International Journal of Food Science and Technology 2006, 41, 295–302
Original article
Effect of food ingredients and selected lipids on the
physical properties of extruded edible films/casings
Li Liu, John F. Kerry & Joe P. Kerry*
Department of Food and Nutritional Sciences, University College Cork, Cork, Ireland
(Received 14 December 2004; Accepted in revised form 29 April 2005)
Summary
Films/casings produced using pectin (PN) and gelatin/sodium alginate blends (GSAB)
containing 2.5 and 5% added corn oil (CO) or olive oil (OO) and non-oil containing were
manufactured using extrusion technology. Films/casings formed from PN and GSAB had
great difference in properties of tensile strength, elongation, Young’s modulus and
puncture resistance. Quality and stability of films/casings were greatly enhanced on
addition of oils. However, an oil addition of 2.5% was identified as optimal while 5%
decreased the final films/casings quality. Oil addition increased (P < 0.05) films/casings
thicknesses. Oil addition also decreased (P < 0.05) tensile strength, puncture resistance,
Young’s modulus and tear resistance of films/casings but increased their elongation
properties, with the exception of converse effect of 2.5% CO on two films/casings. Olive oil
and CO both reduced (P < 0.05) water vapour transfer rate (WVTR) of films/casings.
However, no significant difference between films using different concentrations of oil was
observed with respect to WVTR.
Keywords
Edible films/casings, extrusion, mechanical properties, water barrier properties.
Introduction
Interest in the development of edible films and
coatings has increased in recent years because of
their potential to improve the quality and stability
of wide range of food products (Baldwin et al.,
1997; Cuq et al., 1998; Debeaufort et al., 1998). A
range of processing methods suitable for the fabrication of edible films and coatings including; plate
casting, spraying, dipping, etc. have been reported
(Donhowe & Fennema, 1994). Successful application of extrusion technology in the manufacture of
stable edible films merits further investigation
because of its ability to manipulate food polymer
structure and functionality (Liu et al., 2004).
Different materials could form films with different properties, for example, polysaccharides and
proteins have been proved to be able to form
strong films but possess poor water vapour barrier
*Correspondent: Fax: 00-353-4270213;
e-mail: [email protected]
doi:10.1111/j.1365-2621.2005.01063.x
2005 Institute of Food Science and Technology Trust Fund
properties. However, most single hydrophobic
films or coatings (made from lipids) while possessing high water barrier properties, form brittle
films. However, moisture barrier properties of
hydrophilic films can be improved through the
incorporation of hydrophobic materials such as
waxes, long-chain saturated fatty acids, or
through emulsion or lamination technology
(Fennema et al., 1994; Handa et al., 1999; Garcia
et al., 2000). Thus, composite films and casings
can be manipulated to combine the advantages of
both lipid and hydrocolloid components (Kester &
Fennema, 1989b; Krochta & Johnson, 1997).
Lipid addition during film and casing formulation
can serve as an effective barrier to water vapour
while the hydrocolloid component can impart
both oxygen and carbon dioxide barrier properties, as well as form the necessary supporting
matrix of the film or casing (Wong et al., 1994;
Baldwin et al., 1997; Garcia et al., 2000).
The water vapour transfer through films and
casings generally occurs via the hydrophilic
295
296
Effect of food ingredients and selected lipids L. Liu et al.
portion within the system, with the water vapour
permeability (WVP) dependent upon the hydrophilic–hydrophobic ratio of the matrix components (Hernandez, 1994). WVP increases with
polarity, unsaturation, lipid branching/structure
and is further dependent upon the water absorption properties of the polar components of the film
or casing (Gontard et al., 1994; Garcia et al.,
2000). Barrier and mechanical properties depend
on film microstructure, which in turn can influence
the films composition, formation and method of
product containment (Cuq et al., 1995).
Further studies have also shown that when
lipids are employed as laminating agents or within
a hydrophilic film and form a bilayer or emulsified
to form a composite emulsion, the barrier against
water vapour transfer were both improved. Studies, where long-chain fatty acids and solid lipids
were utilized in the preparation of composite films,
were shown to possess an effective water vapour
resistance (Kamper & Fennema, 1984; Kester &
Fennema, 1989a).
Other than reports on manufacturing patents
for the production of edible/biodegradable extruded casings (Lieberman, 1967; Shank & Shank,
1972), little research appears to be available in the
current scientific literature relating to this form of
packaging material, and in particular, on the use
of food grade oils in the formation of extruded
edible films/casings.
