Ethanol production from cassava using a newly isolated thermotolerant yeast strain

Abstract

Efficient ethanol production using thermotolerant yeast strains was carried out with a newly isolated yeast strain called 267. Strains were isolated with an enriched technique consisting of a blackstrap molasses medium supplemented with 40 ml/l of ethanol at 25–28 °C. The results revealed that 33 strains produced ethanol at 45 °C in the cassava starch hydrolysate medium, pH 4.5, which was composed of 180 g/l reducing sugar and 0.5 g/l (NH4)2SO4, with a shaking speed of 120 rpm. The highest ethanol concentrations (26.2, 26.2, 23.6, and 22.5 g/l) were found from Pichia kudriavzevii strains PBB511-1, TM512-2, CPY514-1, and TG514-2, respectively. The yeast strain PBB511-1, which produced the highest ethanol yield, was selected for ethanol production in the shaking flask at 45 °C. Ethanol production reached the highest level after 36 h in a medium composed of 180 g/l reducing sugar, 0.5 g/l (NH4)2SO4, 0.5 g/l KH2PO4, 0.5 g/l MgSO4 · 7 H2O, and 1 g/l yeast extract. It produced 37 g/l ethanol, with a productivity of 1.03 (g/l)/h and a yield of 40% of the theoretical yield. The ethanol production by batch fermentation at 45 °C was performed in a 7-l jar fermenter with an agitation speed of 300 rpm and an aeration rate of 0.2 vvm throughout the fermentation. The results implied that the maximum ethanol concentration was 42.4 g/l after 48 h, at a rate of 0.88 (g/l)/h and a yield of 46% of the theoretical yield.