The number of S. cerevisiae in Fig. 2a begins to rise rapidly
within 24 h. After 48 h, the growth of S. cerevisiae remains relatively
stable. This may be due to exhaustion of sugar as the population
of yeast increases. Overliming at pH 10 produces the highest
colony forming units of yeast, which implies that pH 10 can adequately
eliminate degradation products while still maintaining a
sufficient amount of glucose for the yeast’s growth, as also confirmed
by the SEM image (Not shown) of the MG hydrolyzate after
overliming at pH 10, showing no presence of lignin residues. Overliming
at pH 11 produces the second highest colony forming units
of yeast. This could be due to the elimination of many toxic compounds
along with the loss of usable glucose. A similar explanation
also applies when overliming is performed at pH 12. Even though
overliming at pH 8 and 9 does not decrease the glucose concentration
as severely, compared to that at higher pHs, the process does
not eliminate amounts of degradation and inhibitory compounds.
The remaining compounds become toxic to the fermenting yeasts,
which results in a less yeast growth and potentially lower ethanol
yields (Eliana et al., 2014).