Preliminary experiment
The result of the preliminary experiment has been
presented in Figs 1–3. Based on the earlier work of
Gnanasambandam et al. (1997) that RBP could form
film at glycerol concentration of 2% (w/v), the glycerol
concentration was kept constant at 2% (w/v) against pH
as variables (Fig. 1). It was observed that pH has effect
on the PS of protein films. In the case of casein films, the
elasticity increased when pH increased from 7 to 9,
because of an increase in intermolecular disulphide
bonds (Shimada & Cheftel, 1988). Monahan et al.
(1995) showed that whey protein solutions yielded less
thiol groups at pH 9–11, than at lower pH values. The
PS of SP films increased as pH increased from 6 to 11,
while PS of RBP did not change significantly after pH
8.0. As pH increases towards the alkaline region, RBP
becomes more soluble; this may be the reason for the
gradual but insignificant increase in PS. In addition, at
higher pH, some non-protein nitrogen could be solubilised,
thus influencing the protein quality and purity.
About 16% of the nitrogen in rice bran has been
attributed to non-protein nitrogen (Baldi et al., 1976).
To prevent denaturation of protein at higher pH and
consequent increase in non-protein nitrogen, lower
alkaline pH of 8.0 was suggested as appropriate.
In Fig. 2, the pH of the protein solution was kept
constant at 8.0 and the effect of glycerol concentrationexamined. The PS of protein films was markedly affected
by the concentrations of glycerol. SP films gave much
higher PS than RBP films at each concentration. When
the concentration was below 1.5% (w/v), the films
tended to be brittle and it was difficult to peel from the
casting plate. Addition of glycerol at and above 3%
resulted in weaker protein films. Similar results were
reported for cellulose-based films by Park et al. (1993)
and for wheat gluten films by Gontard et al. (1993).
A large number of hydroxyl groups and carboxyl groups
along protein molecules could be responsible for numerous
hydrogen bonds between the protein molecular
chains. These extensive interchain interactions contribute
to the mechanical strength of films (Lieberman &
Gilbert, 1973). Higher concentration of glycerol may
disturb the hydrogen bonding between protein molecules,
thus weakening the mechanical strength of protein
films. Hence, glycerol concentration considered for RBP
films preparation was 2.0% (w/v), thus confirming the
observation of Gnanasambandam et al. (1997).
Effect of heating temperature of protein solution on
PS has been presented in Fig. 3. The pH and glycerol
concentration of the protein solution were kept constant
at 8.0% and 2% (w/v), respectively. Heat
treatment resulted in aggregation of proteins by
hydrophobic and disulphide bonding (Ali et al.,
1997). In the presence of heat, protein denaturation
commonly defined as any non-covalent change in the
structure of a protein may occur. This change may
alter the secondary, tertiary or quaternary structure of
the molecules. Exposure of most proteins to high
temperatures results in irreversible denaturation.
Depending upon the protein studied and the severity
of heating, these changes may or may not be reversible
and several phenomena may occur, thus influencing the
structure and mechanical property of protein films.
From the result of Fig. 3, the PS of SP films increased
significantly than that of RBP films. Significant
increase was observed only between the PS of RBP
solution heated at 50 C and higher temperatures. This
may be attributed to protein denaturation at higher
temperatures, which resulted in tighter protein networks
and compact structures. Hence, higher temperature
of 80 C and shorter time of 30 min were
considered suitable.
Biopolymer films show a variety of functionality
depending on polymer structures, material properties
and film compositions. The SP films had higher PS than
that of RBP films in all the three parameters examined.
RBP used in this study was not a highly purified one,
which probably makes the difference in the mechanical
property of the RBP and SP films. Moreover, the RBP
films in the preliminary experiment had lower PS than
that in the main experiment (Figs 7 and 8). This
observation may be due to the effect of the degree of
RBP quality and purity used.
Yield and protein content of RBP for film formation
Protein content of rice bran, crude and refined RBP was
15.8%, 58.6% and 94.1% (on dry basis), respectively.
The yield of crude RBP used for the preliminary
experiment and refined RBP used for main investigation
were 9.7 and 3.0 g from 100 g of DRB, respectively. The
higher yield and lower protein content in the crude RBP
may be due to the presence of several non-protein
components such as fibre, cellulose, hemicellulose,
pentosans and lignin (Gnanasambandam & Hettiarachchy,
1995). However, the lower yield and higher protein
content could be due to the removal of these nonproteinous
substances. As the observed PS of films
produced with the crude RBP was lower than that from
refined RBP, this may be a justification for the
preparation of refined RBP.
