Salt-Tolerant Acid Proteases: Purification, Identification, Enzyme Characteristics, and Applications for Soybean Paste and Sauce Industry

Soybean, the most cultivated plant in the world, is rich in proteins (40–50%) and contributes many essential nutrients and health-promoting bioactive compounds. Fermented soybean products including soybean paste and soybean curd, etc. are consumed in considerable amounts in Asian countries such as China, Japan, and Korea for a long history. Nowadays, they are considered as healthy foods and soy sauce become quite popular all over the world. The fermentation process improves the nutritional quality of the soybeans and contributes to the elimination of trypsin inhibitors (Kim et al., 2010). They are known to be highly digestible and nutritious, and affect a number of physiological activities such as antioxidative activity (Lin et al., 2006), fibrinolytic activity, lowering of blood pressure, and prevention of osteoporosis (Kim et al., 2011). Fermentation starters are essential for the production of fermented soybean products. Microorganisms including Bacillus, Aspergillus, and Rhizopus species and proteases are commonly used as a starter for fermenting soybeans. Proteases produced from microorganisms are widely used for baking, photographic, brewing, fermentation, protein hydrolysates, gelatin industries, meat, leather, and detergents, etc. (Murakami et al., 1991). In this study, salt-tolerant acid proteases and their application in the manufacture of fermented soybean products are discussed.