The bioconversion of crops and residues to
fuels and chemicals is receiving increased interest
due to the perceived need for the reduction of
consumption and importation of petroleum fuels.
Many of the biomass feedstocks contain significant
quantities of lignocellulose, which upon hydrolysis
yield sugars. A more complete utilization of
biomass can be achieved by controlled hydrolysis
of the cellulose and hemicellulose fractions and
bacterial fermentation of the pentose and hexose
sugars. Chemical hydrolysis, especially
acid hydrolysis, is one of a number of a viable
technologies being developed as biomass
conversion process. Dilute acids can be also used
for hydrolysis of agricultural residues. It consists
of the hydrolysis of starch, hemicellulose, cellulose
and lignin fractions. Sulfuric acid , hydrochloric
acid, HF or acetic acid are commonly
employed in acid hydrolysis . The acids release
protons that break the heterocyclic ether bonds
between the sugar monomers in the polymeric
chains formed by the starch, hemicellulose and the
cellulose. The breaking of these bonds releases
several compounds, mainly sugars such as xylose,
glucose and arabinose. Other compounds released
are oligomers, furfural and acetic acid .
Hydrolysis reactions of sugar polymers in a
dilute-acid medium are very complex. The mechanism
of the hydrolysis reaction includes: (i)
diffusion of protons through the wet lignocellulosic
matrix; (ii) protonation of the oxygen of a
heterocyclic ether bond between the sugar
monomers; (iii) breaking of the ether bond; (iv)
generation of a carbocation as intermediate; (v)
solvation of the carbocation with water; (vi)
regeneration of the proton with cogeneration of the
sugar monooligomer or polymer depending on the
position of the ether bond; (vii) diffusion of the
reaction products in the liquid phase if it is permit
for their form and size; (viii) restarting of the
second step.
After acid hydrolysis, the predominant
monosaccharides from lignocellulose are glucose
and xylose (about 40% glucose and 20% xylose
of dry matter). The most efficient microorganisms
for converting glucose into ethanol are
industrial yeast strains of Saccharomyces cerevisiae
and bacterial strains of Zymomonas mobilis, but none
of these are able to utilize xylose and arabinose.
Recently, recombinant strains of S. cerevisiae, Z.
mobilis, and Escherichia coli have received the
genes coding of enzymes for conversion of xylose
into ethanol. Cassava is an important source of
carbohydrate and cassava peel is the waste material
obtained from cassava starch production. However
cassava peel consisted of starch, cellulose,
hemicellulose and lignin is discarded. Cassava peels,
the main by- product from processing tuberous roots
of cassava for human consumption, could be used
to be the source of fermentable sugars for ethanol
production. The aim of this work was to investigate
the ethanol production from the solubilized
reducing sugars obtained by dilute- acid hydrolysis
of cassava peel using S. cerevisiae.