ADDIS ABABA UNIVERSITY SCHOOL OF GRADUATE STUDIES INSTITUTE OF BIOTECHNOLOGY Characterization of Wild Yeasts Isolated from Selected Fruits for their Bread Leavening Capacity BY Eshet Lakew

Leavening agents are important in raising flour dough. Biological leavening agents are microorganisms that have the ability to produce carbon dioxide from the utilization of Sugar and thereby ferment and raise the dough. The present study was carried out to characterize yeast isolates isolated from selected fruits and to assess their leavening potential of wheat dough under laboratory scale. The collected fruit samples were processed to isolate yeasts using Potato Dextrose Agar (PDA) amended with 0.1 g/L chloramphenicol. Initially, 88 yeasts were isolated from the fruits and were first tested for their carbohydrate fermentation in yeast extract peptone dextrose (YEPD) broth medium. Six yeast isolates with their sugar fermentative abilities were selected and tested for H2S production. Among them, AAUGr5, AAUOr7 and AAUPi3 found not produce undesirable H2S for bread baking quality on both Kligler Iron Agar (KIA) and Bismuth Sulfite Agar (BSA) media. The three yeast isolates were identified as Saccharomyces using colonial, morphological parameters and biochemical tests. The optimum growth pH and temperature values for the three selected yeast isolates were recorded as 5 and 30 C, respectively, in YEPD medium. In addition, 30% (w/v) D-glucose and 5 %( w/v) NaCl concentrations showed optimum growth of the three selected yeast isolates in yeast extract peptone broth medium. In all the cases, the maximum biomass was achieved at 96 hrs of incubation and there was a rapid decrease in biomass for all the yeast isolates after 96 hrs of incubation. In terms of CO2 and biomass production as well as leavening potential, starter cultures which were formulated from the combination of the three yeast isolates (AAUGr5+AAUOr7+AAUPi3) showed better performance than starter cultures formulated from paired combination of the three isolates or each of the three isolates separately. However, isolate AAUGr5 was found to be satisfactorily potent for leavening action from the single isolates. The present study could therefore be important with respect to screening of wild yeast isolates that possess better bread leavening potential for extending the use of indigenous microbes as starter culture in bakery sector. Keywords/Phrases: biomass, carbon dioxide, fruits, hydrogen sulphide, laboratory scale leavening, yeast isolation