3.6 Foaming properties
The foaming properties of proteins are important in the production of a variety of foods (Hettiarachchy et al. , 2012). Foaming properties can be defined as two parts, the ability to form a two-phase system consisting of air cells surrounded by a thin continuous liquid layer (the lamellar phase) and the ability to stabilize the foams. All canola protein products in this study regardless of the source of meal or fermentation treatment showed FC of 131.1-480.0 % and moderate FS ranging from 68.0-89.0% at pH 3, 5, and 7.
Among the controls, the defatted AE-IP CP control (306.7% at pH 3 and 243.8% at pH 7) had higher FC than the CP (224.4% pH 3 and 165% at pH 7) and the HE control (244.4% at pH 3 and 154.4% at pH 7) possibly due to the residual oil in the CP control and a low protein content (~60%) of both controls. As for the SE products, the CP control had a higher foaming capacity (322.2%) than the defatted CP (246.7%) and the HE (266.7%) control at pH 3. At pH 7, the SE defatted CP control showed the highest FC in all products (480.0%), higher than the SE CP (464.4%) and the HE (145.6%) control. At pH 5, only the CP (241.1% for AE-IP and 211.1% for SE) and the defatted CP control (131.1% for AE-IP and 222.2% for SE) showed the ability to form stable foams for both AE-IP and SE products. However, the poor solubility at pH 5 still had an adverse impact on the foaming properties compared to the values at pH 3 and pH 7. Besides, the high temperature and the exposure to hexane during oil extraction may be the factors that contribute to the loss of foaming ability of the HE control at pH 5. In addition, the SE products showed significantly higher FC values than AE-IP. This might be due to the higher purity of protein in the SE products (>93%) than AE-IP (<80%) and possible differences in protein composition (napin and cruciferin).
Among the AE-IP products, only the AE-IP CP protein products by A. oryzae fermented meals showed the ability to form foams (228.9%) with a low FS of 33.0% at pH 3 after SSF. At pH 7, all fermented AE-IP products showed foaming properties of 230.7% (CP, A. niger ), 180.0% (CP, A. oryzae ) 244.2% (HE, A. niger ), and 136.7% (HE, A. oryzae ) with comparable FC compared to the controls ranging 72.0-84.6%. The AE-IP HE samples pre-treated withA. niger even showed a significantly higher FC value (244.2%) compared to the HE control (154.4%). As for the SE products, all samples were able to form foams at pH 3 and 7. In detail, the FC values of CP products from fermented meals decreased to 196.5% (A. niger at pH 3), 175.6% (A. oryzae at pH 3), 191.1% (A. niger at pH 7), and 153.3% (A. oryzae at pH 7) compared to the CP control at pH 3 and 7. For the HE products, decreases were also found for products at pH 3, at 195.6% (A. niger ) and 135.6% (A. oryzae ), while significant increases were reported at pH 7 as 306.7% (A. niger ) and 155.6% (A. oryzae ) compared to the HE control (266.7% at pH 3 and 155.6% at pH 7). In addition, the FS decreased to ~52% for all products from fermented meals at pH 3, while they remained relatively unchanged (77.6-88.4%) at pH 7. At pH 5, none of the products extracted from fermented meals showed the ability to form stable foams. All proteins had poor foaming properties at pH 3 and 5 and could only form weak foams with large bubbles that disappeared within 5 min. This was possibly due to the extremely low protein solubility (<10%) of proteins extracted from fermented meals at pH 5. The SSF was found to decrease the solubility of canola protein products to <10% at pH 5, which could have further adverse impacts on the foaming properties. Overall, protein products prepared from the fermented HE meals using both extraction methods showed better FC and FS than the HE controls, while opposite results were found for CP canola meal products.
The above results indicate the potential of SSF for improving the foaming properties at pH 7 of protein products extracted from HE meal, especially when using A. niger . In other words, A. nigerwas preferred to provide protein products with better foaming properties compared to A. oryzae . The CP products had better foaming properties demonstrated by some foam when non was observed for HE isolates under certain conditions. Although the FC and FS values decreased after SSF, proteins extracted from the fermented CP meal displayed acceptable foaming properties compared to other plant proteins like soybean, pea, and lentil (~200%) (Chabanonet al. , 2007; Stone et al. , 2015). However, they still could not produce comparable protein products similar to the ones from unfermented CP meals.