Abstract
Aspergillus oryzae is an excellent strain for soy sauce fermentation because of its complicated enzyme system, especially protease. The aim of this study was to investigate the key enzymes and flavors during soy sauce fermentation, and a comparative assessment of extracellular enzymes during various fermentation stages at the proteomic level via iTRAQ analysis is presented. Many important enzymes related to the amino acid and glucose metabolisms participated in the material decomposition under high-salt stress. Dipeptidase, dipeptidyl aminopeptidase, leucine aminopeptidase, aspartic protease pep1, and extracellular metalloproteinase played positive roles during the early stage of soybean mash fermentation, whilst leucine aminopeptidase A and extracellular metalloproteinase NpI were the dominant proteolytic enzymes during the later period of fermentation. At the same time, β-glucosidase and β-xylanase exerted great effects upon glucose metabolism throughout the fermentation process. The results show that protease and amylolytic enzymes are complementary in the formation of flavors such as alcohols, acids, esters, aldehydes, furans, and pyrazines during soy sauce fermentation.
Highlights
Soy sauce is the most popular of the traditional fermented soy foods such as miso, tempeh, natto, tofu, and soymilk products, in Asian cuisine worldwide
This study presents a comparative assessment of extracellular hydrolases during various stages of soy sauce fermentation at the proteomic level using the iTRAQ method
Protein SLE3, protein SLE2, 18 kDa seed maturation protein, 51 kDa seed maturation protein, seed maturation protein PM28, seed maturation protein PM30, sucrose-binding protein and lea protein were difficult to destroy under submerged conditions, but they were decomposed much easier via solid-state koji fermentation
Summary
Soy sauce is the most popular of the traditional fermented soy foods such as miso, tempeh, natto, tofu, and soymilk products, in Asian cuisine worldwide. Soy sauce can enhance the secretion of gastric juice in humans and promote digestion (Kataoka, 2005). Soy sauce is made from a mixture of soybeans and wheat using an established two-step fermentation process, koji and soybean mash (brine) fermentation (Lioe et al, 2007). Soy sauce is an indispensable condiment that is used for cooking in almost every household in China for its unique and abundant flavors. Soy sauce flavors depend on the raw materials and strains of microorganism used (Wanakhachornkrai and Lertsiri, 2003). The flavors come from the degradation of the available materials and the reaction of the degraded products during soy sauce fermentation, and nearly 300 compounds have been identified (Steinhaus and Schieberle, 2007)
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