Preparation of refined RBP for film formation
As presented in Fig. 4, the appropriate alkaline pH for
RBP isolation is 9.5. (Gnanasambandam & Hettiarachchy,
1995). Glass wool was used as an aid to remove
insoluble solid particles before centrifugation. The
suspension was filtered through a bed of celite clay.
Celite is a diatomaceous earth also known as DE,
diatomite and Kieselghur, commonly used as a filtration
aid to remove suspended particles. It has a more
intricate particle shape, and thus provides a moretortuous path for suspended particles to be trapped. Itcould remove fine suspended insoluble proteins and
contaminants, which otherwise cannot be removed by
simple centrifugation. The pH of the filtrate was then
adjusted to 4.5 with 1 m HCl, which is the isoelectric
point of RBP. At the pI (isoelectric point), the protein
remains as insoluble and thus precipitates after about
1 h rest in the cold room. It was then centrifuged to
obtain the protein as sediment and supernatant was
discarded. The sediment was washed by suspending the
sediment in distilled water (pH 7.0) using centrifugation;
this allowed the salt formed during pH adjustment to be
removed. The refined RBP was neutralised by dispersing
in small amount of distilled water (pH 7.0). The
dispersed product was lyophilised and stored at 5 C.
Characteristics of protein films
The thicknesses of films in the preliminary experiment
were in the range 0.217–0.239 mm. This is higher than
the thicknesses of films produced with the refined RBP
(0.110–0.178 mm). However, the PS was lower. Similarly,
in their experiment, Gnanasambandam et al.
(1997) compared the mechanical properties of RBP film
of thickness 190 lm with PVDC film of thickness
40 lm. It was observed that the PVDC film has higher
puncture and tensile strength than that of RBP film.
Gnanasambandam et al. (1997) stated that tensile
strength is a measurement of maximum load per unit
cross-sectional area, and a positive correlation (r ¼ 0.91)
was observed between PS and tensile strength
(P < 0.01).
For possible comparison (Fig. 1), under similar conditions
of 2% (w/v) glycerol and pH 9.0, the thicknesses
of RBP films (0.217–0.239 mm, PS of 1.7 · 104 N mm)2
equivalent to tensile strength of 13.345 MPa) in the
preliminary experiment were higher than that produced
by Gnanasambandam et al. (1997) prepared at pH 9.5
(thickness 190 lm and tensile strength 16.441 MPa);
however, the equivalent tensile strength was lower.
Furthermore, our data (Fig. 7) for refined RBP film at
pH 9.5 showed the PS of 3.6 · 104 N mm)2 equivalent
to tensile strength of 28.26 MPa. Protein solution heat
treatment at 80 C for 30 min and pH 8.0 produced
RBP film of PS 7.96 · 104 N mm)2 with possible
equivalent tensile strength of 62.486 MPa (Fig. 7).
In Fig. 5, protein and glycerol concentrations were
5% (w/v) and 2% (w/v), respectively. The thickness of
the protein films ranged from 0.110 to 0.145 mm. There
were variations in the thickness of RBP films, but no
significant variation in that of SP. At pH 3 and 9.5, the
RBP films cast from unheated solutions were thicker
than the films formed from heated solutions. Herald
et al. (1995) suggested increase in protein solubility as a
possible reason for increased thickness of wheat gluten
films. Such effect was not observed by Gnanasambandam
et al. (1997) on RBP films prepared at pH 3.0 and
9.5 of consistent thickness of 190 lm. The combination
of effect of higher pH and ionic strength, rather than pH
alone was given as possible reason. Our experiment is in
agreement with that of Herald et al. (1995).
In Fig. 6, protein concentration and pH were 5%(w/v)
and 8.0, respectively. The thickness of the protein films
ranged from 0.110 to 0.178 mm. There was significant
difference in protein film thickness as the glycerol
concentration increased. The SP and RBP films cast
from heated and unheated protein solutions with 4%
glycerol were thicker than those with 2% or 3% glycerol.
The increase in film thickness results from an increase in
total solid in film composition, by increasing the glycerol
content.
Mechanical properties of protein films provide an
indication of film integrity under stress conditions that
may occur during processing, handling and storage. PS
was evaluated as the hardness of protein films, which
was the maximum force exhibited by the films under test
conditions. In Fig. 7, protein and glycerol concentrations
were 5% (w/v) and 2% (w/v), respectively. The PS
of SP films increased as pH increased from 3.0 to 9.5,
while that of RBP films increased up to pH 8.0 and then
decreased. The glycerol concentration effected the PS ofprotein films (Fig. 8; protein concentration 5% (w/v) at
pH 8.0). Higher concentration of glycerol (4%) produced
weaker protein films, probably because of
increase in water content of the films as glycerol
concentration increased. The heating