3.3 In vitro protein digestibility
The IVPD of control and fermented meals is presented in Table 4. Meal
type, strain used, fermentation time and all their interactions were
significant (p <0.05) factors affecting IVPD. At time 0
h, HE meals and the CP meal fermented by A. oryzae had similar
IVPD of ~72.7%, while that for CP by A. nigerwas much higher (80.2%). The A. niger fermented CP meal also
followed a different trend than the other three meals after 24 h of
fermentation as it had a decrease in IVPD (a ~5.7%
decrease) whereas the IVPD of all other meals increased (increases of
~4.8, 10.6 and 11.9% for CP by A. oryzae , and HE
by A. oryzae and A. niger , respectively). The three
aforementioned meals reached their maximum IVPD value after 24 h, after
which the values started decreasing as fermentation continued (towards
48 and 72 h). At the end of the fermentation period (72 h), all
fermented samples had similar IVPD values of ~69.7%,
statistically lower compared to the respective controls for CP meals and
the HE meal fermented by A. oryzae , whereas that for HE byA. niger remained relatively unchanged. The most substantial
decrease was for A. niger fermented CP meal (by
~12.8%), while those for CP meal fermented withA. oryzae (by ~3.6%) and HE meal with A.
niger (by ~5.2%) were less consequential.
Despite the rather substantial drop in ANF contents (TPC and PAC with
~65-85% reductions, Table 3), the IVPD decreased
overall upon fermentation in the present study. Literature concerning
changes in protein digestibility of fermented canola meals is scarce.
Shi et al. (2015) reported a significantly enhanced amino acid in
vitro digestibility of rapeseed meal (from China) after SSF withA. niger ; however, wheat bran was included with the meal as a
carbon substrate for fungal growth. Fermentation, in general, has been
reported to improve protein digestibility in various legume, cereal, and
oilseed samples. Employing a similar fermentation approach (SSF byA. oryzae and A. niger ), Kumitch et al. (2020) reported an
increased IVPD of pea-protein enriched flour (from ~74.8
to ~80.2%) over the fermentation period (6 h). Natural
fermentation (16-20 h) using a yogurt culture containingLactobacillus delbrueckii subsp. bulgaricus andStreptococcus thermophilus improved the IVPD (a
~9.5% increase) of desi chickpea flour; however, the
fermentation process did not alter the values for kabuli chickpea or
faba bean flours, according to Chandra-Hioe et al. (2016). An increased
IVPD of sorghum flours coupled with reduced phytic acid, trypsin
inhibitors, and tannins after traditional (natural) fermentation (24 h)
was observed by Osman (2004).
The improved protein digestibility upon fermentation in the
aforementioned studies is likely due to the breakdown of protein
molecular structures that enhanced the accessibility of digestive
enzymes, while the reduction/elimination of ANF also resulted in
weakened crosslinking activities, allowing more intensive proteolytic
attacks. However, depending on the nature of processing (e.g., types of
fermentation and strains utilized) and the starting material (e.g.,
structural differences among species and cultivars), such microbial
modification of matrix structures may also cause blockage in certain
passages and limit the function of digestive enzymes (Chandra-Hioe et
al., 2016; Emkani et al., 2022; Skalickova et al., 2022) which is
hypothesized to have occurred in the present study. It is interesting to
note that during the period where the IVPD of most samples initially
increased (0-24 h), the DH was also increasing at its fastest rate
(Table 1). We therefore hypothesize that within that time frame,
effective hydrolysis of large protein molecules into smaller subunits
might have overcome the influence of passage blockage of the digestive
enzymes and as such an overall positive effect on the protein
digestibility of the fermented meals was seen. The primary factor
influencing IVPD in the canola meals was not the presence of ANF in our
case but the specific actions of the selected fungal strains and the
sensitivity of sample materials to fungal fermentation using SSF.