3.1 Protein content of canola protein products
The protein products obtained using AE-IP from unfermented HE and CP (non-defatted) meals had protein levels of 86.5% and 68.9%, respectively (Table 1). After the defatting process, an 81.6% protein content was achieved for the CP meal, indicating that the high oil content negatively influenced protein extraction by AE-IP. The canola protein products obtained from the CP meal fermented with A. niger NRRL 334 and A. oryzae NRRL 5590 had protein levels of 62.7% and 57.9%, respectively. The protein products from the HE meals fermented with A. niger and A. oryzae had protein levels of 56.3% and 58.4%, respectively. Both the CP and HE meals, when fermented with either of the fungal strains, gave products with lower protein content upon enrichment using AE-IP.
In contrast, the SE method resulted in higher protein levels (>90%) for all unfermented and fermented canola protein products than those produced by AE-IP (56.3%-86.5%). The lower protein content found in the AE-IP samples may be due to the heterogeneous nature of canola proteins (cruciferin and napin), which were proven to have different isoelectric points that complicate their precipitation (Wu & Muir, 2008). Fermentation did not increase the protein content of SE products for either type of meal (CP and HE). Of the protein products extracted from fermented meals, the SE process using the CP meal fermented with A. oryzae resulted in the highest protein content (99.0%).
As these findings refute the hypothesis that SSF could improve the protein extraction process, a few theories are proposed. Fermentation can lead to partial hydrolysis of the protein, which is hypothesized to increase the protein surface hydrophobicity (decrease in protein solubility) and in turn create stronger protein-lipid interactions. Also, the large quantity of hyphae produced by fungi may act like a ‘glue’ to connect the solid substrates tightly and affect the protein extraction by limiting the soluble protein dissolving into the surrounding aquatic system. After the SSF, the degradation of fibre might also release insoluble or soluble carbohydrates (polysaccharides or starch) and make it difficult to separate the proteins from the complex (protein-carbohydrate interaction). In addition, the oil could largely affect the protein extraction process, which may be the case for the CP meals (Krause & Schwenke, 2001), although adding a defatting step to the unfermented meal resulted in slightly lower protein content (99.6 vs. 96.5%) of the SE products.