Potential Hexose Fermenting Yeast for Conversion of Sugary and Starchy Raw Materials into Ethanol

Abstract

Nineteen yeast isolates retrieved from molasses and grapes were studied for tolerance to temperature, ethanol and sugar along with optimization of fermentation parameter and ethanol production using different carbon sources. All the isolates were able to grow up to 40 °C temperature. Total 11 isolates, i.e., M1, M5, M6, M7, M8, M11, M12, M15, M18, M19 and G1 tolerated 18–20% (w/v) glucose, while isolates M1, M2, M5, M12, M15 and M20 were able to tolerate 12–15% (v/v) ethanol. Maximum 1.72 g/100 g_substrate/h ethanol productivity, using 15% (w/v) glucose at 30 °C, was obtained in case of isolate M12 followed by 1.37, 1.29, 1.25 and 1.20g/100 g_substrate/h by isolates M3, M6, M5 and M15, respectively. Total eight promising isolates were selected for optimization of fermentation parameters. The temperature shift from 30 to 35 °C, increased ethanol production in case of all the isolates by a factor ranging from minimum 8.85% in isolate M3 to maximum 26.22% in isolate M7. Isolate M15 showed 8.43% (v/v) ethanol at 35 °C as compared to 7.25% (v/v) at 30 °C. Maximum ethanol production was achieved at pH 4.8 in all the promising isolates which was decreasing with increasing pH from 4.8 to 5.5 and above. Isolate M20 produced 9.30% ethanol with molasses as compared to 8.1% (v/v) achieved using pure glucose as carbon source under optimized conditions. Maximum 10.72% and 9.65% (v/v) ethanol were produced by isolate M20 using hydrolysates of rice and wheat, respectively. Appearance of 101.51% fermentation efficiency during bioconversion of rice hydrolysate into ethanol by isolate M20 clearly indicated the occurrence of simultaneous saccharification and fermentation.