The objectives of this study were to generate test
composite edible films/casings utilizing an emulsified solution containing oils and food polymers
using extrusion technologies previously reported
(Liu et al., 2004), and to evaluate appearance,
mechanical properties and WVPs of extruded test
films/casings containing different food ingredient
substrates. The effects of addition of different oils
fractions on properties above against non-oil
containing controls were also carried out.
Materials and methods
Food ingredients
The following materials were used in this study:
sodium alginate (Manucol DM; International
Specialty Products HQ, Tadworth, UK); gelatin
(bloom 264, Klippan, Sweden); high methoxy
pectin (CP Kelco, Lille Skensved, Denmark),
glycerol and lecithin (Sigma Chemical Company,
St Louis, MO, USA), corn oil (CO) (Mazola) and
olive oil (OO) (Don Carlos) were purchased from
Pallas Foods, Limerick, Ireland.
Film/casing manufacture
All films/casings were formed according to the
methods outlined by Liu et al. (2004) with the
exception of 2.5 and 5% OO and CO being
incorporated into the mixture of water, glycerol
and 1% lecithin for emulsification purposes. The
composition of each film/casing generated in this
study is shown in Table 1.
Some casings were cut on one side of the die
after extrusion, thus allowing for the formation of
a continuous strip of film. The remaining films/
casings were stored for additional tests. All test
Table 1 Film/casing composition
Composition
Lipid (%)
Film/casing types
Protein
(%)
Carbohydrate
(%)
Water
(%)
Glycerol
Oil
PN without oil
GSAB without oil
PN with 2.5% CO
PN with 5% CO
GSAB with 2.5% CO
GSAB with 5% CO
PN with 2.5% OO
PN with 5% OO
GSAB with 2.5% OO
GSAB with 5% OO
0
7.82
0
0
7.82
7.82
0
0
7.82
7.82
54.14
31.26
54.14
54.14
31.26
31.26
54.14
54.14
31.26
31.26
22.93
30.46
21.79
20.64
28.94
27.42
21.79
20.64
28.94
27.42
22.93
30.46
22.93
22.93
30.46
30.46
22.93
22.93
30.46
30.46
0
0
1.15
2.29
1.52
3.05
1.15
2.29
1.52
3.05
International Journal of Food Science and Technology 2006, 41, 295–302
2005 Institute of Food Science and Technology Trust Fund
Effect of food ingredients and selected lipids L. Liu et al.
films/casings were placed in a drying room at
23 ± 2 C (50% ± 5% RH) for 3 days.
three independent samples, from each film/casing
type was used to establish permeability values.
Appearance evaluation of extruded casings
Statistical analysis
Appearances of films/casings manufactured both
with and without oil were assessed after 3 days
storage in drying chamber. Film/casing samples
were then transferred to a chill (4 C) room and
displayed for 2 days to evaluate practical environmental effects on films/casings appearance.
Data were analysed by one-way analysis of variance, using SPSS software (version 11.0; SPSS,
Dublin, Ireland). Post hoc multiple comparisons
were determined by least significant difference
(LSD). All comparisons were made at a 5% level
of significance.
Film thickness and mechanical properties
Results and discussion
Film thickness and mechanical properties were
measured using the methodologies described by
Liu et al. (2004).
Water vapour transfer rate (WVTR) of films/
casings
Water vapour transfer rate of films was measured
using WPA-100 (VTI Corporation, Hialeah, FL,
USA). Three samples with a surface area over
6.8 cm2 were taken from each film/casing. Each
sample was then attached to an adhesive mask (to
prevent the film/casing being damaged during the
test) which had a circular hole of 6.8 cm2 in the
centre, over which the film was placed. The mask
with the test film/casing sample was mounted onto
instrumental cylinders, followed by analysis.
Instrumental
operating
parameters
were:
23 ± 2 C temperature, 50 ± 5% relative humidity (RH), 500 cc min flow rate and 0.005 WVTR in
5 min equilibrium. The reading obtained was the
WVTR value for that sample. The average value of
Appearance assessment
Both pectin (PN) and gelatin/sodium alginate
blend (GSAB) without oil formed stable films/
casings (Table 2). However, after holding test
films/casings under refrigerated conditions, samples quickly absorbed moisture, expanded, became
weak and split.
Addition of oil to film/casing formulations
containing PN resulted in very stable packaging
materials. Upon refrigerated storage, PN based
films/casings were stable, remained intact and had
higher flexibility compared with similar films/
casings formed without the use of oil. Therefore,
oil addition successfully reduced water absorption
by PN-based films/casings, thereby, improving
extruded film/casing properties. However, addition of 5% CO or OO to PN formulations reduced
film/casing strength compared with those manufactured using concentration of 2.5% oil.
