This document reviews research on edible films and their potential as barriers to oxygen and aroma transport. It begins by introducing the theoretical basis for determining oxygen and aroma barrier properties. It then provides historical context on the development of barrier polymers from early packaging materials to modern synthetic polymers. The document examines how a polymer's structure and composition affect its mass transport properties. It surveys research on edible films and their oxygen and aroma barrier performance. It suggestsareas for further research and potential applications of edible films to provide barrier properties while reducing packaging waste.

1. Review
A
Oxygen and aroma barrier
properties of edible films:
A review
K.S. Miller
Interest in maintaining
and J.M. Krochta
food quality while reducing packaging
waste has encouraged the exploration of the oxygen and
aroma transport properties of edible films. This review article
introduces the theoretical
basis for oxygen and aroma barrier
property determination and presents a brief historical perspective of the development of barrier polymers. The effects
of structure and composition on mass transport in edible
films are examined and compared with those of the more
thoroughly
investigated synthetic polymers. A survey of edible
film oxygen and aroma barrier research is presented; areas requiring additional investigation are suggested, for applications
as well as basic research. The potential
of edible films and
coatings to provide excellent aroma retention and superior
oxygen barrier properties makes this quite a promising area of
research for both food and packaging scientists.
Food quality is easily diminished by the deleterious
transport of aroma compoundsand oxygen. Food is required to satisfy the biological need for a source of nutrition; however, it is the flavor and aroma of a food that
provide the impetus for its consumption. In fact, a large
segment of commercial manufacturing deals with the
production of packaging that extends the shelf life of
food by controlling oxygen and aroma transport. A
food’s characteristic flavor and aroma are the result of a
complex construct of hundredsof individual constituent
compounds interacting to produce a recognizable taste
and aroma. Thus, if one or more flavor constituents are
altered or diminished, food quality may be reduced. A
reduction in food quality may result from the oxidation
of aroma componentsdue to the ingressof oxygen, or it
may be the result of the loss of specific aroma compounds to the packaging material or environment.
Therefore, it is critical to identify both the oxygen and
aroma masstransfer properties of food packaging.
K.S. Miller
(formerly
Engineering,
University
Frito-Lay,
(corresponding
Engineering
CA 95616,
228
Inc.,
7701
author]
of the
Department
of California,
Legacy
Drive,
of Biological
Davis,
Plano,
is at the Departments
and Food Science
& Technology,
USA (fax: +l-916-752-4759;
CA
95616,
TX 75024,
of Biological
University
e-mail:
Copyright 01997. Elsevw Science Ltd All rights resewed
PI,: 50924.2244(97~01O51-0
and
Agricultural
USA)
USA.
is now
and
Agricultural
of California,
jmkrochta@ucdavis.edu).
0924.2244/97~$17.00
at
J.M. Krochta
Davis,
Origin and definition of edible polymer films
Foods such as fruit and nuts have natural built-in
packaging protection in the form of skins and shells.
These natural barriers regulate the transport of oxygen,
carbon dioxide and moisture and also reduce flavor and
aroma loss. However, processedfoods dominate today’s
diet; and no such natural barriers exist for processed
foods.
Humankind’s instinct to cover food (perhaps stemming from a desire to hide this precious commodity)
may have inadvertently led to the implementation of
food packaging. The very first package probably consisted of leaves, animal skin or the shell of a nut or
gourd’. Around SOOOBC, different types of packagthe
ing materialsthat were available included sacks,baskets
and bagsmade from plant or animal material, as well as
primitive pottery and ceramic vessels’.
By -15OOBC, hollow glass objects had begun to appear, but it was not until -AD~OO that the woven,
pressedsheetsthat eventually became known as paper
appearedr. Lard or wax was used to enrobe fruit and
other food items in 16th-century England2. The first
plastic, a cellulose-basedpolymer, was introduced in
1856; then in 1907, phenol formaldehyde plastic
(Bakelite) was discovered’. From then on, a series of
discoveries and inventions led to today’s multitude of
primarily synthetic polymer packaging materials.
Polymer scientists have produced a variety of synthetic polymers and polymer laminatesthat are excellent
barriers to both oxygen and aroma compounds. However, despite the availability of these synthetic barriers,
the food industry is now considering natural packaging
biopolymers such as edible and biodegradable polysaccharide or protein films. Although these biopolymers
share their origins with the early, all-natural packaging
materials, they have many of the same properties and
are as convenient as the synthetic polymers that they
augment. Environmental and economic reasonsas well
as product development and consumer trends have
pushed food and packaging scientists along this cyclic
path.
Edible polymer films may be formed as either food
coatings or stand-alonefilm wraps and pouches. These
biopolymer films have potential for use with food as
oxygen and/or aroma barriers2. This reduces the requirements of the synthetic polymer to the provision
of a barrier to moisture loss and protection of the food
from external contamination. Thus, the amount of synthetic packaging is reduced and recyclability is increasedbecausethe need for synthetic laminates, often
usedto improve oxygen and aroma barrier properties, is
diminished.
Regardlessof whether it is a synthetic polymer or
biopolymer, a polymer’s mass transport properties are
influenced by similar factors; theseinclude composition
and structure, which directly affect a film’s performance
as a barrier to quality loss. For these reasons,environmental and processingconditions that affect the composition and structure of polymer films are of great interest
to both food and polymer scientists.
Trends
in Food Science
&Technology
July 1997 [Vol.
