**2.2. Medium composition for acetoin production**

The fermentation medium used for this work was the significant optimized medium of Xiao et al. [16], which was then modified. The nutrients used comprised the following (g/L): glucose 150, K<sup>2</sup> HP04 0.5 g, CH3 COONa 0.5 g, NaCl 5 g, and MgSO4 .7H<sup>2</sup> O 0.5 g, while 1 g/L of three different nitrogen sources were used in the preliminary experiment (corn steep liquor, yeast extract, and beef extract). The medium was adjusted to pH 7.0 and autoclaved at 121° C for 15 min. The flasks were incubated at 37° C with an orbital shaker at 150 rpm for 7 days.

### **2.3. Batch fermentation study**

The preliminary experimental study of acetoin production was performed with 50 mL of glucose solution (100 g/L) measured into 250-mL Duran flask, and the growth effect of three complex nitrogen sources (yeast extract, beef extract, and corn steep liquor) were tested for acetoin fermentation nutrients. The pH of the medium was adjusted with 120 g/L of NaOH and 36.5 g/L of HCl buffer solutions. Subsequently, 5% volume fraction of inoculum size was added aseptically to the flask. The flasks were transferred into the environment-controlled incubator shaker (platform shaker, model: FSIM SP016) at 30° C and 150 rpm. Fermentation was performed for 168 h with 12-h sampling interval.
