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Low-cost inoculum preparation (IP) media for ethanol production were developed. It was found that sweet sorghum juice (SSJ) containing 100 g l-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 °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°C and the initial yeast cell concentration was 1×108 cells ml-1. In the batch fermentation, the concentration (P), productivity (Qp) and yield (Yp/s) of ethanol were 110.09 ± 0.81 g l-1, 2.29 ± 0.01 g l-1 h-1 and 0.51 ± 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-1). The average P, Qp and Yp/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-1 were 93.30 ± 9.44 g l-1, 1.21 ± 0.43 g l-1 h-1 and 0.48 ± 0.03, respectively.
Key words: Repeated-batch, ethanol fermentation, low-cost nutrient, sweet sorghum juice, Saccharomyces cerevisiae.