81

2. Box 1. Polymer
film mass transport
1
properties
The diffusion coefficient
describes the movement
of permeant molecules through a polymer, and thus represents a kinetic property of
the polymer-permeant
system. Figure 1 shows the activated diffusion process used to describe permeant movement
in a polymer.
Activated diffusion
is described as the opening of a void space
among a series of segments of a polymer chain due to oscillations
of the segments (an ‘active state’), followed by translational
motion
of the permeant within the void space before the segments return
to their ‘normal state’3. DiBenedetto
pointed out that both the active and normal states are long-lived,
as compared with the translational rate of the permeant.
Fick’s first law in one dimension defines the diffusion coefficient:
,=-D$
When S is independent
of the sorbed permeant
concentration
and vapor pressure (i.e. at sufficiently
low permeant
concentrations), then the relationship
between c and p becomes linear and
S is referred to as the Henry’s law solubility
coefficient.
This relationship
is often used to calculate the solubility
coefficient
from
sorption isotherms, which are plots of the permeant concentration
in the headspace above a polymer versus the concentration
of the
permeant within the polymer.
The permeability
coefficient
incorporates
both kinetic
and
thermodynamic
properties
of the polymer-permeant
system, and
thus provides a gross mass transport
property.
The permeability
coefficient,
P, is most commonly
related to D and S as:
P=DS
(1)
where / is the diffusive mass transfer rate of permeant per unit area,
c is the concentration
of permeant, x is the length and D is the diffusion coefficient.
The solubility
coefficient
describesthe
dissolution
of a permeant
in a polymer, and thus represents a thermodynamic
property of the
polymer-permeant
system. The solubility
coefficient
may be defined by an adaptation of the Nernst distribution
function as:
c=sp
(2)
where p is the vapor pressure of the permeant and S is the solubility
coefficient.
The solubility
coefficient
is a function of temperature
and may be a function of the vapor pressure (or concentration
of
dissolved permeant).
This article first reviews the parametersthat are used
to characterize masstransport in polymer films, including the relationship between polymer structure and
those masstransport parameters.The compositions and
structures of edible films are then compared with those
of synthetic polymers, and current researchon oxygen
and aroma transport in edible polymers is summarized.
Finally, potential applications for edible oxygen and
aroma barrier films are examined, and corresponding
basic and applied researchneedsare identified.
(3)
when both D and S are independent
of concentration.
Permeability
is defined at steady state with D and S constant
integrating Eqn 1 and combining
it with Eqns 2 and 3 to obtain:
(dM/dt)
;;$dy
L
P=
(4)
ASP
where M is the quantity of permeant (which can be expressed as
either mass or volume),
t is time, L is the polymer film thickness,
A is the cross-sectional
area of the polymer,
Ap is the partial pressure difference across the polymer,
and P is the permeability
coefficient. The term (dM/dt) is the slope of the transmission
curve and
is required to be at steady state for the permeability
coefficient
calculation.
essentialfor activated diffusion. Factors affecting a polymer’s structure have a direct effect on segmental
mobility
and, therefore, influence its mass transport properties.
Several polymer properties influence permeability:
chemical structure, method of polymer preparation, polymer processing conditions, free volume, crystallinity,
polarity, tacticity, ciosslinking and grafting, orientation,
presence of additives, and use of polymer blends4.
Researchers
have shown that an increasein crystallinity,
density, orientation, molecular weight or crosslinking
resultsin decreased
polymer permeability5x6.
A barrier polymer inhibits permeantprogress,thereby
presenting a greater barrier to mass transport than the
permeant would otherwise meet in the absenceof the
polymer. The necessary
characteristics a barrier polymer
of
include: a degreeof polarity, high chain stiffness,inertness
Structural influences on polymer masstransport
properties
A film’s mass
transportpropertiesareoften described
by
three common coefficients: diffusion (the rate of movement of a permeantmoleculethrough the tangledpolymer
matrix, basedon, for example, the size
of the permeantmoleculeand the strucPermeant
molecule
ture of the polymer matrix), solubility
Segments
(the partitioning behavior of a permeant
I
I
of polymer
I
molecule between the surface of the
chains
I
polymer and the surroundingheadspace)
I
4
2L
andpermeability (the rate of transportof
a permeantmolecule through a polymer
=$x-I
asa resultof the combinedeffects of difI
Reference
I
I
fusion and solubility). These are forI ----position
mally defined in Box 1.
Activated
Normal
Figure 1 depicts the activated diffustate
state
sion processand clearly showsthe importanceof polymer structurefor perme- F;n ,
ant transport. The ability of a segment “@ ’
of the polymer chain to relax and shift The activation process for diffusion. Adapted from ‘Molecular Properties
its structure, allowing the permeant An Interpretation of Gaseous Diffusion Through Polymers’ in). Polym.
accessto newly formed void spaces,is A.T. DiBennedetto, and reproduced with permission from John Wiley
l
Trends in Food Science
& Technology
July 1997 [Vol. 81
by
I
I
I
gg%J-;
gg
I
I
I
Normal
state
one diffusional
of Amorphous
SC;.: Part Al,
after
jump
High Polymers.
Copyright
II.
0 1963,
& Sons, Inc.
