3.2 Contents of phytic acid and total phenolic compounds
The levels of PAC and TPC in the canola meals are presented in Table 3. All fermented samples showed decreases in PAC and TPC. Significant influences were observed only from fermentation time for PAC (p <0.001), whereas fermentation time, strain used, meal type × strain, meal type × time, strain × time, and the three-way interaction of meal type × strain × time were all statistical factors affecting TPC (p <0.05). The reduction of PAC from ~5.9 to ~1.3% (a 74-85% reduction) was achieved by phytase synthesized by A. niger and A. oryzaeduring SSF. In contrast, decreases in TPC upon fermentation as well as the corresponding increases have both been reported in the literature as the enzymatic breakdown of polyphenol complexes (e.g., lignins, tannins, flavonoids) leads to the accumulation of smaller units, and sometimes the formation of new bioactive compounds (Adebo & Medina-Meza, 2020). Depending on seed composition and structure (e.g., ease of cell wall breakdown), microbial strains employed, and the choices of the assay (and standard) for TPC determination, fermentation can bring different outcomes to the contents of ANF. All fermented samples showed a decrease in TPC from 2.7-3.1 to 1.0 mg GAE/g DM (a ~65% reduction) except for the HE meal fermented by A. niger . This sample had an even greater decrease in TPC from 3.1 to 0.6 mg GAE/g DM (a ~81% reduction). Overall, the reduction in PAC and TPC was correlated with fungal growth, indicating that the high residual oil in the CP meal was still suitable for A. niger NRRL 334 andA. oryzae NRRL 5590 to grow without affecting the reduction of ANF levels.
A lower reduction in PAC and TPC was reported by Olukomaiya et al. (2020) where a ~20% reduction in PAC and a ~17% reduction in TPC were achieved using A. sojae ATCC 9362 and A. ficuum ATCC 66876 on solvent-extracted (45% moisture, 30℃) canola meal, even though their fermentation period (7 days) was longer than in the current study. The initial values of PAC and TPC in their meals were 27.06 mg/g DM and 9.31 mg GAE/g DM, respectively, which were significantly lower and higher than those in samples from the present study (~59 mg/g DM and ~2.9 mg GAE/g DM), respectively. Pal Vig and Walia (2001) also reported a similar result that a maximum decrease in PAC (a 42.4% reduction from the initial content of 29.3 mg/g DM) occurred after a 10-day fermentation using Rhizopus oligosporus with a water:rapeseed meal ratio of 1:3 at 25℃. The degradation of ANF in canola meals might require multiple enzymes. The lower reduction in the ANF levels in those studies compared to the present study could be explained by the lack of certain enzymes specific for degrading the particular ANF or the insufficient level of enzymes produced when using other strains. Cultivar differences in the starting materials (meals) should also be considered as cultivar and growing environment can greatly influence the composition and distribution of these compounds in the seed and therefore meals (Bueckert et al., 2011).