Enhancement of ethanol production efficiency in repeated-batch fermentation from sweet sorghum stem juice: effect of initial sugar, nitrogen and aeration

BACKGROUND: Ethanol concentration (PE), ethanol productivity (QP) and sugar consumption (SC) are important values in industrial ethanol production. In this study, initial sugar and nitrogen (urea) concentrations in sweet sorghum stem juice (SSJ) were optimized for high PE (≥10%, v/v), QP, (≥2.5 g/L·h) and SC (≥90%) by Saccharomyces cerevisiae SSJKKU01. Then, repeated-batch fermentations under normal gravity (NG) and high gravity (HG) conditions were studied. RESULTS: The initial sugar at 208 g/L and urea at 2.75 g/L were the optimum values to meet the criteria. At the initial yeast cell concentration of ~1 × 108 cells/mL, the PE, QP and SC were 97.06 g/L, 3.24 g/L·h and 95.43%, respectively. Repeated-batch fermentations showed that the ethanol production efficiency of eight successive cycles with and without aeration were not significantly different when the initial sugar of cycles 2 to 8 was under NG conditions (~140 g/L). Positive effects of aeration were observed when the initial sugar from cycle 2 was under HG conditions (180­200 g/L). The PE and QP under no aeration were consecutively lower from cycle 1 to cycle 6. Additionally, aeration affected ergosterol formation in yeast cell membrane at high ethanol concentrations, whereas trehalose content under all conditions was not different. CONCLUSION: Initial sugar, sufficient nitrogen and appropriated aeration are necessary for promoting yeast growth and ethanol fermentation. The SSJ was successfully used as an ethanol production medium for a high level of ethanol production. Aeration was not essential for repeated-batch fermentation under NG conditions, but it was beneficial under HG conditions.

Influence of the accumulation of phosphate and magnesium ions in the yeast cells on the ethanol productivity in batch ethanol fermentation

The accumulation of phosphate and magnesium in the yeast cells is not necessary to assure the ethanol productivity of batch ethanol fermentations. To avoid the decrease of the ethanol productivity it was sufficient to use a fermentation medium containing calculated concentrations of phosphorus and magnesium sources in order to maintain practically constant the phosphorus and magnesium initial contents of the biomass during the fermentation.

O acúmulo de fosfato e magnésio nas células de levedura não é necessário para assegurar a produtividade em etanol de fermentações alcoólicas descontínuas. Para evitar a diminuição da produtividade em etanol, foi suficiente utilizar um meio de fermentação contendo concentrações calculadas das fontes de fósforo e de magnésio de modo a manter praticamente constantes, durante a fermentação, os teores iniciais de fósforo e magnésio da biomassa.