Full text of DOI:10.1111/j.1365-2621.2001.tb11339.x

JFS:Food Engineering and Physical Properties
Physical and Mechanical Properties of
Pea-Protein-based Edible Films
WON-SEOK CHOI AND JUNG H. HAN
ABSTRACT: Edible films produced from denatured pea protein concentrate (PPC) solution possessed the strength
and elasticity to resist handling. Increasing the concentration of the plasticizer (glycerol) in the film decreased tensile
strength and elastic modulus, and increased elongation and water vapor permeability (WVP).Very strong and stretch-
able films were obtained from 70/30 and 60/40 of PPC/glycerol composition, respectively. The low WVP value was
maintained over a range of glycerol concentration from 20% to 40%, in the dry film. Film solubility was not affected
significantly by the amount of the plasticizer. The physical and mechanical properties of the PPC films were compa-
rable with those of soy protein and whey protein films.
Key words: yellow pea (Pisum sativum L. Miranda), pea protein concentrate, edible film, tensile property, water
vapor permeability
Pea protein concentrate and isolate possess good nutri-
tional quality (that is, protein efficiency ratio, essential amino
acid content) and has potential as a dietary protein fortifier
(Linder 1985). However, the relatively poor functionality has
been reported for protein solubility, emulsion stability and
gelation property (Jackman and Yada 1989; Patel and others
1981; Sumner and others 1981; Johnson and Brekke 1983;
Naczk and others 1986; Klein and Raidl 1986). To increase the
utilization of pea protein concentrate or isolate, the function-
al properties of pea protein have to be developed and modi-
fied to meet the requirements for use as a functional ingredi-
ent, and other applications need to be explored.
Edible polymer films have been studied for food applica-
tions because of their potential for providing a barrier against
hazards, carrying active ingredients, and improving the me-
chanical integrity of coated foods (Perez-Gago and others
1999). Edible films can also reduce and simplify the packag-
ing materials required for the protection of food products
(Han and Krochta 1999; Perez-Gago and others 1999). Edible
films and coatings afford numerous advantages over con-
ventional nonedible polymeric packaging because of their
biodegradability, and have been used to protect pharmaceu-
tical ingredients and extend the shelf-life of food products
(Gontard and others 1993). Soy protein, whey protein, wheat
glutens, corn zein protein, collagen and gelatin have been
studied intensively for edible film applications.
Compared to the price of whey protein isolate ($13.5 to
27/Kg), soy protein isolate ($3 to 3.8/Kg) and corn zein ($23
to 35/Kg) (Krochta and De Mulder-Johnston 1997), pea pro-
tein concentrate ($2.5 to 2.8/Kg) provides an excellent oppor-
tunity for use of pea protein in food and other industries.
Like most synthetic plastic polymers, edible coatings and
film materials require property modifiers to improve the
physical and mechanical properties of the film and to pre-
vent chemical changes. As with synthetic plastics, plasticiz-
ers are incorporated into the edible coating/film materials,
which overcomes the brittleness caused by extensive inter-
molecular forces. The plasticizers reduced these forces and
increase the flexibility and extensibility of the film (Guilbert
and Biquet 1989). On the other hand, the addition of plasti-
cizers generally increases the permeability of both plastic
Introduction
EAS HAVE RECEIVED SPECIAL ATTENTION FOR THE UTILIZATION
P
of legume protein because they are already consumed
as a human dietary resource, have a high nutritional value
and possess various functionalities. Increased legume pro-
tein utilization by the food industry has led to research on
the functionality of legume and seed protein in foods (Kin-
sella 1979; Won and others 2000; Kim and others 1990). Al-
though soybean products are still dominant, efforts have
been made to explore the potential uses of other legumes
for their functional properties including field peas, horse-
beans, faba beans, Great Northern beans, chick-peas and
cowpeas (Vose 1980; Sosulski and McCurdy 1987; Sathe and
others 1981). Currently, the protein sources of edible films
and coatings include a variety of protein-rich legumes. For
example, peanut protein has recently been studied for use
in edible film materials. The mechanical and physical prop-
erties of the peanut protein films were determined by Jang-
chud and Chinnan (1999).