Both 2.5% CO and OO improved the overall
quality of films/casings containing GSAB. Films/
Table 2 Description of appearances of films/casings formed from PN and GSAB
Films/casings types
Appearances of films/casings following chilled storage
PN without oil
GSAB without oil
PN with 2.5% CO
PN with 5% CO
GSAB with 2.5% CO
GSAB with 5% CO
PN with 2.5% OO
PN with 5% OO
GSAB with 2.5% OO
GSAB with 5% OO
Casings became wet, suffered from cracking and broke easily on handling
Casings became wet, suffered from cracking and, broke easily on handling
Casings became wet but were soft and flexible with good integrity
Casings became wet and slightly brittle but were soft and flexible with moderate integrity
Casings became wet but were soft and flexible with good integrity
Did not form acceptable casings
Casings became wet but were soft and flexible with good integrity
Casings became wet and slightly brittle but were soft and flexible with moderate integrity
Casings became wet but were soft and flexible with good integrity
Did not form acceptable casings
2005 Institute of Food Science and Technology Trust Fund
International Journal of Food Science and Technology 2006, 41, 295–302
297
International Journal of Food Science and Technology 2006, 41, 295–302
4.21bcd
2.51cdef
2.91bc
1.17gh
1.24g
1.53cde
1.36a
2.63ab
±
±
±
±
±
±
±
±
0.04bcd
0.05bc
40.56
37.45
42.37
24.63
25.87
37.98
47.27
44.20
0.04a
0.02ab
0.01e
0.01ef
0.01efg
0.09 ±
0.07 ±
0.02 ±
0.02 ±
0.02 ±
–
0.05 ±
0.06 ±
0.78a
0.20fg
0.40ab
0.39f
0.12h
0.48de
0.63d
0.25c
±
±
±
±
±
±
±
±
7.79
3.23
7.23
3.51
2.31
4.15
4.17
6.43
0.02a
0.01d
0.01de
0.01gh
0.01fg
0.01bc
0.05b
0.01f
±
±
±
±
±
±
±
±
Values within rows followed by the same letter are not significantly different at P £ 0.05.
0.58
0.17
0.14
0.05
0.07
0.24
0.30
0.08
2.26fg
3.95fgh
5.31bc
8.58d
6.48de
9.59ab
5.38ef
5.55a
±
±
±
±
±
±
±
±
25.01
24.57
44.96
36.09
31.33
50.66
25.99
57.93
0.39a
0.27e
0.99b
0.49f
0.32fg
0.17fgh
1.16c
0.52cd
±
±
±
±
±
±
±
±
7.26
2.74
6.29
1.96
1.82
1.03
5.55
5.31
0.02bc
0.03a
0.02bcd
0.02b
0.02bcde
0.01bcdef
0.02g
0.01gh
±
±
±
±
±
±
±
±
0.51
0.57
0.51
0.52
0.51
0.50
0.36
0.36
PN with 2.5% CO
PN with 5% CO
GSAB with 2.5% CO
PN with 2.5% OO
PN with 5% OO
GSAB with 2.5% OO
PN without oil
GSAB withou oil
Tear
resistance (kg)
Puncture
resistance (kg)
Young’s
modulus (MPa)
Percentage of
elongation (%)
Tensile
strength (MPa)
No significant differences in thickness were observed between films/casings manufactured from
PN and GSAB without the use of oil (Table 3).
The thickness for films/casings using oil was
higher (P < 0.05) than films manufactured without oil. The 5% CO PN film/casing had a higher
(P < 0.05) thickness value than all other experimental films/casings manufactured with oil. Kester
& Fennema (1986) and Greener & Fennema
(1989) reported that edible films containing
hydrophobic substances such as waxes and oils
can form thicker films. The possible reasoning for
this may be because of the water content present in
lipid-emulsified films. Under similar ageing conditions, the amount of water evaporated from
composite films ere less compared with that
evaporated from films manufactured without
lipids.
Thickness
(mm)
Thickness values for films/casings
Films
casings manufactured with oil were stronger and
less likely to tear or split than those manufactured
without oil following storage at 4 C. While all
films/casings were visually observed to swell
because of increased water uptake during chilled
storage, they maintained acceptable integrity and
strength. Like PN, the use of a 5% oil concentration negatively affected the formation of stable
GSAB films/casings, which showed that film/
casing formed from GSAB was less tolerant to
high level oil (including CO and OO) compared
with film/casing formed from PN.
