of a w ) at 30 0 C
inoculated with Aspergillus niger(a) or Staphylococcus
rouxii{h)
Delay for first apparent spoilage
(days)
aw
0.71
aw
0.75
aw
0.84
aw
0.90
Control (non coated)
(a)
(b)
>40
>40
>40
>40
13
3
4
1
Coated with casein
(a)
(b)
>40
>40
>40
>40
12
2
4
2
Coated with casein and sorbic acid
(a)
(b)
> 40
>40
> 40
>40
22
>40
10
17
Coated with carnauba wax
(a)
(b)
> 40
>40
> 40
>40
> 28
>40
10
14
Coated with carnauba wax and
sorbic acid
(a)
(b)
> 40
> 40
> 40
> 40
32
> 40
10
> 40
(According to Guilbert, 1988.)
by the poor retention properties and the high initial pH of the casein film
(Guilbert, 1988).
The improvement of food microbial stability can also be obtained by
reducing surface pH. This can be achieved by using films or coatings that
immobilize either specific acids or charged macromolecules. Torres and
Karel (1985) succeeded in obtaining a temporary pH difference between the
surface and the core of intermediate moisture foods by adding lactic acid to
zein-based edible films. The development of edible films that immobilize
Contamination: togN (cells / cm2)
(uncoated control)
(reduced pH surface)
Time (days)
Figure 5.10 Effect of reduced surface pH on the microbiological quality of an intermediate moisture
cheese analog coated with a carrageenan and agarose film; challenged with Staphylococcus aureus
S-6 (aw = 0.88 and 35°C) (after Torres and Karel, 1985).
charged macromolecules has made it possible to obtain a pH difference
between the surface and core of enveloped intermediate moisture food
products. A Donnan equilibrium model for semipermeable membranes
containing charged macromolecules can be used to assess the possibility of
obtaining a permanent pH difference between two components separated by
a membrane. The use of agarose- and carrageenan-based edible films (with
charged macromolecules) decreases the surface pH by about 0.5. In addition
to carrageenans, slightly esterified pectins could probably be used for similar
applications (Guilbert and Biquet, 1989). A decrease in surface pH can also
improve the stability and efficiency of antimicrobial agents such as sorbic
acid.
Microbiological analyses have confirmed the antimicrobial efficiency of
this type of surface treatment (Torres and Karel, 1985). Studies of bacterial
growth (Staphylococcus aureus) on intermediate moisture foods revealed
improved microbiological stability with the use of carrageenan-based edible
films (Figure 5.10).
Edible films and coatings are a useful mean to reduce the rate of some
deteriorative reactions such as surface microbial development or oxidation.
They could also be used to achieve the slow release of flavor compounds
during food storage or consumption. Initial choices of film-forming
materials and of fabrication conditions allow modification of solute retention
properties of these edible active layers, and allow selection of the efficient
functional properties in relation to each specific application, according to
specific needs.
5.5
Conclusion
Edible films and coatings can be used to control gas exchange (water vapor,
oxygen, carbon dioxide, etc.) between the food product and the ambient
atmosphere, or between components in a mixed food product, and to modify
and control food surface conditions (pH, level of specific functional agents,
etc.). It should be stressed that the characteristics of the film or coating and
the application technique must be adapted to each specific utilization.
Edible superficial layers provide supplementary and sometimes essential
means to control physiological, microbiological and physicochemical changes in food products. The active edible layers concept can thus be extended
to new fully adapted superficial or internal applications for food products.
Among these new potential applications, layers enriched with susceptors for
microwave treatments or with catalyst for specific reactions on the one hand,
or with flavor, preservative, ethanol, etc., for slow-release systems on the
other hand, can be mentioned. The development of an active edible layer is
mainly limited by formulation constraints, i.e. the layer composition must be
compatible with the product characteristics and regulation, and by industrial
production constraints, i.e. coatings or preformed films applications procedure must be realizable on the industrial scale and be easily integratable in
the food processing line.
Acknowledgements
Dalle Ore Florence (CIRAD-SAR, Montpellier, France), Thibaut
Romain (CIRAD-SAR, Montpellier, France), Aymard Christian (CNRS,
Montpellier- CIRAD-SAR, Montpellier, France) and Cuq Jean Louis
(Universite de Montpellier II, France) are gratefully acknowledged for
helpful discussions and technical assistance. Part of the work presented here
was supported by the EC project: AIRl - CT92 - 0125.
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