3.1 Selection of appropriate enzyme for AEE
Fig. 1 showed the effect of five
different enzymes (Celluclast 1.5L, Hemicellulase, Pectinex Ultra SP-L,
Viscozyme L and Alcalase 2.4L) on RBO extraction
yields.
It could be observed from
Fig.
1a that more RBO was extracted using hydrolytic enzymes (28.59-50.37%)
compared with the control
(21.30-40.12%).
Alcalase
2.4L was found to be the optimum enzyme obtaining the highest
RBO
yields with
50.37%.
The
efficiency of Alcalase 2.4L for RBO extraction was consistent with the
results of Hanmoungjai et al. (2002).
Indeed, protease treatment can
hydrolyze proteins into peptides,
leading to effective
dissolution
and hydrolysis of proteins,
then
break down the protein networks of cotyledon cells and protein based
membranes surrounding the lipid bodies, thereby liberating more oils
(Jiao et al., 2014; Latif & Anwar, 2011). It was also found that the
oil extraction yields of control at pH 9.0 was significantly higher than
that of the
carbohydrase
treated rice bran.
This
might be due to the increased solubility of proteins under alkaline
conditions and saponification of available free fatty acids,
which
improved the separation efficiency of oil bodies from their original
location at high pH and low surface tension (Amarasinghe et al., 2009).
The
effect of the combination of each carbohydrase with Alcalase 2.4L on oil
yields was shown in Fig. 1b. It exhibited that the combination of
carbohydrases and protease did not significantly increase the extraction
yields of RBO.
The
use of cellulose, pectinase and other carbohydrase can hydrolyze
cellulose, hemicellulose, pectin and other components forming cell wall
thus breaking down the structural integrity of the cell walls and
improving the oil extraction yields. However, the rice bran material
selected in this study had been expanded, which could damage the cell
wall to a certain extent, therefore, the carbohydrases had little effect
on the release and extraction of RBO as shown in the result.
3.2 Physicochemical properties of extracted RBO
The
physicochemical properties of AEE-extracted oil (AEEO) and SE-extracted
oil (SEO) were listed in Table 1.
Acid
value and peroxide value, respectively, indicated the oxidation and
rancidity of RBO
and
oil samples with low acid value and peroxide value were considered to be
a high quality. The acid value in the AEEO were found to be slightly
lower than the SEO, but the difference were not significant (p>
0.05).
Moreover, the PV of AEEO (8.15 ± 0.44 mmol/kg) was found to be
significantly (p < 0.05) lower than 8.80 ± 0.00 mmol/kg
for SEO. These values are slightly different from those reported by
Phan,
Junyusen, Liplap & Junyusen (2018),
which
may be due to climate conditions and different cultivars of rice
bran.
Additionally, iodine value
(106.31
± 0.44 g I2/100 g oil) in the AEEO was found to be
higher than the SEO (102.22 ± 0.56 g I2/100 g oil) which
can be due to a higher content of unsaturated fatty acids in the AEEO.
The saponification value of SEO was significantly (p <
0.05) lower than AEEO and the values were similar to those reported by
Amarasinghe et al. (2009) and Sengupta et al. (1996).
Wax
content of AEEO was found to be significantly less than the SEO.
Wax
in RBO seriously affects the quality of RBO. However, as an important
by-product of RBO refining process, rice bran wax is rich in
octacosanol. Octacosanol has been widely used in various food production
due to the function of lowering blood lipid or cholesterol (Chen et al.,
2007). A significantly
(p< 0.05) higher concentration of phospholipids was determined
in the SEO compared with the AEEO. According to Balachandran, Mayamol,
Thomas, Sukumar, Sundaresan & Arumughan (2008),
the
phospholipid content in RBO extracted with n-hexane ranged from 1% to
5%, which was agreement with the results of this
study.
While,
no phospholipids were detected in RBO extracted
by AEE. The result was consistent
with the report of Rovaris et al. (2012) that more than 90% lipids
existed in the residue phase and the rest in the aqueous phase during
the enzymatic extraction of soybean oil. Because of the presence of
phospholipids and effective emulsifiers such as lecithin, oils are
emulsified and not easy to be separated from raw materials.
Phospholipids have a polar phosphate group attached to a lipid group by
electrostatic forces and hydrogen bonds, which can be transferred to the
oil when the polar solvent, able to break these bonds and release them,
is used as extraction medium (Capellini, Giacomini, Cuevas & Rodrigues,
2017). It can be concluded that the
contents of wax and phospholipid in AEEO was low, which can simplify the
following refining steps.
It was also found that the L-value
and b-value of AEEO were higher than SEO. It indicated that AEEO were
more yellow and lighter-coloured than
SEO.
It
can be attributed to that the organic solvent extracted pigments such as
carotenes and chlorophylls from the bran.
Consequently,
the results of physicochemical properties showed that the quality of
AEEO was better than that of SEO.