Water has been reported to be the most
ubiquitous and uncontrollable plasticizer used in
the manufacture of edible films and its presence
decreases the mechanical strength of edible films
(Gontard et al., 1993). Mechanical properties of
edible/biodegradable films like tensile strength and
puncture resistance have been shown to be negatively affected by environmental RH and film water
content (Greener-Donhowe & Fennema, 1993;
Debeaufort & Voilley, 1997). Findings reported
in this study support the observations made by a
number of researchers (Gontard et al., 1993;
Debeaufort et al., 1998). However, results from
this study also showed that the problems created
by the unwanted presence of moisture in edible/
biodegradable films can be circumvented or prevented through the use of oil.
WVP (g H2O/
day/100 in2)
Effect of food ingredients and selected lipids L. Liu et al.
Table 3 Properties of extruded films/casings formed from PN and GSAB both with and without oil
298
2005 Institute of Food Science and Technology Trust Fund
Effect of food ingredients and selected lipids L. Liu et al.
Mechanical properties of films/casings
No significant difference in tensile strength was
determined between films/casings manufactured
from PN and GSAB without the use of oil. CO
(2.5%) improved (P < 0.05) the tensile strength
of both films/casings and also had higher
(P < 0.05) tensile strength than other oil emulsified films/casings. PN films/casings with 5% CO
also had higher (P < 0.05) tensile strength than
films/casings containing OO.
Elongation of film/casing of GSAB without oil
was higher (P < 0.05) than that of film/casing
formed from PN without oil. Elongation of PN
films/casings was unaffected when CO was used
while, 2.5% CO reduced (P < 0.05) elongation of
films/casings manufactured using GSAB. Both 2.5
and 5% OO improved (P < 0.05) elongation
properties of PN films/casings. OO reduced
elongation properties of films/casings manufactured from GSAB.
Film/casing of PN without oil had higher
(P < 0.05) Young’s Modulus than film/casing of
GSAN without oil. CO (2.5%) improved
(P < 0.05) Young’s modulus of PN films/casings.
The use of 5% CO and 2.5% and 5% OO reduced
(P < 0.05) Young’s modulus values for PN films/
casings. Films/casings manufactured from GSAB
using both CO and OO improved (P < 0.05)
Young’s modulus values. Young’s modulus values
for films/casings formed from PN using OO were
lower (P < 0.05) than all other films.
Film/casing based on PN without oil had lower
puncture resistance (P < 0.05) compared with
film/casing based on GSAB without oil. CO
(2.5%) improved (P < 0.05) puncture resistance
of both films/casings types to the same degree.
However, the use of 5% CO, and 2.5% and 5%
OO reduced (P < 0.05) puncture resistance of
PN-based films/casings. OO (2.5%) decreased
(P < 0.05) puncture resistances of films/casings
formed from GSAB.
No significant difference of tear resistance was
found between films/casing formed from PN and
GSAB without addition of oils. PN films/casings
manufactured with 2.5 and 5% CO had higher
(P < 0.05) tear resistances than controls without
oil. Films/casings formed from PN with 2.5 and
5% OO and films/casings manufactured from
GSAB with 2.5% CO had the similar tear
2005 Institute of Food Science and Technology Trust Fund
resistance values. These values were lower
(P < 0.05) than tear resistance values for films/
casings formed from PN and GSAB without oil.
Composition of films/casings had great effect on
film mechanical properties. Film/casing formed
from PN had higher tensile strength, Young’s
Modulus but lower elongation and puncture
resistance compared with film/casing formed from
GSAB.
Debeaufort et al. (2000) reported that lipids
strongly affected the mechanical properties of film
when they were mixed to the hydrocolloid as an
emulsion. Tensile strength (Banerjee & Chen,
1995; Gennadios et al., 1996) and puncture resistance (Gontard et al., 1994) decreased when lipid
sources have been added to edible/biodegradable
film formulations. According to Lai et al. (1997)
and Gallo et al. (2000), the use of liquid lipids in
films enhanced elongation properties since lipid
oils produce lubrication in emulsified films, however, this in turn reduces other mechanical properties. In this study, OO similarly to those lipid
sources cited above in extruded edible/biodegradable films such as decreased the tensile strength,
puncture resistance and tear resistance for both
PN and GSAB films/casings and increased the
elongation of PN based films/casings. CO (5%)
also had similar effects to OO in PN-based films.
The reasons for the negative effects of lipids on
mechanical properties of composite films were
given by other researchers as the lack of selfsupporting and lubrication of lipids. Yang &
Paulson (2000) even supposed that in composite
films, the main ingredient polymer were partially
replaced by lipid and the interaction between nonpolar lipid molecules and between polar polymer
and no-polar lipid molecules are certainly much
lower than those between the polar polymer
molecules.
