Page 123 - Lignocellulosic Biomass to Liquid Biofuels

Bioconversion of lignocellulosic biomass to bioethanol and biobutanol 97

Figure 3.5 Fermentation inhibitors.

lignocellulosic materials cause increased formation of the undesirable by-
products derived from lignocellulose that has the potential to inhibit
microbial growth and, consequently, reduces fermentation yields [230].
The potential fermentation inhibitors, present in the liquid fraction after
lignocellulosic biomass pretreatment, include furan derivatives (i.e., furfu-
ral, or 2-furaldehyde and HMF or 5-hydroxymethylfurfural), aliphatic
acids (i.e., LA, FA, and acetic acid), and phenolic compounds (i.e.,
syringaldehyde and vanillin). These fermentation inhibitors can be
grouped in two main categories (Fig. 3.5): (1) process-derived inhibitors,
produced during pretreatment process (such as furan derivatives), and (2)
feedstock-inherited inhibitors, in other words, naturally occurring inhibi-
tors from the lignocellulosic biomass (primarily acetic acid and phenolic
compounds) [229].

3.5.1 Furfural and 5-hydroxymethylfurfural

Pretreatment of lignocellulosic biomass is an inevitable process for the
depolymerization of the holocellulosic content of biomass [231]: glucose
is the only simple sugar produced by cellulose decomposition; on the
other hand, the xylose is the main soluble sugar produced by hemicellu-
loses degradation [232]. Minor amounts of other monosaccharides,