2. Materials and methods
2.1. Yeast strains, culture media and cell preparation
Saccharomyces cerevisiae M30, a high ethanol-producing flocculent strain, was used in this study. The stock culture was stored in potato dextrose agar (PDA) slants at 4 °C. Starter cultures were prepared by transferring a loop of the stock culture to 100 mL of the sterilized pre-culture medium, which was composed of 50 g/L reducing sugar from palm sugar, 0.05% w/v (NH4)2SO4, 0.01% w/v KH2PO4 and 0.0035% w/v MgSO4.7H2O. The initial pH of the medium was adjusted to 5.0. Cell cultivation was carried out in Innova 4330 refrigerated incubator shaker (New Brunswick Scientific, Edison, NJ, USA) at 150 rpm and 33 °C for about 20 h. The late exponential-phase cells were harvested by decantation to obtain the stock cell suspension