3.3. Fermentation of okara hydrolysate
A 120.5 g/L (70.5 g/L glucose and 50.0 g/L galactose) sugar solution was obtained using vacuum evaporation of low concentration hydrolysates. Fermentation was performed in a fermentor containing 1 L of fermentation medium inoculated with high density cell inoculums (10%, 6.9 × 107/mL). During fermentation, S. cerevisiae exhibited preferential consumption of glucose over galactose as indicated by the fermentation of glucose during the first 12–24 h, while galactose was fermented during 12–36 h ( Fig. 3). S. cerevisiae preferentially utilizes glucose, mannose, and fructose via the Embden–Meyerhof glycolysis pathway, and the presence of glucose appears to retard the fermentation of galactose via the Leloir pathway [35] and [36]. The ethanol results were calculated based on the total FS (glucose and galactose) of pretreated okara. The concentration of ethanol produced was 59.1 g/L and ethanol yield was 96.2% (g ethanol/g initial glucose and galactose) at 36 h (Table 4).