Page 133 - Lignocellulosic Biomass to Liquid Biofuels

Bioconversion of lignocellulosic biomass to bioethanol and biobutanol 107

different treatments that include (1) the use of chemical additives (e.g.,
alkali), reducing agents, and polymers; (2) enzymatic treatment; (3)
liquid liquid extraction; and (4) liquid solid extraction (e.g., ion-
exchange and treatment with activated carbon) [234]. The detoxifica-
tion is selective and targets inhibitors rather than fermentable sugars
[284]. However, many detoxification methods require more chemicals
and frequently a separate process step [237].
3. The bioabatement, or microbial treatment, is another potential
method to remove inhibitory compounds from lignocellulose hydroly-
zate, improving both fermentability and enzymatic hydrolysis. Nichols
et al. [285] reported the abatement of hydrolyzate by inoculation with
the C. ligniaria NRRL30616, an Ascomycete able to metabolize furfu-
ral and HMF, allowed subsequent yeast fermentation of cellulose to
ethanol [285].
4. An important approach to tackle the inhibition problem is applying
various culture schemes. The strategies as SSF, fed batch, and CBP are
being employed for SSF. In these methods the fermenting microor-
ganism also contributes to supply of enzyme. However, the costs
related to designing fermentation process, fermenting microorganism,
enzyme loading, inoculums size, and time of fermentation under the
inhibitory conditions have significant impact on the bioconversion
process [234].
5. Another possible approach for counteracting problems of inhibitors in
processing of lignocellulose is the screening of microorganisms col-
lected from natural or industrial environments with high resistance to
inhibitors. The strains of S. cerevisiae, highly resistant for aliphatic car-
boxylic acids and furans aldehydes, were isolated from the grape marc
in a winery [286]. An important drawback of this approach is the spe-
cific productivity of the microorganism: the selection of microorgan-
ism, indeed, ought to be made primarily on the basis of their specific
productivity and product yields.
6. Evolutionary engineering is a technique that permits to generate ethanol-
fermenting microorganisms with enhanced inhibitor resistance [287].
7. Genetic engineering can be used to obtain recombinant microorgan-
isms with enhanced resistance to hydrolyzates of lignocellulose [55].
Even if the various approaches proposed can be used to tackle the
inhibitors problems, the chemical detoxification is currently the most
effective way to achieve a fermentation level comparable with that of a
medium devoid of inhibitor compounds.