3.3. Hydrogen and bioethanol production
The results of the hydrogen production by S. cerevisiae
fermentation are illustrated in Table 3.
The thermal and acid hydrolysis and the different pretreatments
contributed to the degradation of the predominant and recalcitrant
components (lignin and cellulose), both of which are found in the
skin, pulp and seeds of olives. The hexose and pentose monomers,
primarily glucose and xylose, are the most relevant monomers of
lignin and cellulose. Glucose monomers are partially solubilized
after pretreatments, and degrade more easily to hydrogen and
ethanol by means of the strain [17]. The BP and UP pretreatments
resulted in the best hydrogen production, that is, 1.98 and 1.09 NL/L,
respectively. Fig. 4 reports the average concentrations of ethanol
obtained after the different pretreatments.
All of the tests lead to a rapid increase in the ethanol concentration
in the mixture in the first hours of the fermentation until an
asymptotic value was reached after 150e200 h. Although similar
kinetics were displayed, the final ethanol concentrations obtained
in the tests were different. The lowest ethanol production was
recorded for NPah, with 2.67 g/L and a process efficacy of 8.11%
(Table 3). This low performance of NPah resulted was due to the
small soluble sugar concentration in the mixture, because of the
absence of any pretreatments. The lignocellulosic substances in OP
displayed a recalcitrant nature, and was resistant to microorganism