Page 98 - Lignocellulosic Biomass to Liquid Biofuels

74 Lignocellulosic Biomass to Liquid Biofuels

Hemicellulases are frequently classified according to their action on
distinct substrates and are isolated from a number of bacteria. Commonly
occurring hemicelluloses are xylans, arabinoxylan, glucomannan, galacto-
glucomann, and so on. In xylan degradation, endo-1,4-β-xylanase,
β-xylosidase, α-glucuronidase, α-L-arabinofuranosidase, and acetylxylan
esterase act on different heteropolymers, while during glucomannan deg-
radation, β-mannanase and β-mannosidase cleave the polymer backbone
[30]. Nevertheless, the knowledge and understanding of hemicellulose
systems is far less as compared to the cellulase systems in the thermo-
philic/thermotolerant bacteria.
Indeed, existing enzymatic hydrolysis technologies—carried out at tem-
peratures lower than 50°C—exhibit slow reaction rates, require high dosages
of enzymes, generate low yields of sugars from lignocellulose and are prone
to microbial contamination problems. Therefore thermostable enzymes have
been investigated to overcome the limitations of existing lignocellulosic bio-
mass conversion processes. Among thermostable enzymes, Bhalla et al. [31]
focused on the following groups:
• Cellulases, from thermophilic and mesophilic fungal genera (belonging
to the Aspergillus, Chaetomium thermophile, C. ligniaria Rhizopus, Sclerotium,
Sporotrichum thermophile, Thermoascus thermophile var. coprophile,and
Trichoderma), thermophilic bacteria (belonging to the genera Acidothermus,
Bacillus, Caldibacillus, Caldocellum, Clostridium,and Geobacillus), and hyper-
thermophilic microorganisms (Anaerocellum, Caldicellulosiruptor obsidiansis
sp. nov., Rhodothermus, Thermotoga, and archaea Pyrococcus and Sulfolobus)
• Xylanases, from thermophilic and hyperthermophilic bacteria
(Acidothermus, Actinomadura, Alicyclobacillus, Anoxybacillus, Bacillus,
Cellulomonas, Enterobacter, Geobacillus, Nesterenkonia, Paenibacillus,
Thermoanaerobacterium, and Thermotoga) and fungi (Laetiporus sulphureus,
Nonomuraea flexuosa, Rhizomucor miehei, Talaromyces thermophiles,
Thermoascus aurantiacus, Thermomyces lanuginosus)

3.2.1.2 Fermentation
Hydrolyzed substrate can be fermented to ethanol by different microor-
ganisms [32], major advantages and drawbacks of which were reported in
literature [33]. The search of robust microorganisms is essential to design
sustainable processes of second-generation bioethanol. A number of
S. cerevisiae and Kluyveromyces marxianus strains isolated from industrial
environments and laboratory background strains were investigated [34].