2.3. Batch ethanol fermentation
Mineral nutrients, including KH2PO4, MgSO4·7H2O and either (NH4)2SO4 or (NH2)2CO as nitrogen source, were added to 90 mL of sweet sorghum juice at the designated level (Table 1). After sterilization, 10 mL of inoculum was added into the juice to start fermentation. Batch ethanol fermentation was performed in a 300- mL conical flask. The flask was immersed into a water bath (30 ◦C), and the cultivation medium was stirred using a magnetic stirrer for 24 h. When evaluating (NH4)2SO4 or urea as nitrogen source, the batch ethanol fermentation was carried out for 32 h without