229

3. to permeants, high chain-to-chain packing, some intermolecular crosslinking and a high glass transition temperature’. The effects of the previously mentioned polymer properties on mass transport have been defined
primarily in terms of oxygen and moisture transport. The
diversity of aroma compounds has impeded the thorough
investigation of their myriad polymer-permeant interactions and of the associated effects on aroma permeability.
them open to engage in hydrogen bonding even when the
cohesive energy density is relatively high. In the case of
a polymer with a simple carbon repeating unit, a hydrogen substituent results in an oxygen permeability coefficient that is 117500 times greater than that of the same
backbone with a hydroxyl group substituent. One would
also expect polymers with a higher cohesive energy density to be better barriers to nonpolar aroma compounds.
Chemical structures
Knowledge of the effects of differing chemical structures on a polymer’s mass transport properties is important for today’s packaging industry. The types of
substituent groups present in a polymer can have a
tremendous effect on the variability of the permeability
coefficient by influencing two main factors: how tightly
the polymer chains are bound together and how much
free volume exists between the chain9.
Free volume
Free volume is a measure of the degree of interstitial
space between the molecules in a polymer. The diffusion
coefficient and the permeability coefficient both decrease
with a decrease in free volume for carbon dioxide, helium and methane in various polymers’. Maeda and Paul’
pointed out that the addition of plasticizers to increase the
free volume resulted in lower glass transition temperatures, whereas the addition of anti-plasticizers to decrease
the free volume increased the glass transition temperature.
Table 2 shows the dramatic effect of free volume on the
permeability of oxygen. As the fractional free volume decreases from 0.204 for poly(4-methyl pentene-1) to 0.03
for poly(viny1 alcohol) (PVOH; see Box 2 for all polymer
abbreviations), the oxygen permeability diminishes by six
orders of magnitude. Stiff-chained polymers that have a
high glass transition temperature generally have low gas
permeability, unless they also have a high free volumea.
These results suggest that nonpolar aroma compounds
would also have low permeability coefficients in polymers
with a low free volume.
Cohesive energy density
Cohesive energy density is a measure of the polarity of
a polymer and of the energy binding the polymer chains together. In general, the higher a polymer’s cohesive energy
density, the more difficult it is for the polymer chains to
open and allow a permeant to pass (highly polar permeants
such as water being an exception to this rule). An empirical correlating parameter, dubbed the Permachor value, can
be used to predict gas permeation, if free volume and cohesive energy density are known8. The effects of various
substituent groups on polymer permeability are shown in
Table 1. As increasingly polar substituents are added to the
same carbon backbone (thus increasing the cohesive energy density), oxygen permeability decreases by five orders
of magnitude. However, water, being highly polar, does
not rely on the polymer chains to ‘open’ and can force
Crystallinity
Crystallinity is a measure of the degree of order of the
molecules in a polymer. Polymer properties that affect
crystallinity include the structural regularity of the poly, mer chains; polymer chain mobility,
which allows variable conformation;
Table 1. The effects of cohesive energy density on permeability”
the repeating presence of side chains,
which engage in intermolecular bondBackbone:
CCH,-CHj
ing; and the absence of bulky side
I
chains, which interfere with the crystal
X
Permeability
at 25”Cb
lattice formation”‘. The mass transfer
Substituent
CED
of a gas or aroma in a semi-crystalline
(cal/cm3)
Oxygen
Water
Polymer
group C4
polymer is primarily a function of the
66
0.188
100
amorphous phase, because the crystal-H
Polyethylene
line phase is usually assumed to be
Polystyrene
85
0.168
1100
impermeable. Table 3 illustrates the ef/
-0
fects of crystallinity on oxygen perme88
0.023
8500
-OCOCH,
Polybinyl
acetate)
ability. As the percent crystallinity of a
Poly(vinyl
chloride)
94
0.0036
250
-Cl
polymer increases, the oxygen perme-CN
Polyacrylonitrile’
180
0.000039
300
ability decreases. The degree to which
d
oxygen permeability is affected is highly
Poly(vinyl
alcohol)
220
0.0000016
(dry)
-OH
dependent on polymer structure. An increase in the crystallinity of polyethyl“Adapted
from Ref. 8; reproduced
with permission
from Technomic
Publishing
Co., Inc.
ene from 43% to 74% results in a five“Units for permeability
are cm’ pm/(m? d.kPa), whereby
a given volume of permeant
(cm3) moves through a
specified
cross-sectional
area @‘polymer
(m’), which is of a given thickness
(km), in a certain time interval (d)
fold decrease in oxygen permeability,
with a defined pressure driving force (kPa) across that polymer thickness
whereas an increase in the crystallin‘Unannealed
film
ity of poly(ethylene terephthalate) from
dPoly(vinyl
alcohol)
is soluble in water
~10% to 45% yields a threefold deCED, Cohesive energy density
crease in oxygen permeability. The low
230
Trends in Food Science
& Technology
July 1997 [Vol. 81

4. Table 2. The effects of free volume
diffusion coefficients for aroma compounds in glassy
polymers suggest that the permeability coefficients for
polymers with a high crystallinity would be correspondingly low”.
Orientation
Orientation refers to the alignment of the polymer chains
in the plane of the polymer backbone, and is a by-product
of the processing operation. Sha and Harrison’” mentioned
several mechanisms for these orientation effects. They
reported that the decrease in the fractional free volume
of the amorphous region with orientation correlated well
with the decrease in permeability, solubility and diffusivity coefficients. However, others contend that the alignment of the polymer’s crystallites increases the tortuosity
of the permeant’s path, thus significantly reducing the
permeability only in the case of semi-crystalline polymer?. The minimal reduction in oxygen permeability
following 300% orientation of completely amorphous
polystyrene is cited in support of this observationx.
