2.1.2 Factors affecting extraction rates
The reverse micelle method can effectively increase the extraction rate of proteins compared to the traditional alkali solution–acid precipitation method. As shown in Table 1 , the percentage of soy protein extracted by the reverse micelle method can reach up to 95%. It has been shown in numerous studies that the extraction rate of proteins is influenced by many factors including molar ratio W0, reverse micelle diameter, aqueous phase pH and ionic strength (Rho, 2004), surfactant type and concentration (Shin, 2002), and co-surfactant (Lee, 2004). These factors affect the degree of protein solubility. Among these factors, the extraction efficiency of the proteins increases with increasing W0 (Harikrishna, 2002). W0 is the ratio of water to surfactant and can be altered by an aqueous buffer containing a certain amount of salt, which determines the size and molar ratio of the aqueous core (Ghazi, 2006). As W0 increases, the core radius increases, indicating that not only small molecules of protein can enter the micelles, but also large molecules of protein. Nevertheless, when W0is too high, the surfactant molecules are released from the micelles into the organic phase due to hydrophobic effects, thus reducing the number of micelle aggregates and the efficiency of protein extraction (Bu, 2012). Bu et al. (2012) emphasized that the efficiency of forwarding extraction of soy protein in the AOT reverse micelle system increased with increasing reverse micelle diameter.
Interactions between reverse micelles and proteins lead to structural changes in proteins, the main driving forces of which are hydrophobic effects, hydrogen bonding and electrostatic interactions (Correa, 1998.). Forward extraction at a pH above the isoelectric point suggests a hydrophobic interaction between the soy protein and AOT. Due to reduced hydrophobic interactions, the addition of Triton-X-100 to AOT reduced the extraction efficiency compared to AOT alone. The reason for the reduced extraction efficiency of the Triton-X-100 may be the lack of a strong driving force to diffuse the soy protein into the nonionic reverse micelle core (Zhao et al., 2010a). Zhao et al. (Zhao et al., 2011b) observed that, due to the reverse micelle function, the amino groups near the surface of the 7S and 11S globulin powders were exposed through bond breakage, increasing the surface N atomic percentage in the 7S and 11S globulin powders. Small changes in powder surface composition or bulk composition have the potential to change the functional properties of 7S and 11S globulin powders. Zhao et al. (Zhao et al., 2010b) concluded that the interaction between soybean protein and surfactant was the main factor determining the extraction rate of protein from reverse micelles. Forward extraction was controlled not only by electrostatic interactions between the charged protein and the polar head of the surfactant (Luisi, 1988) but also by hydrophobic interactions between the non-polar region of the protein and the surfactant tail (Rajib, 2005). In particular, the alcohol molecule has an influence on the formation and destruction of reverse micelles and improves the efficiency of protein extraction (Hong DP, 1999).