Page 129 - Lignocellulosic Biomass to Liquid Biofuels

Bioconversion of lignocellulosic biomass to bioethanol and biobutanol 103

on acetic acid medium. Under aerobic conditions the AcH is found pri-
marily in a dissociated form as acetate anion. The acetate enters the plasma
membrane by two specific transport proteins encoded by ADY2 (electro-
neutral proton symport transporter) and JEN1 (monocarboxylate trans-
porter). The peroxisomal or cytosolic acetyl CoA synthetases convert
acetate to acetyl CoA, that is, in turn oxidized in mitochondria through
the Krebs cycle or consumed in the anabolic pathway, called glyoxylate
cycle [267]. However, S. cerevisiae cells in a glucose-containing media are
sensitive to AcH stress and unable to metabolize this weak acid, due to
the activation of pathways for glucose repression [267]. Differently, the
spoilage yeast Zygosaccharomyces bailii showed high resistance to weak acids
in media containing glucose and, in particular, the capacity to metabolize
AcH in the presence of glucose [268].

3.5.2.2 Formic acid
FA (HCOOH), contained in lignocellulosic hydrolyzates at a typical con-
centration of about 1.4 g/L (30 mM), can be formed when furfural and
HMF are broken down at high temperatures of hydrolysis [269]. The pK a
value of FA, equal to 3.75 at 20°C, is considerably lower than those of
AcH (4.75 at 25°C) and LA (4.66 at 25°C); therefore FA toxicity in yeast
strains is more severe than that observed for AcH and LA [269 271]. The
toxic effect on S. cerevisiae increases in the order AcH , LA , FA. The
undissociated form of FA should be present in lower concentrations at
the same pHi, and subsequently be less harmless to the cells. However,
the increased FA toxicity seems to be related to a smaller molecule size
that facilitates its diffusion across the plasma membrane, causing its higher
anion toxicity [269]. Oshoma et al. [269] investigated the FA tolerance in
7 non S. cerevisiae yeast strains. In particular, S. arboricolus exhibited a
higher FA tolerance than other strains, confirmed by a series of fermenta-
tions. Then, this yeast could be used as a novel bioethanol producing
strain or as a new source of gene donor to other inhibitor-sensitive strains
that are able to produce high ethanol levels [269].

3.5.2.3 Levulinic acid
According to the National Renewable Energy Laboratory (Denver, CO,
United States), LA C 5 H 8 O 3 represents one of the 12 keys sugar-derived
platform chemicals that can be produced from biomass, in order to obtain
a series of biochemicals, including solvents, biofuels, food/flavoring/fra-
grance components, chemical intermediates, plasticizers, polymers,