Peas, generally various cultivars of Pisum sativum L., be-
long to the family Leguminosae (Pomeranz 1985). Dried
Peas are generally composed of carbohydrates (35 %), pro-
teins (27 %), fibers (27 %) and a very small amount of lipids
(Boulter 1983). Compared to other pulse products, peas are
high in carbohydrates and fibers, and low in lipids. While
the fiber and starch fraction of peas are highly utilized by
bakery and meat industry, the protein fraction has been un-
derutilized with only a limited use as animal feed (Betker
1990). Pea protein has experimentally been used as a meat
extender in sausage and as a protein fortifier for bread
(Delaquis 1983; Grant 1983). Pea protein consists primarily
of globulins mainly and a small fraction of albumins. The
globulins (Ͼ 80% of total proteins) are storage proteins lo-
cated in the cotyledons and consist of legumin, vicilin and
convicilin (Pate 1977; Boulter 1983). Legumin is usually the
major, and vicilin is the second major globulin fraction
(Gatehouse and others 1981). Albumins, which compose 13
– 14% of the total proteins, are cytoplasmic proteins con-
sisting of many kinds of subunits and contain more sulfur
amino acid residues than the globulins (Mosse and Pernol-
let 1983; Pate 1977; Grant and others 1976).
© 2001 Institute of Food Technologists
Vol. 66, No. 2, 2001—JOURNAL OF FOOD SCIENCE 319

Properties of Pea Protein Edible Films . . .
and edible films to gas, water vapor and solute (Krochta and change in the length of the specimen to the original length
De Mulder-Johnston 1997; Han and Krochta 1999) and can between the grip (5 cm). Each tensile strength (TS), elonga-
decrease elasticity and cohesion (Delporte 1981). The most tion-at-break (E) and elastic modulus was obtained from 6
common edible plasticizers are polyols, mono/di- or oligo- replications of samples taken from the same film.
saccharides, lipids, and their derivatives.
Water Vapor Permeability (WVP)
The objectives of this research are: (1) to produce edible
protein films for food coating and packaging from pea pro-
tein concentrate; and (2) to determine their tensile proper-
ties, water solubility and water vapor permeability and to
evaluate the potential of pea protein for use as a coating/film
material. The ultimate goal of this study is to simplify the to-
tal packaging structure by substituting synthetic packaging
materials with a pea protein coating/film to reduce packag-
ing requirements, cost and contribution to solid wastes.
The modified ASTM E96-92 gravimetric method of
McHugh and others (1993) was used to determine the WVP
and relative humidity (RH) at the film underside. Distilled
water (6mL) was dispensed into flat-bottom acrylic (Plexi-
glas®) cups with wide rims. The cup was covered with a film,
sealed with a ring and silicon sealant (High Vacuum Grease,
Dow Corning, Midland, MI), and placed in a desiccator cabi-
net containing fans to be held at 0% RH using anhydrous
calcium sulfate (W.A. Hammond Drierite Co., Xenia, OH) at
25 ЊC. Weight changes were taken periodically after the
steady state of weight loss was achieved and used to calcu-
late the %RH at the film underside and the resulting water
vapor transmission rate (WVTR). The WVP was calculated
by multiplying WVTR by the thickness of the film.
Materials and Methods
Materials
Pea protein concentrate (PPC) extracted from yellow field
peas (Pisum sativum L. Miranda), was supplied by Parrheim
Foods Co. (Portage-La-Prairie, MB, Canada) as a liquid. This
PPC was produced by acidified protein curds from the soluble
fractions of ground yellow peas. The PPC contained 83% pro-
tein, 3% fat, 4% ash, and 10% non-protein soluble fractions by
Soluble Protein Content (%SP) and Total Soluble
Matter (%SM)
Film specimens (1.5 cm ϫ 1.5 cm) were dried for 3 days
total solid content. The pH of the liquid PPC was increased to 9 in a desiccator over anhydrous calcium sulfate. The speci-
by the addition of 2N NaOH solution which increased the solu- mens (approximately 15 mg) were accurately weighed and
bility of the PPC. The PPC solution was freeze-dried and stored immersed in test tubes containing 5 mL distilled water
at 4 ЊC. Glycerol (Sigma Chemical Co., St. Louis, MO) was used (DW). The tubes were covered with aluminum foil and incu-
as a plasticizer. The pH-adjusted liquid PPC are generally bated at ambient temperature for 24 h with occasional gen-
spray-dried for the commercial products. However, to main- tle agitation. After dissolving water-soluble components
tain the high water-solubility of PPC powder, we used a lab- which may include soluble proteins, soluble carbohydrates
scale freeze-dryer instead of a commercial-scale spray-dryer.
and other soluble ingredients, the soluble protein contents
(%SP) of the films in the DW were determined by measuring
absorbance at 280 nm using a spectrophotometer (Ultro-
spec2000, Biochrom Ltd., Cambridge, England) and quanti-
fied with a standard concentration curve (R2 ϭ 0.9933) con-
structed from pea protein isolate.
