Fermentation using bagasse hydrolysate
Previous results show that LA2 exhibited a strong ability to tolerate
acidic conditions and produce lactic acid without a neutralizing agent.
Subsequently, the robustness of this strain against
lignocellulose-derived inhibitors was examined. The hydrolysate obtained
from hot water pre-treatment was directly used as a medium without prior
treatment of enzymatic hydrolysis, detoxification or neutralization. In
addition to glucose, xylose was also present in the hydrolysate.
However, our yeast strain possesses no active genes that connect the
lactic acid generation and xylose metabolism pathways. Therefore, the
concentrations of xylose were not included in any of the calculations.
As can be seen in Figure 5 , LA2 strain generated lactic acid in
a concentration of 25.34 + 3.25 g·L-1 at 15 h
from an initial glucose concentration of 49.57 + 0.49
g·L-1 (yield = 0.51 g·g-1 glucose;
productivity = 1.69 g·L-1·h-1). The
productivity of our strain was slightly lower than that of other reports
(shown in Table 3 ). In those reports, however, sodium hydroxide
had to be added to maintain the pH of the medium during fermentation.
Meanwhile, in this experiment, cultivation was performed without the
neutralizing treatment at any stage of fermentation. Interestingly,
despite a decline in the rate of glucose uptake and productivity, LA2
strain converted about 51% of the available glucose when SCB
hydrolysate was used as a medium, whereas cultivation using the YPD
medium only converted 33% of the glucose (non-neutralized condition).
Also, the ratio of lactic acid to ethanol was remarkably improved (2.67
at 15 h) using SCB hydrolysate compared with cultivation using a YPD
medium (0.91 at 9 h). This result indicates that the stress of chemical
inhibitors up to particular concentration levels could instead provide a
positive impact by shifting the metabolism and accumulating lactic acid
in higher concentration.