Tacticity
Tacticity refers to the stereochemical arrangement of the
substituted groups in relation to the plane of the polymer
backbone. Isotacticity occurs when all of the substituent
groups lie on one side of the plane of the main chain. If
substituent groups alternate on either side of the plane,
the polymer is considered to be syndiotactic, and atactic if
the substituent groups are randomly configured. Min and
PaulI examined the influence of tacticity on the permeability of carbon dioxide, oxygen and nitrogen in poly(methy1
methacrylate) (PMMA). It was concluded that permeability increased as the percentage of syndiotactic substituents increased. Jasse et ~1.~ suggestedthat these results
might be indicative of a more densely packed polymer
structure for isotactically substituted polymers.
Crosslinking
Crosslinking is the formation of intermolecular bonds
among the chains of a polymer. Researchhas examined
Box 2. Polymer
on permeability”
Fractional free
volumeb
Polymer
Poly(4-methyl
pentene-1)
Oxygen permeability
at 25°C’
0.204
1.56
Polystyrene
0.176
0.17
Polycarbonate
0.168
0.097
0.132
0.0065
0.120
0.0029
0.098
0.0019
0.080
0.000039
(annealed)
0.050
0.000016
(a= 1)
0.030
0.0000016
Poly(methyl
Nylon
methacrylate)
6 (a= 1 .O)
Poly(vinylidene
fluoride)
Poly(acrylonitrile)
Poly(acrylonitrile)
Poly(vinyl
alcohol)
‘Adapted
from Ref. 8; reproduced
‘Fractional
free volume
with permission
from Technomic
is the ratio of the interstitial
space behveen
volume of the polymer
-Units for permeability
fraction
(dry)
Publishing
Co., Inc.
molecules
to the
at a temperature
of absolute zero
are cm’ pm/(mVkPa)
(see Table 1)
~1, Amorphous
(dry)
amorphous
volume
state, as opposed
(the ratio of the volume
to a crystalline
of the polymer
state, to the total volume
that exists in an
of the polymer)
the effects on masstransport of polymer crosslinking
induced by heat curing and irradiation of a variety of
polymers and by enzymatic treatment of protein-based
edible polymersI&-]’. Heat curing of biopolymers resulted in decreasedwater vapor permeability for soy
proteinI and whey protein isolate15.
These effects were
attributed to an increase in intermolecular crosslinking
amongthe protein strandsduring heating.
Polymer chemists have made great advances in producing synthetic polymers that have very specific properties and characteristics; however, predicting and controlling the structure of biopolymer films are both very
difficult tasks. Food scientists have begun fleshing out
the properties and characteristics of edible films, but
many significant topics pertaining to the application of
Table 3. The effects of crystallinity
on permeability”
abbreviations
CMC:
Carboxymethylcellulose
EVOH:
Ethylene
HDPE:
High-density
HPC:
Hydroxypropylcellulose
HPMC:
Hydroxypropyl
LDPE:
Low-density
MC:
Methylcellulose
PEG:
Poly(ethylene
PMMA:
Poly(methyl
PVDC:
Poly(vinylidene
PVOH:
Poly(vinyl
VOH:
Vinyl
(d = 0.92)
43
0.19
Polyethylene
(d = 0.955)
74
0.038
Poly(ethylene
terephthalate)
Poly(ethylene
terephthalate)
30
0.0024
Poly(ethylene
terephthalate)
45
0.0014
! Polyethylene
vinyl alcohol
copolymer
polyethylene
methylcellulose
polyethylene
0.0049
6
0
0.0029
Nylon
glycol)
Nylon
6
60
0.00045
(dry)
Polybutadiene
methacrylate)
0
Polybutadiene
60
(dry)
0.97
0.27
chloride)
“Adapted
alcohol)
‘Units
Trends in Food Science
40
alcohol
& Technology
from Ref. 8; reproduced
for permeability
d, Density
July 1997 [Vol. 81
with permissron
are cm3 p,m/(m’d
from Technomic
kPa) (see Table
1)
Publishing
Co., Inc.

5. edible films remain unexplored. Examination of the influences of the composition of synthetic polymers on
oxygen and aroma barrier properties suggests that the
polar nature of edible polymer films should yield excellent oxygen and aroma barrier properties.
Edible polymer film composition and structure
Edible polymer films include polysaccharides and/or
proteins. Kester and Fennema? have produced an excellent overview of the types, methods of preparation,
properties and applications of all types of edible polymers, and pointed out the rationale for developing these
films as packaging supplements. The authors noted that
possible functional properties include the retardation of
moisture migration, gas transport (oxygen and carbon
dioxide), oil and fat migration and solute transport, as
well as improved mechanical handling properties, additional structural integrity, use as a vector for food
additives, and retention of volatile flavor compounds.
Recently, Krochta and De Mulder-Johnston’” provided
a synopsis of the research on edible polymer films and
their potential applications. They also touched on nutritional, safety and health issues associated with edible
polymers. Edible polymer films prepared from celluloses,
starches, other polysaccharides (alginates, carrageenans
and pectinates) and proteins (collagen, gelatin, zein, gluten, soy protein, casein and whey protein) were reviewed.