The remaining pieces of film after soluble protein tests
were dried in the oven at 70 ЊC for 24h to obtain the final dry
weight of the film (total solid matter). The percentage of to-
tal soluble matter (%SM) of the films was calculated using
the following equation:
Film Preparation
Aqueous solutions of 10%(w/w) PPC were prepared and
glycerol (Gly) was added at a ratio of 80/20 to 50/50 of PPC/
Gly. The film-forming solutions were degassed under vacu-
um and heat-denatured at 90 ЊC for 25min in a water bath.
The heat-denatured solutions were cooled to room temper-
ature. Before film casting the solution were degassed under
vacuum again to remove any dissolved air. Native film solu-
tions were prepared by the same process with the exception
of heat denaturating treatments.
%SM ϭ [(Initial dry wt Ϫ Final dry wt) / Initial dry wt] ϫ 100
Edible PPC films were cast by transferring 5 - 6g of the
film-forming solution onto poly styrene petri-dishes (10cm
dia) resting on a well-leveled wood slab. The film-forming so-
lution was allowed to dry at room temperature over 24h. The
films were then peeled off from the casting dishes and their
thickness were measured with a caliper micrometer (B.C.
Ames Co., Waltham, MA) at 5 random positions of the film.
The average thickness was used for the proceeding mechani-
cal test and water vapor permeability determination.
Results and Discussions
Film Formation
After drying the film-forming solutions, free peeled trans-
parent films were obtained. All film forming processes were
conducted under the %RH lower than 20. The films were
slightly yellowish, but the color was negligible due to their
transparency. The films were strong and flexible enough to be
handled, except for the 80/20 (PPC/Gly) films which were very
brittle and fragile. This brittleness was increased by the lower
humidity level. Because the physical and mechanical proper-
ties of PPC films are related to the storage humidity, like most
other protein films, the film specimens had been stored in a
desiccator until examinations to maintain the same dry condi-
tion. It is also recommended to store PPC film specimens at
constant humidity in order to compare the physical and me-
chanical characteristics between different films.
Tensile Test
Film specimens, 1 cm wide and 8 cm long, were cut after
conditioning in a desiccator for 3 days. Tensile strength (TS),
elongation-at-break (E) and elastic modulus were deter-
mined from a stress-strain curve using texture analyzing in-
strument (Texture Analyser, TA-XT2, Texture Technologies,
Corp., NY) following the procedure outlined in ASTM meth-
od D882-91 (ASTM, 1991). Initial grip distance and cross-
head speed were 5 cm and 100 mm/min, respectively. Ten-
sile strength (TS) was calculated by dividing the peak load
by the cross-sectional area (thickness ϫ 1 cm) of the initial
specimen. Elongation was expressed as the percentage of
Tensile Properties
Table 1 shows the tensile properties of heat-denatured
320 JOURNAL OF FOOD SCIENCE—Vol. 66, No. 2, 2001

Properties of Pea Protein Edible Films . . .
Table 1. Tensile properties of pea protein concentrate films.
and native PPC films. Increasing the concentration of a plas-
ticizer (glycerol) decreased tensile strength (TS) and elastic
modulus, while increasing the elongation-at-break (E), for
native PPC films (Table 1b). Like most other protein films,
the addition of glycerol made the film more ductile, which
indicates that glycerol takes a place between protein mole-
cules and interferes with the intra-molecular forces. In the
case of heat-denatured films, the 80/20 film was so brittle
that the TS of the film was weaker than that of a 70/30 film.
The 70/30 film showed the largest value of TS among tested
specimens. In terms of elongation properties, the 60/40 film
was the most stretchable of both heat-denatured and native
films. No data for the tensile test was obtained for the na-
tive 80/20 film due to its extreme brittleness.
a. Heat-denatured film.
