2.1.3 Advantages of the reverse micelle extraction
Reverse micelle has a variety of advantages such as enormous interfacial
area, thermodynamically stable and optically transparent, low cost due
to the recovery of surfactant and nonpolar solvents, ease of scale-up
and simple control of the reaction variables (Sereti V, 2014). Most
importantly, due to the similarity of its aqueous cores to the
physiological environment, the reverse micelles could prevent the
denaturation of encapsulated biomolecules (Bu, 2014). A recent study has
demonstrated that the reverse micelle method can better prevent breakage
of the natural molecular structure of proteins compared to the
traditional alkali solution–acid precipitation method (ASAPM) (Yao et
al., 2021). Specifically, 11S globulin extracted by ASAPM had a higher
β-fold content compared to 11S globulin isolated by reverse micelles
containing more hydrophobic amino acids and fewer sulfur-containing
amino acids. As a result, the surface hydrophobicity of 11S globulins
obtained by ASAPM was increased. In addition, the 11S globulins
separated using reverse micelles were more
resistant to high temperatures.
Similarly, Du (Du et al., 2020b) reported that more β-sheets but less
turn structure was observed in the 7S globulin extracted by reverse
micelle, indicating that the native folded structure of protein could be
protected by the reverse micelle environment and 7S globulin formed a
more compact conformation. It is well known that the functional
properties of proteins are influenced by their structure. The low
denaturation temperature, the poor thermal stability, and the strong
hydrophobic interaction of 7S globulin prepared by the reverse micelle
method affect its gelation process. Therefore, an improvement in the
quality of thermally induced gelatin of 7S globulin was observed in the
reverse micelle environment (Du et al., 2020b). As an advanced soybean
protein extraction method, reverse micelles can not only separates and
purifies soy protein but also improves the functionality, nutritional
properties, and flavour of soy protein and reduces undesirable beany
flavor. Zhao et al (Zhao et al., 2018b) concluded that the protein oil
absorption capacity, solubility index, emulsification capacity, and
stability as well as foaming capacity obtained by AOT reverse micelles
were significantly higher than those obtained by alkali extraction
isoelectric precipitation (AEIP). Soy is an essential source of amino
acids (Zarkadas, 2007). Current research has shown that AOT reverse
micelle extracted soy protein is a superior source of protein nutrition
suitable for human consumption. For 11S globulins, the total amino acid
content of the AOT reverse micelle extract was increased by 5.98%
compared to the amino acid composition of the aqueous buffer extract,
but the content of 7S globulins was similar. For both 7S and 11S
globulins, the major amino acid content in the aqueous buffer solution
was lower than that in the AOT reverse micelles (Zhao et al., 2011a).
2.2 Enzyme-assisted
extraction
Enzyme-assisted extraction uses water and protease to extract protein
from soybeans and is considered an alternative extraction method to
alkaline extraction which involves pollution (Campbell KA, 2011). As a
mild extraction method, enzyme-assisted techniques minimize side
reactions (Sari et al., 2013). Enzyme-assisted extractions are
considered environmentally friendly technologies as they offer a green
chemistry possibility for the food industry looking for cleaner routes.
Recent studies on enzyme-assisted extraction have shown that it offers
faster extraction rates, higher recoveries, less solvent use, and lower
energy consumption than non-enzymatic methods and therefore represents a
potential alternative to traditional solvent extraction methods
(Vergara-Barber, 2015. ). Compared to alkaline extraction, the addition
of enzymes results in a reduction in protein size due to protein
hydrolysis. As a result, proteins are more easily extracted. In
addition, the use of enzymes can also be used to lower the processing
pH, thus avoiding severe conditions of protein denaturation (Sari et
al., 2013). Enzyme-assisted countercurrent extraction significantly
increased the protein yield compared to alkaline extraction and acid
precipitation. The protein had a larger molecular weight distribution,
reduce flavor volatiles, higher thermal stability, and surface
hydrophobicity as evidenced by the denaturation temperature and enthalpy
change of the protein (Wei et al., 2017). Under alkaline pH conditions,
80% of soybean meal protein is extracted without the addition of
enzymes, while the addition of enzymes increases the protein extraction
yield of soybean meal to 90% (Sari et al., 2013). Many studies have
shown that enzyme-assisted extraction has been used to enhance the
nutritional value and alter the structural properties of proteins (Lu,
2016). The process of enzymatic hydrolysis of soy proteins has obtained
many peptides in cancer prevention, anti-hypertension, and reducing
blood cholesterol (Hoa N T, 2014, ). Compared to natural SPI, SPI
prepared by enzyme-assisted treatment has higher hydrophobic amino acid,
surface hydrophobicity, and interfacial adsorption properties. This is
due to the formation of small soluble aggregates accompanied by protein
unfolding. In addition, the significant improvement in emulsification
capacity and physical stability of the emulsions may be related to the
higher surface protein loading. These results provide a viable route for
the production of nutrient-enhanced soy proteins with excellent
emulsification properties for application in the food industry as novel
functional ingredients (Lu et al., 2016).