All variables were analyzed for 10 h of hydrolysis and 4 h of
the fermentation process. The initial pH was adjusted to 4.5 for
the hydrolysis and fermentation. The yield (Y1) ranged from 73.7%
to 80.5%. It was also observed that the central points varied only
slightly for all parameters, indicating good reproducibility. The
highest yield of ethanol occurred under the hydrolysis conditions
at the central point: 0.05% enzyme and 55 ◦C. A significance level
of 95% was employed, i.e., the parameters were considered significant
when p < 0.05. Eq. (6) represents the model with the significant
variables coded to the fermentation yield.
Fig. 4 shows the response surfaces for the enzyme concentration
and the temperature of SM hydrolysis according to feedback
from the fermentation yield (Y2), ethanol productivity (P) and the
concentration of residual sugar (RS).