3.4 Oil holding capacity
Fats act as a flavour carrier and mouthfeel enhancer, so the ability of the protein to absorb and retain oil is important in food formulations (Kinsella, 1982). Fat adsorption of proteins mainly depends on the physical entrapment of oil through a capillary-attraction process (Kinsella, 1982). As shown in Table 3(a), the AE-IP CP control showed a higher OHC value (2.9 g/g) than the HE control (2.3 g/g), possibly due to the thermal and chemical changes of proteins that occurred during solvent extraction, leading to an increase in protein-protein aggregations and limited protein-water or protein-oil interactions. Similar results were observed for the products obtained using SE in Table 3(b). The CP control had a higher OHC value (3.2 g/g) than the products of the HE control (2.4 g/g). The OHC value of the AE-IP defatted CP control (2.4 g/g) was lower than that of the non-defatted CP control (2.9 g/g), whereas the SE defatted CP control (3.9 g/g) had a higher OHC value than the CP non-defatted control (3.2 g/g).
After the 72-h SSF, the OHC of AE-IP CP products increased to 2.8 g/g (both A. niger and A. oryzae ) compared to the defatted CP control (2.4 g/g, p< 0.05), whereas it was similar to that of the non-defatting CP control (2.9 g/g, p>0.05). The AE-IP HE products also showed an increase (p<0.05) from 2.3 g/g to 3.1 g/g (A. niger ) and 3.3 g/g (A. oryzae ). As for the SE products, an increase in OHC was found for both the AE-IP and the SE protein products compared to the controls (p<0.05). For the SE CP products, the OHC values increased from 3.2 g/g (CP control) and 3.9 g/g (defatted CP control) to 4.6 g/g (A. niger ) and 4.7 g/g (A. oryzae ), while the value increased from 2.4 g/g (HE control) to 4.5 /g (A. niger ) and 3.9 g/g (A. oryzae ) for HE products. The partial hydrolysis of proteins during fermentation may change the compact protein structure and expose more hydrophilic and hydrophobic sites, making it easier for proteins to adsorb and hold oil. The above results were agreed by Kinsella (1982) that the increase in fat absorption could be associated with the hydrolysis and denaturation of proteins due to the exposure of non-polar residues buried inside the protein molecules. Comparing the type of strains, there is no significant difference (p>0.05) between A. niger andA. oryzae except for the SE HE products, where A. nigerwas preferred to improve the OHC of products. Similar to the WHC, a higher increase was found in the HE meal compared with the CP meal after SSF.
Whereas the WHC results showed higher capacities with the AE-IP isolates, canola protein products obtained using SE showed higher OHC compared to those prepared by AE-IP. It may indicate that a higher level of protein (~95% for SE products and ~60-80% for AE-IP products) is preferred to obtain protein products with enhanced OHC. It could also be concluded that the proteins of SE might be more hydrophilic than those from AE-IP, hence a higher protein solubility for the SE products. The lower solubility and greater OHC suggest that the proteins recovered from AE-IP could obtain a more hydrophobic nature. The partial hydrolysis of proteins during the fermentation process may change the compact protein structure and expose more hydrophilic and hydrophobic sites (additional binding sites available) buried inside the protein molecule, which may decrease the solubility of canola proteins while increasing the ability of the proteins to adsorb and hold water/oil (Kinsella, 1982).