The ability of Saccharomyces cerevisiae to ferment sweet-stalk sorghum juice depend on the strains of the yeast and sweet sorghum cultivars used in the fermentation process. This research was aimed to evaluate the ethanol production capability of eight yeast strains using sweet-stalk sorghum juice from three sweet sorghum cultivars as a source.
Yeast strains used for ethanol fermentation were OUT 7009, 7027, 7096, 7055, 7921, 7080, and 7903. Sweet-stalk sorghum juice extracted from three cultivars of sweet sorghum (FS501, FS902 and KCS105) with addition of 4,60 g/l (NH4)2SO4 and 0,45 g/l KH2P04 were used to observe the growth and fermentation ability of the yeast strains in batch culture. Yeast extract peptone dextrose (YEPD) broth were used as a standard medium for aerobic growth of the yeast strains. The sugar brix content of the sweet-stalk sorghum juice were measured using refractometer (ASONE, Spittz IPR-101α) and the sugar profile were evaluated by HPLC. Yeast specific growth rates were determined by observing the growth curve of the yeast strains in aerobic condition and measuring the µ value at the exponential phase. The fermentation ability of the yeast strains determined by measuring the final ethanol concentration, ethanol yield and sugar conversions efficiency (SCE).
The highest specific growth rate of the yeast strain grown aerobically were achieved by OUT7027 yeast strain grown in FS902 medium (1,053 h-1), followed by OUT7913 yeast strain in the KCS105 (0,956 h-1) then OUT7903 yeast strain in the FS501 medium (0,955 h-1) and YEPD broth (0,730 h-1). However, the highest specific CO2 evolution rate of the yeast strain during fermentation of the juice was achieved by OUT7027 yeast strain in the FS902 medium (0,525 h-1), OUT7055 yeast strain in the FS501 medium (0,477 h-1), and OUT7096 yeast strain in the KCS105 medium (0,469 h-1). The highest ethanol concentration, ethanol yield, and sugar conversions efficiency (SCE) were obtained by strain OUT7921 when it was employed to ferment sweet-stem sorghum juice of FS902 cultivar. It was also observed that the juice extracted from sweet stalk sorghum of FS902 cultivar is the most suitable medium for all yeast strains to achieve their highest fermentation abilities.