Water-insoluble cellulose is brought into aqueous solution by etherification with methyl chloride, propylene
oxide or sodium monochloroacetate to yield the non-ionic
methylcellulose (MC), hydroxypropyl
methylcellulose
(HPMC) and hydroxypropylcellulose (HPC) films and the
ionic sodium carboxymethylcellulose (CMC) filmst9. The
degree of substitution that occurs during these etherification reactions affects the properties of a film such as
water retention, sensitivity to electrolytes and other solutes,
dissolution temperatures, gelation properties and solubility in non-aqueous systems. Cellulose ether films are resistant to fats and oils, and are therefore likely to be good
aroma barriers”. The cellulose ethers produce moisturesensitive films that are effective oxygen barriers, and
when applied to various fresh commodities, they have
been shown to retain flavor components during storage,
thus indicating their potential aroma barrier properties”.
The linear starch polymer amylose produces a hydrophilic film with low oxygen permeability; hydroxypropylated amylose also yields films with very low oxygen
permeability19. Plasticization, chemical crosslinking and
esterification all affect the final structure of the starch
film to varying degrees. Coating apple slices and dried
apricots with starch hydrolysates resulted in a better flavor, indicating their potential aroma barrier properties”.
Alginate films are composed of polymer segments of
pOly@-D-InaI’IUUrOniC
acid), poly(a-r.-guluronic
acid)
and of a segment of alternating D-mannuronic and
L-guluronic acid residents2. Alginate films have been
shown to reduce oxygen transport and aroma loss in various food products 19.Alginate film structure is affected by
the concentration of polyvalent cations in the gel (such as
calcium), rate of cation addition, time of cation exposure,
232
pH, temperature and presence of other constituents such as
hydrocolloids2. The calcium ions pull the alginate polymer
chains together via ionic bonding and thus allow for increased hydrogen bonding. The same effect occurs with
pectin films. Carrageenan films are thought to form a threedimensional polymer structure via the formation of a
double-helix structure, which is also thought to be effected by inter-chain salt bridges’. The oxygen and aroma
barrier properties of films from pectins, carrageenans
and other polysaccharides have not been examined in the
literature.
Krochta?” discussed the effects of protein structure and
composition on edible film barrier properties. The proteins
must be in an open or extended form to allow the molecular interaction that is necessary for film formation.
The extent of this interaction depends on the protein
structure (degree of chain extension) and the sequence
of hydrophobic and hydrophilic amino acid residues in
the protein. Increased molecular interaction results in a
film that is strong but less flexible and less permeable.
The degree of hydrophilicity of the amino acid residues
in a protein controls the influence of moisture on the protein film’s mass transport properties*“. Most edible films
are quite moisture sensitive, but this inherent hydrophilicity
makes them excellent barriers to nonpolar substances such
as oxygen and some aroma compounds. As mentioned
previously, an increase in crystallinity, density, orientation,
molecular weight or crosslinking results in a decrease in
polymer permeability. Complicated protein structures
make the control of these factors quite challenging.
Researchers studying edible polymers have significant obstacles to surmount in simply producing a usable
film. Only of late have investigations of edible polymers
included the examination of barrier properties for permeants other than moisture. The promise of using a renewable resource to simplify packaging and extend food
shelf life has encouraged researchers to explore the oxygen and aroma barrier properties of edible polymers.
Oxygen and aroma barrier properties of edible
polymer films
Oxygen barrier properties
Oxygen permeability is the next most commonly studied transport property of edible polymer films after water
vapor permeability. Commercial data2’ on MC and HPMC
films indicate that they are moderate barriers to oxygen;
their oxygen permeability is approximately an order of
magnitude lower than that of low-density polyethylene
(LDPE), but two to three orders of magnitude greater than
that of poly(vinylidene chloride) (PVDC) and ethylene
vinyl alcohol copolymer (EVOH) at -24°C and 50%
relative humidity (Table 4). Although cellulose ethers
possess a chemical formula similar to that of EVOH, their
repeating ring and side-group structures probably produce
a smaller cohesive energy density, larger free volume
and smaller crystallinity relative to those of the linear
EVOH. The higher oxygen permeability of HPMC compared with that of MC can probably be attributed to the
larger HPMC side group, which results in HPMC having
a smaller cohesive energy density, larger free volume and
Trends in Food Science
& Technology
July 1997 [Vol. 81

6. Table 4. Comparison
polymer
of the oxygen
permeability
of edible polymer
films and conventional
synthetic
films
lower crystallinity than MC. Donhowe
Film type”
Test conditions
Permeabilityb
and FennemaZ2 found that compared
with other water or water-ethanol solCellulose-based:
vents, oxygen permeability was miniMC
24”C, 50% RH
97
mized when an MC film was formed
HPMC
24”C, 50% RH
272
from a water-ethanol
solvent in the
ratio of 75% : 25% at elevated temperaMC
25”C, 52% RH
90
ture (Table 4). Films formed in this
manner also had greater crystallinity,
Starch-based:
lower water vapor permeability, higher
Amylomaize
starch
25”C, <lOO% RH
<65
tensile strength and higher elongation.