PPC/Gly
TS (MPa)
Modulus (MPa)
E (%)
50/50
60/40
70/30
80/20
0.692 Ϯ 0.073
2.896 Ϯ 0.244
7.317 Ϯ 0.436
4.938 Ϯ 1.024
3.32 Ϯ 0.53
8.15 Ϯ 2.35
204.86 Ϯ 66.48
49,749 Ϯ 21,916
92.0 Ϯ 21.5
147.9 Ϯ 13.6
46.8 Ϯ 5.8
0.6 Ϯ 0.2
b. Native film
PPC/Gly
TS (MPa)
Modulus (MPa)
E (%)
50/50
60/40
70/30
80/20
0.317 Ϯ 0.024
1.105 Ϯ 0.092
4.109 Ϯ 1.732
ND*
1.39 Ϯ 0.29
7.44 Ϯ 1.79
1,015.2 Ϯ 187.4
ND
95.3 Ϯ 20.3
93.7 Ϯ 11.2
7.6 Ϯ 3.0
ND
With the same composition of PPC/Gly in the film, dena-
tured PPC films were stiffer, stronger and more extensible
* Not Detectable
than native films due to the formation of intra-molecular Table 2. Water vapor permeability (WVP) of heat-denatured pea
protein concentrate films at 25°C.
chemical bonds between protein molecules after denatur-
ation. Heat-denaturation of film-forming solutions affects
remarkably the physical properties of wheat gluten films
(Rangavajhyala and others 1997). Films prepared from
heat-denatured soy protein solutions had lower WVP than
those prepared from unheated film-forming solution
(Stuchell and Krochta 1994). Heating soy protein solution at
80 ЊC or 95 ЊC for various period of time results in the films
with increased TS, more dark yellow color and lowered E
and decreased WVP values (Gennadios and others 1996). As
with other edible protein films, heat-denaturation at 90 ЊC
for 25 min enhances the strength of PPC edible films.
PPC/Gly
Thickness
(mm)
WVP
RH
(g mm m-2 hr-1 kPa-1)
(%)
50/50
60/40
70/30
80/20
5.83 Ϯ 0.85
4.45 Ϯ 0.60
4.65 Ϯ 0.98
5.60 Ϯ 0.14
7.42 Ϯ 0.69
4.98 Ϯ 0.50
4.10 Ϯ 0.58
4.30 Ϯ 1.16
64.52 Ϯ 1.92
67.48 Ϯ 1.42
72.55 Ϯ 1.57
75.70 Ϯ 4.72
Table 3. Protein solubility (%SP) and total soluble matter (%SM)
of heat-denatured pea protein concentrate (PPC) film with dif-
ferent levels of glycerol (Gly).
PPC/Gly
Protein solubility (%SP) Total soluble matter (%SM)
50/50
60/40
70/30
80/20
37.8 Ϯ 4.5
34.1 Ϯ 3.7
32.2 Ϯ 9.8
33.8 Ϯ 9.1
43.5 Ϯ 4.0
41.3 Ϯ 7.9
38.7 Ϯ 4.0
44.6 Ϯ 7.0
Water Vapor Permeability
The water vapor permeability of PPC film was not affected
by the amount of plasticizer (Gly) added to the dry film over
the range of 20% to 40% (Table 2). Only the 50/50 film has a
significantly higher value for WVP compared to the other
PPC/Gly combinations. The independence of WVP values on
the amount of plasticizer, within the range of 20% to 40% of
glycerol, may be of value in applying PPC/Gly film for use
with food coating or packaging applications because the film
flexibility can be modified by addition of a plasticizer, without
significant changes in the moisture barrier properties. How-
ever, the 80/20 PPC film which contains less plasticizer pos-
sesses a similar WVP value to that of the 70/30 film in spite of
the more humid testing condition (higher %RH) at the under-
side of the film. Therefore, it can be assumed that with the
same conditions of relative humidity, the 80/20 film may
have a lower WVP value than that of the 70/30 film, as the
WVP of edible films generally increases with increasing RH.
Like other edible films, increasing the amount of the plasti-
cizer in the film reduces the moisture barrier properties of
the PPC film. Overall, the WVP of the PPC film is affected by
the relative humidity and the amount of plasticizers.
were compared to the data of Perez-Gago and others (1999).
This indicates that the PPC film has weaker intra-molecular
interactions in the aqueous condition compared to those be-
tween whey protein (mainly ␤–lactoglobulin) molecules in
heat denatured whey protein isolate films. Table 4 compares
the physical and mechanical properties of pea protein con-
centrate film, soy protein isolate film and whey protein isolate
film. The properties of the PPC film were comparable with
those of other protein films. Therefore, the pea protein con-
centrate may be utilized as a replacement for soy protein and
whey protein in forming food coatings or edible films. The rel-
atively high solubility of pea protein films in water, especially
native PPC films, could possibly make the films appropriate
for hot water soluble pouches, like the cellulose ether-based
soluble pouch which is commercially available currently
(Kunte and others 1997).
