Carob waste is a useful raw material for the second-generation ethanol because 50% of its dry
weight is sucrose, glucose, and fructose. To optimize the process, we have studied the
influence of the initial concentration of sugars on the fermentation performance of
Saccharomyces cerevisiae. With initial sugar concentrations (S0) of 20 g/l, the yeasts were
derepressed and the ethanol produced during the exponential phase was consumed in a
diauxic phase. The rate of ethanol consumption decreased with increasing S0 and disappeared
at 250 g/l when the Crabtree effect was complete and almost all the sugar consumed was
transformed into ethanol with a yield factor of 0.42 g/g. Sucrose hydrolysis was delayed at
high S0 because of glucose repression of invertase synthesis, which was triggered at
concentrations above 40 g/l. At S0 higher than 250 g/l, even when glucose had been
exhausted, sucrose was hydrolyzed very slowly, probably due to an inhibition at this low
water activity. Although with lower metabolic rates and longer times of fermentation, 250 g/l
is considered the optimal initial concentration because it avoids the diauxic consumption of
ethanol and maintains enough invertase activity to consume all the sucrose, and also avoids
the inhibitions due to lower water activities at higher S0.

Keywords: Bioethanol, Carob pod, Fermentation, Invertase synthesis, Saccharomyces, 2nd generation biofuels