Selection of yeast stain and optimum conditions for scale-up ethanol production from sweet sorghum juice

Abstract

This research aims to select yeast strains giving high ethanol production efficiency from sweet sorghum juice as well as to study batch ethanol production under sterile and non-sterile conditions for industrial scale productions. When Saccharomyces cerevisiae SC90 and S. cerevisiae TISTR5048 were grown in sterile yeast extract malt extract (YM) medium and in sterile juice containing 15 ?Brix of total soluble solid (TSS), the maximum specific growth rates (?max) of the two strains in both media were not different. The ethanol production from the sterile juice containing 15, 18 and 24 ?Brix of TSS (corresponding to 130, 162 and 225 g/l of total sugar, respectively) by the two yeast strains was further carried out in a 500-ml air-locked flask at 30 ?C, 100 rev/min with the initial yeast cell concentration of 2?107 cells/ml. The results showed that the rates of sugar consumption and ethanol production of SC90 were higher than those of TISTR5048 at all conditions. Therefore, SC90 was selected for the ethanol production from the juice containing 15 ?Brix of TSS in a 2-litre stirred tank bioreactor. Three sterilization methods were tested as follows: (1) autoclave at 110?C 40 min, (2) no sterilization and (3) penicillin addition at 0.05 g/l. The results showed that the juice sterilized by method (1) gave the highest ethanol production efficiencies in terms of ethanol concentration (P, 60.87 ? 4.58 g/l), productivity (Qp, 1.27 ? 0.10 g/l.h) and yield (Yp/s, 0.49 ? 0.00), while the ethanol production efficiencies of using the juices sterilized by other two methods were decreased about 14 to 17 % compared to those of method (1). When the ethanol production from the sterile juice was scaled up in a 50-litre stirred tank bioreactor, P, Qp and Yp/s were similar to those in the 2-litre stirred tank bioreactor.