3.2 Protein solubility
The protein solubility is one of the most important physicochemical
properties, as it is generally related to other functional properties,
which may affect the rheological, hydrodynamic, and surface activity
characteristics (Zayas, 1997). As shown in Table 2, the AE-IP CP control
had a significantly higher (p <0.05) solubility at pH 3
where it was 88.9% soluble protein compared to the defatted CP and HE
controls (76.2% and 81.4%, respectively). Similar to the results at pH
3, at pH 5 the AE-IP CP control showed higher solubility (53.5%) than
the defatted CP (36.9%) and HE (2.0%) controls, whereas at pH 7 the HE
control (70.0%) had a higher solubility than both the CP control
(27.7%) and the defatted CP control (37.8%). As for the SE products,
all of the controls had a higher solubility (≥94%) at pH 3 than the
controls produced by AE-IP. At pH 5, a higher solubility was found for
the SE HE control (29.4%) compared to AE-IP (2.0%), whereas it was
lower than the SE CP control (37.7%) and defatted CP control (37.3%),
which had similar solubility values. At pH 7, both the SE CP (51.8%)
and defatted CP control (49.3%) had a higher solubility than the
respective AE-IP product however, the SE HE control (43.2%) was less
soluble than the AE-IP HE control. Overall, the SE products showed
higher solubility at pH 3 and 7. Stone et al. (2015) also found a
lower solubility of canola protein products obtained by AE-IP compared
to the SE process. Similar solubility values were found for both the CP
and defatted CP controls at every pH and tended to be higher than the HE
control.
The canola protein products prepared from fermented meals showed a lower
solubility at each pH compared to the control products. This is
hypothesized to be due to partial hydrolysis of protein during
fermentation which led to the exposure of previously buried hydrophobic
groups. At pH 3, all AE-IP and SE controls showed higher solubility than
products from fermented meals. However, the solubility of products fromA. oryzae fermented meals was significantly higher than products
from A. niger fermented meals. This indicated that fermentation
with A. oryzae was able to maintain the solubility (pH 3) at a
relatively higher level than A. niger after SSF. As such, usingA. oryzae may be favorable to obtain a more soluble protein
product at pH 3. After fermentation, most extracted samples showed
significant decreases (p <0.05) in solubility compared
to the controls at pH 5 and had overall low solubility values ranging
from 4.5% to 11.8%. However, there was an increase in solubility at pH
5 from 2.0% for the AE-IP HE control to 11.8% and 5.8% when fermented
using A. niger and A. oryzae , respectively. According to a
previous study, the isoelectric point at pH 6.2 and low solubility at
both pH 5 (~37%) and pH 7 (~23%) were
reported for a SE canola protein isolate produced from unfermented HE
meal (Chang et al. , 2015). Our results indicated that SSF
processing decreased the protein solubility at pH 5 which may be due to
a change in the isoelectric point (close to pH 5) by partial hydrolysis
of the proteins, causing the shift in solubility with pH value.
At pH 7, the solubility of AE-IP canola protein products showed
significantly lower (p <0.05) solubility than SE
products obtained from CP control meals. The inverse was found for the
products extracted from the HE control meal. Unlike at pH 3 and 5, the
SSF showed the ability to improve the protein solubility of the
extracted products at pH 7. For instance, the AE-IP product from theA. niger fermented CP meal showed a significant
(p<0.05) increase in solubility (to 47.5%) compared to both
the AE-IP CP controls. In addition, the same level of solubility
(70.0%) was found for the AE-IP HE control and the HE product from theA. niger fermented meal. However, there was a decrease in
solubility (p <0.05) for protein products extracted
(AE-IP) from A. oryzae fermented CP and HE meals compared to the
controls, which indicated that A. niger was preferred to maintain
or increase the solubility of AE-IP protein products at pH 7. Some
opposite results were found for SE protein products, as products fromA. oryzae fermented meals resulted in higher solubility values
(90.7% for A. oryzae CP and 55.6% for A. oryzae HE)
compared to the controls (p<0.05). An increase in the
solubility of the SE product from A. niger fermented HE meal
(48.5%) was also observed compared to the HE control
(p<0.05). However, for the CP meals a decrease in solubility
was reported when using SE on the A. niger fermented meal
(37.2%) compared to the CP controls. When extracting proteins via SE,
both A. niger and A. oryzae can be acceptable inoculums
for meals to maintain or increase the protein solubility at pH 7,
whereas A. niger might be a better culture choice for AE-IP
products at pH 7.
The results above indicate the differences between AE-IP and SE canola
protein (protein fraction, the percentage of napin and cruciferin) and
the hydrolysis mechanism of A. niger and A. oryzaeincluding possible different proteinase, length of peptides, structure
of hydrolyzed protein, and synthesized metabolism. The protein products
extracted using SE showed higher solubility than the AE-IP products
mainly due to the low content of non-protein compounds, possible
differences in protein fraction, and possible interactions of protein
and carbohydrate in the AE-IP products. In addition, the protein
products extracted from CP meals showed higher solubility than products
from the HE meal. Protein denaturation during the hexane-extraction
(heat treatment) could explain the solubility reduction. Heat treatments
could result in the exposure of hydrophobic groups, which contribute to
the reduction in protein solubility (Khattab & Arntfield, 2009). The
partial denaturation of proteins can alternate the balance of protein
hydrophobicity/hydrophilicity and further affect solubility (Moureet al. , 2006).