Page 132 - Lignocellulosic Biomass to Liquid Biofuels

106 Lignocellulosic Biomass to Liquid Biofuels

suggesting that acetic acid and furfural interact antagonistically on cell
growth [250]. In the presence of mixtures of acetic acid, FA, LA, furfural,
and HMF in the fermentation medium, the ethanol yield decrease was
only slight compared to a reference fermentation [244], indicating that
these compounds inhibit cell growth more than ethanol formation [250].

3.5.5 Other inhibitor compounds

Other strong inhibitory compounds, generated from lignocellulose feed-
stocks and during pretreatment processes, include quinones and small ali-
phatic aldehydes [281]. The ethanol, generated during fermentation, also
shows a strong inhibitor effect on the viability, growth, glucose transport
systems, and proton fluxes of S. cerevisiae. Nevertheless, the concentrations
of ethanol tolerate from ethanologenic microbes S. cerevisiae and Z. mobilis
correspond to 18% and 12%, respectively [247].
The formation of dark-colored substances, known as humins, during
the thermal hydrolysis of glucose, has been reported relatively recently.
The humins can be formed from reaction of HMF with glucose and/or
via reaction of HMF with 2,5-dioxo-6-hydroxy-hexanal (hydrated HMF)
and subsequent polymerization. However, the humins impact on cellulo-
lytic enzymes and/or yeast during ethanol production remain at present
uncertain [247].

3.5.6 Strategies for minimizing inhibitor effects
Different strategies can be employed to counteract problems with fermen-
tation inhibitors.
1. Several natural varieties of feedstocks with low recalcitrance can be
identified and used for bioconversion of lignocellulose in biofuels. The
use of feedstocks with relatively low recalcitrance allows to perform
pretreatment under mild conditions and without any addition of acid
catalysts, in order to reduce the concentrations of furan aldehydes and
phenol in lignocellulose hydrolyzates [282]. The feedstock engineering
is another strategy to generate novel bioenergy crops with low recalci-
trance and then obtain a reduced release of inhibitors during pretreat-
ment processes.
2. Detoxification or conditioning of lignocellulosic hydrolyzates and slur-
ries is one of the most powerful ways to render strongly inhibitory
hydrolyzates as fermentable as reference fermentations with the same
amounts of sugar but no inhibitors [283]. This process involves