Page 99 - Lignocellulosic Biomass to Liquid Biofuels

Bioconversion of lignocellulosic biomass to bioethanol and biobutanol 75

Ethanologenic thermophilic and thermotolerant microorganisms
[35,36] are of current interest due to their ability to ferment various
components of cellulosic biomass into chemicals and fuels [37]. Potential
thermophilic/thermotolerant ethanologenic bacteria include Aerobacter sp.,
Aeromonas hydrophila, B. macerans, B. polymyxa, Clostridium acetobutylicum,
Clostridium thermohydrosulfurium, Clostridium thermosaccharolyticum, Clostridium
thermosulfurogenes, Clostridium tetani, Erwinia sp., Escherichia sp., Geobacillus
sp., Klebsiella pneumoniae, K. marxianus, Lactobacillus sp., Leuconostoc sp.,
Pichia sp., Thermoanaerobacter ethanolicus, Thermoanaerobacterium saccharolyticum,
and Z. mobilis [29].
Yeast S. cerevisiae is more robust than bacteria and other yeasts, since
it presents higher tolerance to ethanol and inhibitors present in hydroly-
zates and higher efficiencies of sugar conversion into ethanol. S. cerevi-
siae is widely used in economically feasible biomass-to-ethanol
fermentation processes, as it exhibits fast sugar consumption, high yields,
and ethanol tolerance. In addition, S. cerevisiae displays several advanta-
geous characteristics, including its inherent resistance to low pH, high
temperature, and various inhibitors, for which it is used in industrial
applications [38]. S. cerevisiae has the potential to ferment ethanol from
various types of cellulosic biomass and is able to ferment hexoses rapidly
and efficiently. In contrast, it cannot naturally ferment pentose
sugars such as D-xylose and L-arabinose contained in the hemicellulosic
fraction and use these sugars for the growth. The bioconversion of
pentoses to ethanol is still one of the major bottlenecks for ethanol
commercialization effort.
Xylose-fermenting natural strains of bacteria (Clostridium sp., B. macer-
ans, Clostridium saccharolyticum, and T. ethanolicus) and yeasts (Pichia stipitis,
Pachysolen tannophilus, Candida tropicalis [39], Candida shehatae, and K. marx-
ianus) can be used as biocatalysts to convert xylose or cellulose into
ethanol. The disadvantages of using bacteria in large-scale fermentation
are the low ethanol yields, by-product formation, intolerance to high
ethanol concentrations, and growth at narrow and neutral pHs varying
from 6.0 to 8.0 [20,40].
Many fungi and yeasts can aerobically assimilate L-arabinose, but most
of them are unable to ferment it to ethanol or they exhibit only very low
ethanol production rates and yields.
Nevertheless, a lack of microorganisms that will efficiently convert
hexoses and pentoses to ethanol is a major constraint to the economical
conversion of biomass.