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100 Lignocellulosic Biomass to Liquid Biofuels

Moreover, furan derivatives affect cell wall formation, DNA, RNA,
plasmids, and/or protein synthesis, and cause the accumulation of reactive
oxygen species damaging cytoskeleton, nuclear chromatin, mitochondria,
and vacuole membranes [237].
Some microorganisms are capable of converting furfural and HMF to
less toxic compounds [230]. For example, S. cerevisiae and P. stipitis can
reduce furfural and 5-HMF, respectively, into 5-hydroxymethyl furfuryl
alcohol (2,5-bis-hydroxymethylfuran, furan-2,5-dimethanol) and furfuryl
alcohol (2-furanmethanol) by ADH and AlDH [252]. For most microor-
ganisms, such as S. cerevisiae, 5-hydroxymethyl furfuryl- and furfuryl alco-
hols are less toxic than HMF and furfural, causing only slight inhibition,
whereas the inhibitory effect is still acute for other microorganisms such as
Rhodosporidium toruloides [253]. In addition, the inhibition of P. stipitis aer-
obic growth has been reported [254]. The industrially important yeasts
Tepidibacillus fermentans and S. cerevisiae (primarily, under aerobic condi-
tions), the fungi C. ligniaria, and the bacteria Pseudomonas putida and
Cupriavidus basilensis can oxidize furfural and/or HMF to 2-furoic acid,
that has a lower toxicity than furfural and furfural alcohols [230]. The
HMF conversion rate is four times slower than that of furfural, probably
as result of lower membrane permeability, causing a longer lag phase of
the cell growth cycle, as indicated earlier [254].

3.5.2 Weak acids
Acids are categorized as either weak or strong on the basis of their acid
dissociation constant, K a . The pK a is indeed the negative logarithmic of
K a (pK a 52log10 K a ) and corresponds to the pH value at which dissoci-
ated (S) and undissociated (HS) states concentrations of the acid are equiv-
alent. The weak acids contained in lignocellulose hydrolyzates, mainly
LA, FA, and acetic acid, exist in equilibrium between HS and S that
highly depends on both pK a of the particular acid and pH of the hydro-
lyzed [255]. In particular, HS concentration of weak acids increases when
the pH value of the hydrolyzed is lower than pK a value of weak acid, in
according to the Henderson Hasselbalch equation [254]. HS of weak
acids is liposoluble and can transfer from the fermentation medium across
the plasma membrane by a passive diffusion process [255] or enter into
the cell through the Fps1 aquaglyceroporin channel, as observed for acetic
acid [256]. In according to the uncoupling theory proposed by Russell in
1992 [257], HS of weak acids dissociates inside the microbial cells into