Page 128 - Lignocellulosic Biomass to Liquid Biofuels

102 Lignocellulosic Biomass to Liquid Biofuels

cassette anion efflux pump Pdr12p to export the remaining anions out of
the cell. Moreover, S. cerevisiae seems to induce certain heat shock genes at
low PHi and in the presence of a weak acid as sorbic acid [250].
The main weak acids contained in lignocellulosic hydrolyzates are
acetic acid, FA, and LA. The formation of acetic acid (AcH) is primarily
due to the hemicellulose deacetylation, while FA and LA arise as acid-
catalyzed thermochemical degradation products from polysaccharides: FA
is a degradation product of furfural and HMF, while LA is obtained by
HMF breakdown [247]. The content of AcH, FA, and LA in hydrolyzates
or slurries is very variable and depends on the feedstock and the severity
of the pretreatment process. For example, feedstocks as agricultural resi-
dues and hardwood with a high percentage of acetylated xylan provide
greater concentrations of weak acids than softwood [247]. Differences in
toxicity between AcH, FA, and LA are correlated to differences in perme-
ability of the membrane, or in toxicity of the acids anionic form within
the cell [250].

3.5.2.1 Acetic acid
The pK a of AcH (CH 3 COOH) is 4.75 at 25°C; thus at low pH values of
the medium, it is present in the undissociated form able to diffuse across
the plasma membrane. Inside the cell, HS of AcH dissociates, discharging
protons and anions in the cytoplasm and inhibiting cell activity and
growth [264]. The AcH toxicity varies depending on the cultivation con-
ditions employed in the fermentative process; for this reason, it is neces-
sary to consider not only concentration of weak acid in the hydrolyzates
but also the oxygen concentration and the pH of the medium, and the
presence of other toxic compounds in the medium [264]. A research
work by van Zyl et al. [265], in which P. stipitis was used to obtain etha-
nol from sugarcane bagasse hemicellulosic hydrolyzate, reported a drop to
50% in the ethanol production at pH 6.5 and with AcH concentrations of
about 15 g/L. However, the same drop in ethanol concentration was
observed at pH 5.1 when the AcH concentration was only 1 g/L [265].
According to Felipe et al., AcH concentrations more than 3 g/L were
dangerous for the xylose fermentation to xylitol by Candida guilliermondii,
while ethanol production was stimulated in medium free of other toxic
compounds by AcH concentrations up to 10 g/L [266]. The weak acids
as benzoic and sorbic acid cannot be metabolized by S. cerevisiae, causing
membrane damage and severe oxidative stress under aerobic conditions.
On the contrary, S. cerevisiae cells are characterized by the ability to grow