Conversely, in this study, 2.5% CO was found
to increase tensile strength, Young’s modulus and
puncture resistance in both PN and GSAB films/
casings, but decreased elongation properties. This
was totally different from most of other researches. According to Mor et al. (1999), protein
matrix with dispersed lipid can be classified as two
systems: interactive or non-interactive. In interactive system, lipid emulsified in protein matrix
could be covered by protein and become an
integrate part of the network. These oil globules
International Journal of Food Science and Technology 2006, 41, 295–302
299
300
Effect of food ingredients and selected lipids L. Liu et al.
fitted into the matrix and served as many anchor
points that strengthened the whole network. Many
other researchers (McClements et al., 1993; Line
et al., 2004) also got the same finding to support
this. Compared with protein emulsion system, the
influence of lipid incorporation on mechanical
properties of hydrocolloid systems has not been
extensively investigated. However, it is reasonable
to suppose that the similar principle exists in the
hydrocolloid emulsion system, is that, lipid participated in the formation of the whole network of
films. 2.5% CO under the condition in this work
might be able to cooperate with matrix of PN and
GSAB and became and integrate part of the whole
films/casings, thereby, increased mechanical properties of these two extruded films/casings, with the
exception of elongation.
In this study, the effect of OO on elongation of the
GSAB film was different from results previously
shown in the scientific literature. Most studies
assessing the effect of lipids on properties of films
have concentrated on single ingredient films. The
role of lipid in such kind of films is more defined and
predictable, unlike that of composite films.
Films/casings formed from GSAB without oil
had very high value of elongation because of the
existence of gelatin, which individually produced
very soft and sticky extruded products, with high
elasticity (unpublished work carried out in our
laboratory). The use of gelatin introduced greater
properties of elasticity and elongation and films/
casings manufactured using GSAB. The addition of
OO may have counteracted the function of gelatin
in this composite film/casing, thus reduced the
elongation of films/casings formed from GSAB.
Certainly, further investigations to clearly
understand the effects of low-level CO on film
mechanical properties and OO on elongation of
films formed from GSAB are required.
Water vapour transfer rate (WVTR)
Film/casing formed from PN without oil had
higher WVP compared with GSAB films/casings
without oil, but the difference was not significant.
Both CO and OO decreased (P < 0.05) the
WVTR of PN films/casings. The use of OO
lowered (P < 0.05) WVTR in PN films/casings
compared with those manufactured with CO,
regardless of oil concentration used. Only OO
lowered (P < 0.05) the WVTR of films/casings
manufactured from GSAB.
The form of lipid added to edible films has been
shown to have a pronounced effect on the WVP,
the long chain and solid lipids producing greater
barrier effects than the short chain and liquid
lipids (Kamper & Fennema, 1984). This effect was
hypothesized to be because of the increasing crosslinking at the film interface between the lipid layer
and the hydrocolloid layer (McHugh & Krochta,
1994a). Interestingly, while OO used in this study
had shorter fatty acid chains compared with CO, it
produced more positive effects on the WVTR
properties of extruded films. The reason for this
difference may be because of the extrusion
manufacturing process used.
From extensive review of the literature, the
majority of studies examining the effects of lipids
on moisture barrier property of edible films have
been conducted on films manufactured using the
solution casting method. With this method,
biopolymer solutions were heated using low
temperature (normally 80 C). Compared with
the casting films, extruded films/casings in this
study were heated under much higher temperature
(135 C) and certain pressure. The form and
function of oil and their effects on moisture barrier
property may have been affected and altered by
higher heating temperature and pressure compared with those using lower heating temperature
and without pressure. The details of this effect
need to be studied and understood further.
Besides the type of lipids used in the manufacture of edible/biodegradable films, the amount of
lipid was also reported to be an important factor
for the moisture barrier property of films, the
greater the amount of lipid used, the lower the
WVP (Shellhammer & Krochta, 1997). While
limited by the scope of the extrusion technology
used, no such concurring result was determined in
this study. It is possible that because a lipid
concentration not exceeding 5% was used in this
study, range of oil concentrations was not wide
enough to show the full effect of oil on WVTR of
extruded films/casings.
Acknowledgments
This research was part funded by the Irish
Department of Agriculture under the National
International Journal of Food Science and Technology 2006, 41, 295–302
2005 Institute of Food Science and Technology Trust Fund
Effect of food ingredients and selected lipids L. Liu et al.
Development Plan through the FIRM Research
Programme.
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2005 Institute of Food Science and Technology Trust Fund