In addition, cell activity was also affected by aeration. During fermen- tation at 35 ◦C with a dilution rate of 0.3 h−1, kLa was calculated, and the corresponding cell activity was measured. Plotting cell activ- ity against kLa (Fig. 4a) indicated that cell activity appears to be positively affected by kLa. Fig. 4b shows that cell activity could enhance the ethanol yield. Therefore, it was essential to provide an appropriate air supply into the reactor to obtain a high ethanol yield.