Page 96 - Lignocellulosic Biomass to Liquid Biofuels

72 Lignocellulosic Biomass to Liquid Biofuels

Lignocellulosic biomass mainly comprises cellulose, hemicellulose, and
lignin. The high amounts of sugars present in cellulose and hemicellulose
fractions can be hydrolyzed and converted into biofuels by a fermentation
process. In contrast the recalcitrant structure of lignin is difficult to disrupt,
and a pretreatment step is essential to improve its digestibility and increase
the release of fermentable sugars.
In most cases, pretreatment technologies based on physicochemical
processes involve high-energy demand, high-capital investment, some
sugar degradation, and generation of inhibitory compounds that affect the
downstream hydrolysis and fermentation steps. In contrast, biological
approaches are interesting to improve the efficiency of the bioconversion
processes and to overcome barriers in the scale-up and commercialization
of renewable biorefineries.
In this context, biodelignification is a promising technology, where
the so-called white rot Basidiomycetes (Ceriporiopsis subvermispora, Dichomitus
squalens, Pleurotus ostreatus, and Coriolus versicolor) are able to depolymerize
and mineralize lignin efficiently and extensively [20]. Some Ascomycetes—
such as Trichoderma reesei and Aspergillus terreus—can also colonize lignocel-
lulosic biomass. Apart from fungi, certain bacterial strains, such as Bacillus
macerans, Cellulomonas cartae, Cellulomonas uda, and Zymomonas mobilis, have
also shown delignification abilities yielding lignin degradation up to 50%.
The use of ligninolytic enzymes such as laccases instead of microorganism
populations is another feasible alternative for the delignification of ligno-
cellulose [21].
Biodetoxification reduces the amount of inhibitors produced after the
physicochemical pretreatment during lignocellulosic bioethanol produc-
tion. Fungi, such as T. reesei, Coniochaeta ligniaria, Amorphotheca resinae
ZN1, and Aspergillus nidulans FLZ10, have been studied for microbial
detoxification. Bacteria—such as the thermophilic bacterium Ureibacillus
thermophaercus, Methylobacterium extorquens, Pseudomonas sp., Flavobacterium
indologenes, Acinetobacter sp., and Arthrobacter aurescens—and yeasts—such as
Issatchenkia occidentalis CCTCC M 206097—have also been used for
detoxification purposes to a lesser extent. Besides microbial detoxification,
enzymatic detoxification with laccases and peroxidases is one of the main
biotechnological methods used to diminish the inhibitory compounds of
fermentation broths. Finally, coculture, evolutionary or genetic engineer-
ing modifications, cell retention, encapsulation, and flocculation have
been developed to increase the intrinsic tolerance or the inherent detoxifi-
cation capacity of some strains [21].