Repeated-batch ethanol fermentation from sweet sorghum juice by free cells of Saccharomyces cerevisiae NP 01

Low-cost inoculum preparation (IP) media for ethanol production were developed. It was found that sweet sorghum juice (SSJ) containing 100 g l super(-1) of total sugar without nutrient supplement could be used as the low-cost IP medium instead of the typical IP medium or yeast extract malt extract (YM) medium. Ethanol production from the SSJ (total soluble solids of 24 degree Bx) by the inoculum of Saccharomyces cerevisiae NP 01 in batch and repeated-batch fermentations was then investigated. The fermentations were carried out under static condition in 500 ml air-locked Erlenmeyer flasks at 30 degree C and the initial yeast cell concentration was 1x10 super(8) cells ml super(-1). In the batch fermentation, the concentration (P), productivity (Q sub(p)) and yield (Y sub(p/S)) of ethanol were 110.09 plus or minus 0.81 g l super(-1), 2.29 plus or minus 0.01 g l super(-1) h super(-1) and 0.51 plus or minus 0.02, respectively. In the repeated-batch fermentation, the yeasts could be used at least eight successive batches without a marked decrease in ethanol production. The repeated-batch fermentation with fill and drain volume at 75% of the working volume gave higher Ethanol production efficiencies than those at 50% of the working volume in terms of total Ethanol production rate (g h super(-1)). The average P, Q sub(p) and Y sub(p/S) of the eight successive batches at 75% fill and drain volume in a 2 L bioreactor at the agitation rate of 100 rev min super(-1) were 93.30 plus or minus 9.44 g l super(-1), 1.21 plus or minus 0.43 g l super(-1) h super(-1) and 0.48 plus or minus 0.03, respectively.