3. Results and discussions
3.1. Fermentation of glucose
The glucose concentration before and after the hydrolysis and
pretreatment stages is shown in Table 2. The glucose concentration
over time is shown in Fig. 2.
Glucose consumption is rapid on the first day of fermentation
when the concentration is higher. Therefore, it can be stated that S.
cerevisiae is only able to convert the simplest chemical compounds
present in the mixture into ethanol and hydrogen, and thus results
in an inefficient degradation of the more complex compounds.
Table 2 shows that NPbh produces a glucose concentration of
approximately 25 g/L. The hydrolysis stage (including the heating
to 120 !C and the 0.5% v/v H2SO4 addition to the reaction medium)
increases the glucose concentration to 31.61 g/L (NPah, Table 2). This
increase is the result of the breakdown of the polymeric structures
of cellulose and the consequent release of glucose monomers [23].
The glucose concentration of NPah coincides with the glucose
content obtained in the UP, BP and PCa tests before the respective
pretreatments. Only BP is able to increase the glucose in the mixtures
to a significant level, that is, to approximately 47 g/L. (Table 2).
However, the glucose concentration remains the same for the UP
and PCa tests at 32.60 g/L and 32.54 g/L, respectively.
A basic pretreatment is widely recognized as being effective for
materials with a large percentage of lignin and cellulose, as in OP.
Fan et al. [26] suggested that the base removes the lignin through a
saponification reaction of the intermolecular ester bonds that
cross-link xylose hemicelluloses and other components, such as
lignin, whereas the action mechanism appears to result from the
increase in porosity of the ligno-cellulosic materials. Generally,
NaOH treatment appears to cause different beneficial effects towards
biological attack on ligno-cellulosic materials, in particular
on cellulose, by increasing the internal surface area, decreasing the
degree of polymerization and crystallinity, and promoting the
cracking of the structural links between the carbohydrates, thus
releasing glucose monomers [23]. The PCa and UP tests did not
show a relevant increase in the sugar concentrations, which
remained almost identical to those recorded before the pretreatment.
This pretreatment did not degrade ligno-cellulosic materials
for the CaCO3 case, but contributed to reducing the concentration of
inhibiting polyphenols in the mixture.