Pea protein isolate and concentrate require a heating pe-
riod of 45 min to 60 min at 90 ЊC to form a strong gel (Flem-
ing and Sosulski 1975; Betker 1990). A differential scanning
Film Protein Solubility (%SP) and Total Soluble Matter calorimetry study by Murray and others (1985) indicated
(%SM)
that the denaturation temperature of pea protein isolate
was ranged from 88.9 ЊC to 94.5 ЊC. The gelation properties
of the pea protein suggest that the heating time and tem-
perature distribution of the denaturation process of the PPC
could be important in determining the physical and me-
chanical properties of the PPC films. On the contrary, our
preliminary experiments resulted in identical film strengths
with both 10 min and 50 min heat-denatured specimens at
90 ЊC, while the previous studies revealed that over 45 min
was required to complete the gelation process at the same
temperature. More studies of the conditions and mechanism
Heat-denatured PPC films maintain their integrity during
the film soaking treatment. No difference was found in pro-
tein concentration (%SP) and total soluble matter (%SM) of
the tested specimens over the range of glycerol concentra-
tion from 20% to 50% (Table 3). The soaking water also con-
tained soluble pea protein concentrate fractions, which are
non-protein soluble fractions such as soluble fibers and
sugars. The %SP and %SM of the PPC films were approxi-
mately 20% higher than those of whey protein isolate films
with the same compositions of protein/glycerol, when they
Vol. 66, No. 2, 2001—JOURNAL OF FOOD SCIENCE 321

Properties of Pea Protein Edible Films . . .
Table 4. Physical and mechanical properties of pea protein concentrate, soy protein isolate and whey protein isolate
films.
Pea protein1)
Soy protein2)
Whey protein3)
Tensile strength (MPa)
Elastic modulus (MPa)
Elongation (%)
7.3 Ϯ 0.4
204.9 Ϯ 66.5
46.8 Ϯ 5.8
8.5 Ϯ 0.5
6.9
199
41
31.9 Ϯ 2.4
3.76 Ϯ 0.16 (73.26)
6.5 Ϯ 0.3
WVP5) (% RH)
4.1 Ϯ 0.6 (72.6 Ϯ 1.6)
32.2 Ϯ 9.8
4.96 (71)3), 4.75 (77)4)
Protein solubility (%)
Total soluble matter (%)
Ϸ10
Ϸ30
38.7 Ϯ 4.0
35.1 Ϯ 1.0
1)
2)
pH 9, 10% PPC, 70/30 (PPC/Gly)
pH 10, 5% SPI, 10/3 (SPI/Gly); Kunte and others (1997)
pH 7, 5% WPI, 70/30 (WPI/Gly); Perez-Gago and others (1999)
pH 7, 10% WPI, 70/30 (WPI/Gly); Perez-Gago and others (1999)
3)
4)
5)
-2 -1
-1
)
Unit of WVP is in (g mm m hr kPa
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Conclusion
OTENTIAL APPLICATION OF EDIBLE PROTEIN FILM MADE BY
P
pea protein concentrate was suggested. The pea protein
concentrate film was strong and elastic, and possessed a
good moisture barrier property and physical integrity. Addi-
tion of glycerol plasticized the tough and brittle film charac-
teristics resulting in flexible and stretchable. The character-
istics of the pea protein-based edible films were compara-
ble with other edible protein films, such as soy protein and
whey protein, in terms of the tensile strength, elongation,
moisture barrier property and water solubility. The new film
products may exploit the utilization of pea protein.
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(NSERC) of Canada. Authors thank Dr. M.B. Perez-Gago in the Department of Food Science
and Technology at the University of California-Davis for help with the experiments, and
Parrheim Foods Co. (Portage-La-Prairie, Manitoba, Canada) for the donation of pea pro-
tein. Authors also like to thank Sci Publication & Communication Services (Winnipeg,
Manitoba, Canada) for helping in the preparation of this manuscript.
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Author Choi is affiliated with the Department of Food Science,The University
of Manitoba, Winnipeg, Manitoba, R3T 2N2, Canada. Author Han is with
the Department of Food Science, The University of Manitoba, Winnipeg,
Manitoba, R3T 2N2, Canada. Direct inquiries to author Han (E-mail:
hanjh@ms.umanitoba.ca).
322 JOURNAL OF FOOD SCIENCE—Vol. 66, No. 2, 2001