3.3 Water holding capacity
The ability of a protein to absorb and retain water is critical in stabilizing the structure, improving flavour retention and mouth feel, and reducing moisture loss of food products (Khattab & Arntfield, 2009). As shown in Table 3(a), the AE-IP CP control protein product showed higher WHC (2.1 g/g) than the products of the HE control (1.8 g/g), which may be due to the thermal and chemical changes of HE proteins that occurred during the solvent extraction processing, leading to an increase in the protein-protein aggregations and limiting the protein-water or protein-oil interactions. However, the value of the defatted CP control (2.7 g/g) was higher than that of non-defatted ones (2.1 g/g), which was mainly due to the presence of residual oil that impacted the hydrophilic ability. Similar results were found in the SE controls of which the defatted CP control (2.1 g/g) had a higher WHC value than the HE (1.3 g/g) and non-defatted CP controls (1.1 g/g) as shown in Table 3(b).
After a period of 72-h fermentation of the source meal, a significant increase (p <0.05) in the WHC of AE-IP products was found. The WHC increased from 2.1 g/g (CP control) and 2.7 g/g (non-defatted CP control) to 3.1 g/g (A. niger fermented) and 3.0 g/g (A. oryzae fermented). No significant difference (p >0.05) was found between the strains for AE-IP CP products. In addition, A. oryzae showed a higher increase in WHC of AE-IP HE products (3.2 g/g) than A. niger. (2.9 g/g). Similar results were found for SE HE products as SSF increased the WHC values from 1.3 g/g to 2.3 g/g (A. niger ) and 2.7 g/g (A. oryzae ). The partial protein hydrolysis during the fermentation process may change the compact protein structure and expose more hydrophilic and hydrophobic sites (additional binding sites available for water), which made it easier for the protein to adsorb and hold water (Kinsella, 1982). In contrast, the WHC values of the SE CP products remained stable (p >0.05) after SSF using A. niger (1.3 g/g) and A. oryzae (1.4 g/g) compared to the non-defatted CP control (1.1 g/g) and decreased when compared to the defatted CP control (2.1 g/g). For the type of strains, A. oryzae was preferred to improve the WHC of both AE-IP and SE HE products. In addition, all HE products showed higher increases in WHC than the CP products.
Overall, the defatted CP controls showed higher WHC values than the non-defatted CP controls and the HE controls. As for the difference between the AE-IP and SE products, the AE-IP protein products showed a slightly higher WHC value than the SE ones. This was possibly due to the lower protein level in AE-IP products, which led to a larger sample amount of AE-IP products used for testing than the SE products (corrected by protein content). The non-protein compounds in AE-IP products such as the soluble fibre and polysaccharides may contribute to the ability of water to absorb and bind. Thus, it is important to take the possibly contained fibre and polysaccharides into consideration. The presence of these compounds can enhance the overall water holding capacity of canola products (Aider & Barbana, 2011).