Very high gravity ethanol fermentation by the newly isolated osmotolerant Saccharomyces cerevisiae isolate G2-3-2

Abstract

The high yield ethanol fermenting osmotolerant yeast G2-3-2 was obtained after screening 147 ethanol fermenting yeasts that had been isolated from six sugar factories in Thailand. It was found that high capability of osmotic tolerance did not indicate high fermentation efficiency under that high osmotic condition. The yeast G2-3-2 was identified as Saccharomyces cerevisiae according to its colony and cell morphology, biochemistry, and sequence analysis of the variable D1/D2 region of the large subunit of the rRNA gene. Optimal conditions for ethanol production under a very high gravity (VHG) condition were: (i) inoculum grown in 150 g/L glucose containing medium; (ii) inoculation of late log phase cells to a final concentration of 1 × 109 cells/mL; (iii) key nutrient concentrations of (all (g/L)) glucose, 280; polypeptone, 5; yeast extract, 7.5; (NH4)2HPO4, 1; MgSO4.7H2O, 1.5; KH2PO4 3; and (iv) incubation at 30 °C, pH 5.0 under oxygen limitation for 120 h. Under these conditions the maximum ethanol produced obtained was 134.7 g/L ( 0.48 g ethanol/g glucose utilized) and the ethanol productivity was 1.12 g/L/h. Removal of carbon dioxide from, and the relief of oxygen-stress to, the optimized VHG fermentation decreased the maximum ethanol produced to 125.1 g/L (0.45 g/g glucose) and 122.5 g/L (0.44 g/g glucose), respectively, but increased the maximum ethanol productivity to 1.73 and 1.70 g/L/h, respectively.