The effect of aeration for K. marxianus KD-15 was tested in
1.5 L of media composed of thick juice diluted with whey alone by using a 2-L fermentor at 30 o C at 5 Hz. Aeration increased
may be explained by the characteristics of K. marxianus, which is classified as a facultative fermentative aerobic yeast that requires oxygen limitation for ethanol synthesis [10]. Under an optimized condition with aeration at 15 mL min-1 in 72 h, sugars were almost completely converted to ethanol at
102 mg mL-1, corresponding to 92.9% of the theoretical yield
(Fig. 3). Lactose was consumed during rapid hydrolysis of sucrose and successive uptake of glucose and fructose. Simultaneous fermentation of lactose and sucrose reflects the characteristics of strain KD-15 that is insensitive to catabolite repression [7]. During cultivation, the pH was not so changed at 4.6 to 5.1 (data not shown). The cell number increased from
3 x 108 cells mL-1 and reached 5 x 108 cells mL-1 in 24 h (data
not shown), while their viability drastically decreased in accordance with the accumulation of ethanol after 60 h (Fig. 3). K. marxianus is reported to show lower ethanol toler- ance than S. cerevisiae [10]. Strain KD-15 maintained cell viability in the presence of ethanol up to 90 mg mL-1 (Fig. 3) and seems to be applicable for the cell recycling method used by Brazilian ethanol plants [13] when initial concentration of
sugar was limited less than 180 mg mL-1.