3.5 Emulsifying properties
The emulsifying properties are critical functional attributes of food proteins. Several factors affect the emulsifying properties, including the type and concentration of protein, pH, ionic strength, and viscosity of the solvating medium. Many chemical and physical factors are involved in the formation, stability, and textural properties of oil-water emulsions stabilized by protein (Chang et al. , 2015). The EAI is a measurement of the interfacial area coated by protein during the formation of an emulsion and acts as a good predictor for the surface activity of the protein, whereas the ESI is a measurement of the stability of the diluted emulsion over a fixed period of time (Can Karaca et al. , 2011).
As shown in Table 4(a), the AE-IP HE control had a higher EAI value (13.2 m2/g) than the CP control (11.4 m2/g) and the defatted CP control (8.3 m2/g) at pH 3, which may indicate a higher hydrophobicity value of the HE products compared to the CP controls at pH 3. Similarly for pH 7, a higher EAI value was found for the HE control (18.3 m2/g) than the CP control (11.7 m2/g) and the defatted CP control (11.6 m2/g) at pH 3. As for pH 5, the defatted CP control (5.0 m2/g) had a higher EAI value than the CP control (4.1 m2/g, p >0.05) and the HE control (3.0 m2/g, p <0.05). As for the SE products, the defatted CP control (11.0, 9.4, and 14.5 m2/g at pH 3, 5, and 7, respectively) showed the highest EAI values compared to the CP (5.6, 5.8, and 12.9 m2/g at pH 3, 5, and 7, respectively) and the HE control (5.9, 6.8, and 13.9 m2/g at pH 3, 5, and 7, respectively). The difference between the AE-IP and SE products may be due to the different protein fractions and the nature of products resulting from the two methods. As for pH, the EAI values tended to be low at pH 5, ranging from 1.3-5.5 m2/g due to the low protein solubility. Higher EAI values were found at pH 7 compared to pH 3 for all protein products obtained from unfermented control meals, which was possibly related to the higher solubility at pH 7 than at pH 3.
During SSF, the partial hydrolysis of proteins loosens the compact protein structure and exposes more hydrophobic sites buried inside the protein molecule. A higher hydrophobicity value can be achieved. Townsend and Nakai (1983) suggested a positive correlation between hydrophobicity and the emulsifying properties of proteins. Results of the present study showed that canola products extracted from fermented meals have better or at least unchanged emulsifying properties compared to the controls. Among the AE-IP products, the EAI values increased to 13.8 m2/g (A. niger ) and 21.1 m2/g (A. oryzae ) at pH 3 for the CP products (p<0.05). In addition, only A. oryzae improved the EAI of the HE product (17.6 m2/g), while a decrease (8.5 m2/g) was found using A. niger at pH 3. At pH 7, the AE-IP products from A. niger and A. oryzaefermented meals had either improved or unchanged EAI values. In detail, the EAI values of AE-IP CP products extracted from A. niger andA. oryzae fermented CP meals significantly increased (p <0.05) to 25.8 and 27.6 m2/g, respectively. Similar to the AE-IP HE products, the EAI values were significantly increased to 18.0 and 25.5 m2/g when fermented using A. niger and A. oryzae . The EAI values of the AE-IP products at pH 5 remained relatively low, ranging from 1.3-5.5 m2/g for both the AE-IP CP and the HE products, regardless of whether fermented or not, may be due to the low protein solubility (pH 5). In addition, a low ESI was reported for all protein products and ranged from 1.1-4.5 min. For the SE products, A. niger and A. oryzae improved the EAI values of the CP products at pH 3 to 8.5 m2/g (A. niger ) and 9.7 m2/g (A. oryzae ) compared to the CP control (5.6 m2/g), however, they were lower than the defatted CP control (11.0 m2/g). The increases in the SE HE product at pH 3 after SSF were also reported as 8.5 m2/g (A. niger ) and 11.3 m2/g (A. oryzae ) compared to the HE control (5.9 m2/g). However, SSF decreased the EAI of the CP and HE products to 1.8 m2/g (CP, A. niger ), 4.6 m2/g (CP, A. oryzae ), 4.1 m2/g (HE, A. niger ), and 3.0 m2/g (HE, A. oryzae ) at pH 5. At pH 7, a decrease in the EAI of SE CP and HE products was also reported to 12.2 m2/g (CP, A. niger ), 14.9 m2/g (CP, A. oryzae ), 11.4 m2/g (HE, A. niger ), and 7.6 m2/g (HE, A. oryzae ). The strain A. oryzae was able to more positively modify the EAI values of canola protein products than A. niger while the enhanced properties were more frequently observed in AE-IP isolates than SE.
The above EAI and ESI results reported were lower than those from previous studies by Cheung et al. (2014) and Chang et al.(2015). Cheung et al. (2014) suggested that high solubility was positively correlated with the ESI values due to the additional protein precipitation and adherence to the viscoelastic film surrounding the droplets. A lower solubility was reported compared to previous studies on canola protein, which may result in a reduction in ESI. In contrast, Kinsella and Melachouris (1976) explained that high protein solubility and high fat-adsorption capacity were positively correlated with the ability of emulsifying (form and stabilize emulsions). Wu and Muir (2008) and Cheung et al. (2014) examined the emulsifying properties of two major canola proteins, cruciferin and napin. It has been reported that the emulsion prepared with cruciferin showed a significantly higher specific surface area and a lower particle size than that with napin. The study by Wu and Muir (2008) indicated that the presence of napin could detrimentally affect the emulsion stability of canola protein isolates. The differences may be a result of the canola cultivar, the preparation of canola meals (oil extraction method) and protein products (extraction methods and conditions), and the analytical methods employed. These differences might result in variations in protein fractions and the levels of protein that further affect the emulsifying properties. In addition, the released short peptides from fermentation may also have an impact on the protein functionality such as a reduction in emulsion activity and stability (Kristinsson & Rasco, 2000).