the main causes of such observations. Therefore, experiments were
further conducted in a 3-L bioreactor fed with the FW hydrolyzate
without dilution to investigate the effect of mixing speed on ethanol
fermentation (Fig. 3). Compared to the results presented in
Fig. 2, the glucose utilization rate in bioreactor was significantly
higher than that of flask experiments. The fermentation was completed
within 48 h in the bioreactor, while it took 72 h in the flask
experiments (Fig. 3A).
By increasing the mixing speed, the glucose uptake rate
increased (Fig. 4A). As a result, the fermentation of glucose was
completed in a shorter time at higher agitation speeds. The specific
growth rate using 100 rpm was 0.24 h1, while those with 200 and
300 rpm mixing speeds were 0.3 and 0.32 h1, respectively. These
indicate that a proper agitation improves mass transfer, and
promote cell growth, ultimately enhances ethanol productivity
during the fermentation. It should also be noted that further
increase in agitation speed beyond 200 rpm may not be helpful
for the bioethanol production as shown in Fig. 3. As such, the highest
ethanol concentration of 58 g/L was obtained at 200 rpm versus
57 g/L at 300 rpm (Fig. 4C). These are probably due to the fact that
cell metabolism would be limiting the ethanol production when
sufficient mixing is in place [31]. Thus, the mixing speed of
200 rpm was adopted in all the subsequent experiments.
The ethanol concentration of 58 g/L obtained corresponded to
98% of the theoretical ethanol yield, suggesting that the hydrolyzate
produced from enzymatic pretreatment of mixed FW by the
fungal mash is a perfect substrate for ethanol production. For the
purpose of comparison with the FW hydrolyzate, the experiments