1. Introduction
Soy, an important cash crop, is used in the production of protein. Most importantly, soy protein contains a wide range of essential amino acids for humans and has a high nutritional value. In addition, soy protein is widely used in the food industry due to its high yield, low cost, and appropriate functional properties. Soy protein can be classified into four different components based on different sedimentation factors, namely: 2S, 7S, 11S, and 15S fractions. The 2S fraction includes the majority of soy protein albumins, whereas the 7S, 11S, and 15S fractions mostly contain globulins. Soybean storage proteins consist mainly of β-conglycinin and glycinin, which are 7S and 11S globulins respectively. 11S globulins make up more than 40% of total soybean protein and have better gelation properties than 7S globulins, which are often used as food additives to improve the texture and taste of foods. The 11S globulin molecule is a two-ring hexagonal structure consisting of six acidic and six basic subunits linked by disulfide bonds. Due to the tight structure of the 11S globulin molecule, most of the active groups are wrapped in the globular structure, making it difficult to express its physiological activity. Depending on the protein content, soy protein products are divided into three main categories: soy flour, soy protein concentrate (SPC), and soy protein isolate (SPI) (Wang et al., 2004). Soy flour has a protein content of 50 %. Washing the soy flour in hot water produces SPC with a much higher protein content (60-68 %). SPI has a significantly higher protein content (85-90%) than SPC and contains almost no water-insoluble carbohydrates.
Functional properties affect the use of soy protein in food, attributable to the fact that protein composition and conformation determine its physicochemical properties. The use of soy protein as a food ingredient will be more widespread when it has the appropriate functional properties (i.e. gelation, foaming, solubility, volatile compounds, emulsification, adsorption, etc.). As protein composition and functionality are influenced by processing conditions (Kinsella, 1979), it is necessary to consider preparation methods when preparing soy proteins with specific functionalities.
Currently, the widely used method for soy protein extraction is the alkali solution–acid precipitation method (ASAPM). The first disadvantage of the traditional method is the poor solubility of SPC and SPI when rehydrated (Fisher et al., 1986). This is because the extraction of proteins uses extreme conditions such as acids, bases, heat treatment, or centrifugation, resulting in protein denaturation. In addition, SPC and SPI produced by the traditional process may contain high levels of phytic acid. Phytic acid complexes with divalent cations form phytate minerals or protein mineral phytate complexes, which affect the bioavailability of the minerals (Grynspan and Cheryan, 1989). Phytic acid may also lead to the reduced solubility of proteins. Another disadvantage of traditional methods is the pollution of the environment. Large amounts of acidic and alkaline wastewater are generated during protein extraction leading to water pollution.
To address the drawbacks of traditional extraction methods, researchers have developed emerging extraction techniques for soy protein. This review article outlines three extraction techniques, including reverse micelle extraction, enzyme-assisted extraction, and ultrafiltration membrane extraction. We first examine the principles and technical characteristics of each technique. Next, we discuss their advantages. Finally, we take an integrated overview of the novel applications of each technique in soy protein extraction.