Starch-Degrading Activities of Bulgarian Yeast Isolates

Abstract

ABSTRACTTwenty yeast strains, able to degrade starch as a sole carbon source were isolated from plants and insects, inhabiting Bulgarian mountains, as well as from fermented foods. The probes were taken aseptically and the corresponding microflora was isolated as pure cultures in selective media. The strains' identification was based on classical taxonomy criteria, including cells and colony morphology, physiological requirements, carbohydrate utilization profiles and enzyme activities tests. The new isolates belong to genera Saccharomyces, Geotrichum, Candida, Metschnikowia, Clavispora, Wickerhamiella, Debaryomyces, Kloeckera and Rhodotorula. Aiming to find out strains with possible biotechnological application, the main fermentation products of our isolates were estimated. After aerobic cultivation in medium, containing 20 g/l glucose fourteen strains accumulated high amounts of ethanol; all formed some lactic acid and six strains—succinic acid. The starch-degrading activity was evaluated quantitatively by measuring the light absorption of the iodine-starch complex. The kinetic experiments, carried out using 10 and 20 g/l starch, showed that all strains were capable to degrade it at different levels. In view of the strains' vigorous growth in starch-containing media, the potential of the isolates to produce valuable chemicals from starch was studied.