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).