Hydroxypropylated
amylomaize
starch
25”C, ~78% RH
-0
Donhowe and Fennema31 found that
glycerol, added at 30% (w/w), was a
Protein-based:
more effective plasticizer than propylCollagen
ene glycol in decreasing the tensile
RT, 0% RH
<0.04-0.5’
strength and increasing the elongation
Collagen
RT, 63% RH
23.3
of MC films. Both approximately douCollagen
RT, 93% RH
890
bled the oxygen permeability at -25°C
and 50% relative humidity. Lower
Zein : PEG t glycerol (2.6 : I)
25”C, 0% RH
38.7-90.3
molecular weight (molecular weight
Gluten :glycerol (2.5 : 1)
25”C, 0% RH
6.1
of 400 and 1450) poly(ethylene glySoy protein isolate : glycerol (2.4 :l)
25”C, 0% RH
6.1
~01)s (PEGS) were also good plasticizers but increased oxygen permeability
Whey protein isolate : glycerol (2.3 : 1)
23”C, 50% RH
76.1
by a factor of 4-5. Park et aL3? found
Whey protein isolate : sorbitol (2.3 : 1)
23”C, 50% RH
4.3
that at 0% relative humidity, the oxygen permeability of MC and HPC films
Synthetic:
increased as their molecular weight inLDPE
23”C, 50% RH
1870
creased. Propylene glycol was shown
to be a relatively poor plasticizer and
HDPE
23”C, 50% RH
427
produced a large increase in oxygen
Polyester
23”C, 50% RH
15.6
permeability at 0% relative humidity.
23”C, 0% RH
EVOH (70% VOH)
0.1
Interestingly,
although glycerol and
PEG-400 were found to be good plas23”C, 95% RH
12
EVOH (70% VOH)
ticizers for MC and HPC, they had litPVDC-based
films
23”C, 50% RH
0.4-5.1
tle effect over a range of concentrations on oxygen permeability at 0%
“See Box 2 for polymer abbreviations
relative humidity. On the other hand,
hUnits for oxygen permeability are cm’~~m/(m*~d kPa) (see Table 1)
Park and Chinnan
found that the
’ Based on a percentage of the oxygen permeability of PVDC-based film; Ref. 6
quantity of PEG-400 greatly affected
RT, Room temperature
the oxygen permeability of MC and
RH, Relative humidity
HPC at 0% relative humidity. RicoPeiia and Torres3” found that oxygen
acid composite
Butler et al.36 found that glycerol-plasticized chitosan
transmission through an MC-palmitic
film increased rapidly with relative humidity >57%, films had extremely low oxygen permeability at 0% relacorrelating well with moisture content. Park et al.35
tive humidity. Increasingthe plasticizer content increased
studied MC films laminated with a corn zein-fatty acid the oxygen permeability. Wong et aL3’ found that adding
layer. They found that oxygen permeability increased lauric acid to a chitosan film more than doubled the oxyrate. However, pahnitic acid or acetylated
as the concentration and chain length of the fatty acids gentransmission
monoacylglycerol reduced the oxygen transmissionrate
increased.
High-amylose amylomaize starch films are moderate by an order of magnitude.
As a group, protein films appearto have lower oxygen
to good oxygen barriersz3,with an oxygen permeability
that is lower than that of the cellulose ethers, even at permeabilitiesthan non-ionic polysaccharide films. This
higher relative humidity (Table 4). Oxygen permeability may be related to their more polar nature and more linear
is even higher for high-amylose films than for PVDC or (non-ring) structure, leading to higher cohesive energy
EVOH films. However, hydroxypropylated starch films density and lower free volume. At 0% relative humidity,
may have even lower oxygen permeability24.Apparently collagenfilm hasan oxygen permeability similarto that of
the starch structures in these films produce a combi- PVDC and EVOH filmsZ. However, collagenfii is more
nation of higher cohesive energy density, lower free sensitive to relative humidity; at -50% relative humidity,
volume and higher crystallinity than occurs in cellulose its oxygen permeability is one to two ordersof magnitude
greater than that of PVDC or EVOH films (Table 4).
ethers.
Ref.
21
21
22
23
24
25
25
25
26
27
28
29
29
6
6
30
6
6
6
I
Trends in Food Science
& Technology
July 1997 [Vol. 81
233

7. Films that are based on corn zein, wheat gluten, soy protein
or whey protein appear to possess an oxygen permeability
that is greater than that of collagen-based films (at 0%
relative humidity)‘hm29. This is probably due to the fact
that these globular proteins have a less linear structure and
a greater percentage of larger amino acid side groups than
collagen, resulting in a smaller cohesive energy density
and larger free volume. However, proper selection of plasticizer appears to reduce the oxygen permeability while
maintaining the mechanical properties, presumably by
affecting the polymer free volume (Table 4)29.
Gennadios et aL2’ investigated the effect of temperature on the oxygen permeability of corn zein, wheat gluten
and wheat gluten-soy protein isolate films at 0% relative humidity. Results showed good agreement with the
Arrhenius activation energy model. Based on the lack of
breaks in the Arrhenius plots, no structural transitions were
identified in the 7-35°C temperature range. Brandenburg
et af.‘8 discovered that the oxygen permeability of soy
protein films decreased as the pH of the film solution
preparations increased from 6 to 12. Gennadios et al.‘”
found that replacing glycerol plasticizer with triethylene
glycol in wheat gluten films produced a large increase
in oxygen permeability. This effect was attributed to the
larger size and less polar nature of triethylene glycol,
which would also correlate with an increased free volume and reduced cohesive energy density.
McHugh et aL4” studied the properties of films made
from fruit purCes. Peach puree films were found to be
better oxygen barriers than MC and other polysaccharide
films and comparable to whey-protein-based
films.
