Isolation And Selection Of Ethanol Tolerant Yeasts For The Production Of Ethanol

Twenty yeasts colonies were isolated from Tela, Teji, Honey and teff dough selection forrnethanol tolerance on yeast extract peptone dextrose agar medium (YEPDA). All the isolatesrnwere first tested for carbohydrate fermentation using Durham tube fermentation method inrnyeast extract peptone dextrose broth using common fermentative carbohydrates. Ten isolatesrnwhich were relatively high fermentative in Durham tube fermentation method were selectedrnfor testing of isolates for ethanol tolerance. Ethanol tolerance was tested using differentrnconcentration made from 96 %( v/v) of absolute ethanol in yeast extract peptone dextrosernbroth and their growth was determined by measuring optical density of the cells in brothrnusing spectrophotometer at 615nm. Two strains of isolates showed measurable growth inrnmedium containing above 14% (v/v) of ethanol were selected for further study. Strain TBY1rntolerates15.5% (v/v) ethanol and strain TGY2 tolerates 16% (v/v) of ethanol. Both isolatesrnwere classified under genus saccharomyces based morphological appearance of vegetativerncell under microscope, ascospore production, ascospore character, colony character andrnphysiological testing of carbohydrates using baker yeast as reference. They were found to bernequally sugar tolerant having good growth in medium containing 24-28 % (w/v) sucrose.rnFermentation of sucrose was optimized with respect to temperature, pH and sugarrnconcentration. Results revealed that pH 5 and 28% sucrose concentration as optimum forrnfermentation for the three yeasts. The optimum temperature for selected strains and referencernyeast was 300C. The maximum ethanol produced by TBY1and TGY2 from 28% of sucrose atrnpH 5 and temperature of 30 0C were 11.3 and 11.5% (v/v) of ethanol respectively, andrn9.6%(v/v)ethanol by the standard baker’s yeast. The amount of ethanol produced in thernfermentation broth was measured using ebulliometer. The biomass was determined after thernend of fermentation and the result showed it the biomass of more ethanol tolerant strain isrngreater than the others.rnKeywords/phrases: Baker yeast, biomass, Durham tube, ethanol, ethanol tolerance, opticalrndensity Spectrophotometer, sugar tolerance

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