Isolation and Selection of Ethanol Tolerant Yeasts for the Production of Ethanol
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Date
2010
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Addis Ababa University
Abstract
Twenty yeasts colonies were isolated from Tela, Teji, Honey and teff dough selection for
ethanol tolerance on yeast extract peptone dextrose agar medium (YEPDA). All the isolates
were first tested for carbohydrate fermentation using Durham tube fermentation method in
yeast extract peptone dextrose broth using common fermentative carbohydrates. Ten isolates
which were relatively high fermentative in Durham tube fermentation method were selected
for testing of isolates for ethanol tolerance. Ethanol tolerance was tested using different
concentration made from 96 %( v/v) of absolute ethanol in yeast extract peptone dextrose
broth and their growth was determined by measuring optical density of the cells in broth
using spectrophotometer at 615nm. Two strains of isolates showed measurable growth in
medium containing above 14% (v/v) of ethanol were selected for further study. Strain TBY1
tolerates15.5% (v/v) ethanol and strain TGY2 tolerates 16% (v/v) of ethanol. Both isolates
were classified under genus saccharomyces based morphological appearance of vegetative
cell under microscope, ascospore production, ascospore character, colony character and
physiological testing of carbohydrates using baker yeast as reference. They were found to be
equally sugar tolerant having good growth in medium containing 24-28 % (w/v) sucrose.
Fermentation of sucrose was optimized with respect to temperature, pH and sugar
concentration. Results revealed that pH 5 and 28% sucrose concentration as optimum for
fermentation for the three yeasts. The optimum temperature for selected strains and reference
yeast was 300C. The maximum ethanol produced by TBY1and TGY2 from 28% of sucrose at
pH 5 and temperature of 30 0C were 11.3 and 11.5% (v/v) of ethanol respectively, and
9.6%(v/v)ethanol by the standard baker’s yeast. The amount of ethanol produced in the
fermentation broth was measured using ebulliometer. The biomass was determined after the
end of fermentation and the result showed it the biomass of more ethanol tolerant strain is
greater than the others.
Keywords/phrases: Baker yeast, biomass, Durham tube, ethanol, ethanol tolerance, optical
density Spectrophotometer, sugar tolerance
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Keywords
Baker, yeast, biomass, Durham tube, ethanol, ethanol tolerance, optical density Spectrophotometer, sugar tolerance