In general, the oxygen permeability of edible polymer
films, especially protein films, appears to be quite low.
Optimization of polymer structure by increasing crystallinity, orientation or crosslinking in pre-processing steps or
during film formation may result in further reductions in
the oxygen permeability of a film. Modification of polymer
structure combined with optimized selection of plasticizer
may produce edible films with oxygen barrier properties
that are as good as those of PVDC and EVOH films.
(wheat protein) films. An isostatic gas chromatograph
technique was used with a dual-detection scheme for measuring moisture and aroma transport simultaneously. The
gluten film was a better barrier to 1-octen-3-01 than either
the LDPE or MC film, but not as good a barrier as the
cellophane film.
Continuing this work, Debeaufort et al.“’ attempted to
explain the differences in l-octen-3-01 transport among
LDPE, cellophane, MC and gluten films. However, they
were unable to correlate aroma flux to the amount of
aroma absorbed, the hydrophobicity of the polymer, or to
trends in the diffusion coefficient. It was concluded that
the sorption-diffusion
model, alone, cannot describe the
aroma or moisture permeability in edible films. Furthermore, it was suggested that the variations in aroma permeability were due to a moisture plasticization phenomenon and the ‘sweeping’ action of water vapofls.
Whey protein films have excellent oxygen barrier propertiesz9. However, DeLassus” has shown that a polymer’s
oxygen barrier properties are not necessarily a reliable
indicator of its aroma barrier properties. The author cautioned that oxygen and aroma compounds behave quite
differently in glassy versus rubbery polymers. Glassy
polymers have medium to high oxygen diffusion coefficients but very low aroma diffusion coefficients (at low
permeant concentrations) rl. Rubbery polymers exhibit diffusivities for oxygen and aroma compounds that are of
the same order of magnitude (i.e. permeant size is not as
influential a factor)“. DeLassus” stated that trends in oxygen and aroma permeability are comparable within the
rubbery or glassy polymer categories, but not between
them. Recent work by Miller et a14’ indicates whey protein isolate films to be excellent barriers to d-limonene.
Miller and Krochta47 found whey protein isolate films
containing 25% glycerol (dry basis) plasticizer to be comparable to EVOH films as a barrier to d-limonene under
similar temperature and humidity conditions. Additionally,
d-limonene permeability in 25% glycerol whey protein
isolate films was found to be significantly affected by
temperature and relative humidity but not by permeant
concentrations in the range of 62-226 ppm (v/v)““.
Existing commercial applications of edible films include
Aroma barrier properties
Although a significant body of work concerned with the collagen as a casing for sausages and a wrap for smoked
oxygen barrier properties of edible films exists, the aroma meats, and gelatin and corn zein as encapsulating agents
for food ingredients and pharmaceuticals20. Evaluation of
barrier properties of edible films have not been thoroughly
examined. Recent reviews of the use of proteins as edible the basic barrier properties of edible polymers will pave
the way for additional applied research dealing with spefilms and coatings indicate that the literature is somewhat
cific food applications. Such applied studies of the oxylacking in research pertaining to the aroma barrier properties of edible films20s4’. However, reviews of the literature gen and aroma barrier properties of edible polymers will
on synthetic polymers are valuable resources to the re- aid in defining the limits of specific food applications.
searcher studying the aroma transport properties of edible Current research on edible biopolymers allows for speculation on several food-polymer applications.
films [Refs 42 and 43, and K.S. Miller (1997) Physical
Properties of Whey Protein Isolate Films: d-Limonene
Gas and aroma barrier food applications of edible films
Permeability, Water Vapor Permeability and Mechanical
Oxygen barrier applications
Properties (PhD thesis), University of California, Davis,
Applications that take advantage of the beneficial oxyCA, USA].
In fact, Debeaufort and Voilley4” were the first to ex- gen barrier properties of edible polymer films have been
amine aroma permeability in edible polymers. They explored for many years. Ganz4x found that HPC film
coatings provided peanuts with some protection from oxyexamined the co-permeation of moisture and l-octen-3-01
(mushroom aroma) in LDPE, cellophane, MC and gluten gen, but the effect wasnot well quantified. MC andHPMC
Trends in Food Science
& Technology
July 1997 [Vol. 81

8. coatings are commonly used for pharmaceutical tablets,
providing protection from oxygen, aroma and moisture
transport. Several researchers have found that CMC-based
coatings can delay ripening and improve the quality of
fresh fruit and vegetables by retarding the transport of
oxygenJ9-5’.
Park et al.s’ investigated the application of MC film
laminated with a corn zein-stearic acid-palmitic acid
blend for the packaging of potato chips. Acceptable chip
quality was maintained for up to 25 d at 25°C. The composition of the corn zein-stearic acid-palmitic acid blend
layer had no effect on the results.
Jokay et aL5’ concluded that sensory tests on stored
almond nut meats coated with hydroxypropylated
highamylose starch indicated considerable protection against
the development of oxidative rancidity. However, quantitative data were not presented. Murray and Luft5s found
that starch hydrolysate coating applied to apple slices before drying maintained whiteness more effectively than 2%
ascorbic acid solution, but not as effectively as sulphur
dioxide. However, slices coated with starch hydrolysate
were judged superior in flavor. Murray and Luft5s also
reported that almonds coated with the starch hydrolysate
had improved flavor and shelf life, indicating oxygen
barrier attributes for the coating; however, they did not
present any data.
