Page 125 - Lignocellulosic Biomass to Liquid Biofuels

Bioconversion of lignocellulosic biomass to bioethanol and biobutanol 99

lignocellulose source. For instance, hemicellulose from a monocot source
(grass) mainly includes xylose sugar: the furfural thus is the main furan
generated in pretreated monocot; a significant amount of mannose is
instead contained in the hemicellulose from softwood source: HMF thus
is the most abundant furans in pretreated softwood lignocellulose [242].
According to research studies carried out by Aguilar [243] and Larsson
[244], the furfural concentration in sugarcane bagasse can reach up to 5 g/
L during acid hydrolysis [243], while the HMF can accumulate to 6 g/L
in hydrolyzates from chipped pine wood [244]. Furfural and HMF are
considered potent inhibitors of yeast cells growth and subsequent of fer-
mentation process in a dose-dependent manner [245,246], causing the
inhibition of sugar uptake rate and the decrease of ethanol production
rate [247]. Anyway, they are usually toxic to many other types of organ-
ism, such as mammalian cells, fungi, and bacteria [248]. These inhibitor
compounds act by blocking both different enzymes of the glycolysis path-
way, that is, hexokinase, phosphofructokinase, triose-phosphate dehydro-
genase [239], and nonglycolytic enzymes. Modig et al. [248] reported the
furfural and HMF inhibitory effects on three dehydrogenase enzymes:
alcohol dehydrogenase (ADH), aldehyde dehydrogenase (AlDH), and the
pyruvate dehydrogenase (PDH) complex. The relevant reactions carried
out by these different enzymes are given by the following equations:
ADH
1
1
Acetaldehyde 1 NADH 1 H ! NAD 1 ethanol
AlDH
1
Acetaldehyde 1 NAD ! acetic acid 1 NADH 1 H 1
1 PDH 1
Pyruvic acid 1 CoA 1 NAD ! acetyl-CoA 1 NADH 1 H 1 CO 2
Furfural and HMF can act synergistically, but the microbial activity of
yeast strains seems to be more sensitive to inhibition by furfural than
HMF, at the same concentration [245,249]. For instance, the presence of
furfural negatively affects the activities of the ADH, AlDH, and PDH
enzymes; in contrast, HMF has a less inhibitory effect on AlDH and PDH
enzymes compared to furfural, while the effect on ADH enzyme is similar
to that of furfural [248]. However, HMF can increase the lag phase of
microbial growth, depleting the cell growth [250]. It was reported that in
cultivating S. cerevisiae K35 and CBS 1200, and Z. mobilis ATCC 10988,
5-HMF significantly inhibited cell growth and ethanol production [251].