Systematic Characterization of the Metabolism of Acetoin and Its Derivative Ligustrazine in Bacillus subtilis under Micro-Oxygen Conditions

Bacillus subtilis is an important microorganism for brewing of Chinese Baijiu, which contributes to the formation of flavor chemicals including acetoin and its derivative ligustrazine. The first stage of Baijiu brewing process is under micro-oxygen conditions; however, there are few studies about B. subtilis metabolism under these conditions. Effects of various factors on acetoin and ligustrazine metabolism were investigated under these conditions, including key genes and fermentation conditions. Mutation of bdhA (encoding acetoin reductase) or overexpression of glcU (encoding glucose uptake protein) increased acetoin concentration. Addition of Vigna angularis powder to the culture medium also promoted acetoin production. Optimal culture conditions for ligustrazine synthesis were pH 6.0 and 42 °C. Ammonium phosphate was shown to promote ligustrazine synthesis in situ. This is the first report of acetoin and ligustrazine metabolism in B. subtilis under micro-oxygen conditions, which will ultimately promote the application of B. subtilis for maintaining Baijiu quality.