Wanstedt et a1.j’ found that coating ground pork patties with calcium alginate either before or after precooking improved the quality of the final cooked product, as
measured by the development of oxidative rancidity.
Earle and Snyder 57 found that an alginate coating improved the flavor and color of frozen shrimps, probably
because of a reduction in rancidity. Earle and McKeej8
developed an alginate-based coating with oxygen barrier
properties for breaded and filled-dough food products.
Meyer et a1.s9 found that carrageenan coatings extended
the shelf life of poultry pieces by acting as an oxygen
barrier. Chitosan coatings were found to be effective in
extending the life of fresh fruit by modification of oxygen and carbon dioxide transfer60.h’.
Collagen casings for sausages are known to provide
some protection from oxygen’:. Gelatin coatings have
been found to be effective in protecting several meat
products from oxygen h3.M Zein-based coatings have been
.
used to reduce rancidity in nuts and confections65,6h. Corn
zein films were also shown to affect oxygen and carbon
dioxide exchange in fresh tomatoes, as evidenced by a delay in color change, firmness loss and weight loss during storage6’. The result was an extension of shelf life
by 6d. Coatings based on whey protein were shown to
reduce the oxygen uptake by dry-roasted peanuts68, delaying oxidative rancidity, as measured by the peroxide
value and hexanal content of the peanuts6’.
Aroma barrier applications
Edible films can be used as flavor carriers in addition
to providing a barrier to aroma 10~s~~~“.Andres’O also
pointed out that flavor quality deterioration can include
the loss of characteristic flavor owing to oxidation or
poor oxygen barrier properties. Thus, an edible film can
Trends in Food Science
& Technology
July 1997 [Vol. 81
assist in retaining the characteristic food flavor via its
aroma barrier properties and also limit quality deterioration due to oxidation via its oxygen barrier properties.
Researchers have examined the ability of edible coatings applied to harvested fruit to prevent the loss of
characteristic flavor. The use of edible coatings on citrus fruit resulted in an increase in desirable flavor compounds after storage, as compared with uncoated fruits2.
Cellulose-based composite films including wax seemed
to provide the best balance between flavor retention
and the prevention of weight loss due to moisture
transport5’.
Pervaporation, the removal of organics from an
aqueous solution through a separating membrane, has
been successfully utilized to enrich and recover flavor volatiles”. Understanding the behavior of flavors in
aqueous solutions, such as the systems used in these
pervaporation studies, provides insight into the potential
applications for edible films in environments with a
high water activity.
The sorption characteristics of edible films may allow
the incorporation of desirable flavors and aromas into
a coating for delayed release, thereby enhancing the
food’s flavor profile. Encapsulated flavors and aromas
could be released by heating and/or rehydration, as well
as by mastication. Hydrophilic edible films can be applied to any low-moisture food with a sensitive characteristic flavor to aid in aroma retention. An example
would be fruit-flavored chewing gums, which often lose
their characteristic aroma with time. Dry, fruit-flavored
cereal would be another potential application for edible
films to prolong a product’s shelf life by limiting aroma
transport.
Basic and applied research needs
The effects of factors, identified by Banker’?, that influence film mass transport - polymer structure and orientation, salt concentration, ion ratios, polymer-permeant
interactions, acid and base concentrations, addition of
dispersed solids, and permeant boundary layer thickness
- provide the edible film researcher with boundless avenues of research. Specifically, no work has been done
to optimize the influences of free volume, crystallinity
or orientation on the oxygen and aroma barrier properties of edible polymers.
Before a packaging specialist can take advantage of an
edible polymer’s barrier properties, the polymer must be
successfully applied to the desired food system. Guilbert’
examined the factors influencing the food film coating operation and concluded that the degree of cohesion (interactions among the polymer molecules) and the degree of
adhesion (interactions between the polymer and the food
molecules) are of critical importance to the successful
application of an edible packaging. The author mentioned several formulation and processing parameters
that influence cohesion and adhesion, including solution
temperature, solvent evaporation rate, solvent characteristics and the concentration of the film-forming polymer
molecules in the solution. Few researchers have focused
on the effects of these parameters on both the degree of
235

9. adhesion and the degree of cohesion during food film
coating. Understanding these basic effects is critical to
the successful application of an edible coating to a food.
Gaseous diffusion through polymers has long been
studied by polymer scientists. DiBenedetto3 concluded
that models of such diffusion depend on knowledge of
the physical properties of the polymer and the geometry
of the permeant. Lack of knowledge about these polymer and permeant properties restricts the applicability
of many of the models that have been proposed to predict oxygen and aroma transport.
Knowledge about the physical properties of edible
films is even more limited. Kester and Fennema* concluded that much of the edible film and coating work reported in the literature is of limited value owing to the
‘lack of quantitative data on barrier characteristics of the
coatings’. It is only through the compilation of barrier
properties and their correlation with edible polymer
structure and composition that it will be possible to apply
generalized theories explaining oxygen and aroma mass
transfer behavior to solve food packaging problems.
Finally, microbial stability is an area that will become more important as more edible polymers approach
commercial viability. This will be especially important
for higher-water-activity
applications. The addition of
antimicrobial agents and their migration in MC and
HPMC multi-layer polysaccharide films have been examined with respect to their effect on oxygen permeability34*73. However, other antimicrobial agents and
their effects on both aroma and oxygen permeability in
edible polymers have not been examined.
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