Page 140 - Advances in Eco-Fuels for a Sustainable Environment

Prospective ecofuel feedstocks for sustainable production 111

The production of ethanol from sugar beet is expensive when compared to sugar cane
because it requires greater chemical and energy input. Sweet sorghum has a significant
sucrose content and is not widely grown. The sugar is stored in the main stalk in sweet
sorghum and is recovered by pressing the stalk. An average of 20gal of ethanol can be
obtained per tonnes of stalk [78]. The most widely employed microorganism for
fermenting sugars to ethanol is Saccharomyces cerevisiae. Aeration is required during
the fermentation process carried out by S. cerevisiae. The other yeast that can be
employed for ethanol fermentation is Schizosaccharomyces pombe. Among bacteria,
Zygomonas mobilis is the most promising microorganism. It has low energy efficiency
and yields ethanol in high quantity [78].

4.3.2 Bioethanol from starch
For ethanol production, starch is a high yield feedstock but to produce ethanol by fer-
mentation, starch has to be hydrolyzed. Initially, starch was hydrolyzed using acids
but later was replaced by an enzyme (α-amylase) due to its specificity, inherent mild
reaction conditions, and absence of secondary reactions. Thermoresistant bacteria
such as Bacillus licheniformis or engineered Escherichia coli or Bacillus subtilis
forms the source of α-amylase for starch hydrolysis. Some of the starchy feedstocks
used in bioethanol production are corn, wheat, cassava, rye, barley, sorghum, and trit-
icale [78]. Corn, wheat, and barley are the main grains providing starch. Starch is a
polymer that is mainly made up of a long chain of glucose units containing
1000–6000 monomeric units or more per amylopectin structure. In North America
and Europe, this type of feedstock is widely utilized for bioethanol production, espe-
cially corn and wheat. Starchy crops such as tubers are used for fuel ethanol produc-
tion in tropical countries [77].

4.3.3 Bioethanol from lignocelluloses

Lignocellulosics are the most abundant biopolymer on earth, and can also be used as
feedstock for the production of bioethanol. About 50% of the world biomass is
believed to be comprised of lignocelluloses. For bioethanol production, the lignocel-
lulosic materials can be divided into six groups: crop residues (cane bagasse, wheat
straw, corn stover, rice straw, barley straw, rice hulls, sweet sorghum bagasse, olive
stones, and pulp), hardwood (aspen, poplar), softwood (pine, spruce), cellulose waste
(newsprint, recycled paper, waste paper, sludge), municipal solid waste (MSW), and
herbaceous biomass (alfalfa hay, switchgrass, timothy grass, reed canary grass,
coastal Bermuda grass). The bioethanol production from lignocellulosics requires pre-
treatment followed by fermentation [78]. The main components of lignocellulosics
feedstocks are cellulose, hemicellulose, lignin, and solvent extractives. The structural
rigidity of the cellulosic feedstock is provided by lignin. β-D-Glucopyranose is the
monomeric unit in cellulose. Hemicellulose is a polysaccharide that is less complex
and easily hydrolyzed, containing both C5 (xylose, arabinose and rhamnose) and C6
(glucose, mannose and galactose) sugars. The key factor affecting the efficiency of
biofuel production during the conversion process is the chemical composition of