Selection of thermotolerant Saccharomyces cerevisiae for high temperature ethanol production from molasses and increasing ethanol production by strain improvement

A thermotolerant ethanol fermenting yeast strain is a key requirement for effective ethanol production at high temperature. This work aimed to select a thermotolerant yeast producing a high ethanol concentration from molasses and increasing its ethanol production by mutagenesis. Saccharomyces cerevisiae DMKU 3-S087 was selected from 168 ethanol producing strains because it produced the highest ethanol concentration from molasses at 40 degrees C. Optimization of molasses broth composition was performed by the response surface method using Box-Behnken design. In molasses broth containing optimal total fermentable sugars (TFS) of 200g/L and optimal (NH4)(2)SO4 of 1g/L, with an initial pH of 5.5 by shaking flask cultivation at 40 degrees C ethanol, productivity and yield were 58.4 +/- 0.24g/L, 1.39g/L/h and 0.29g/g, respectively. Batch fermentation in a 5L stirred-tank fermenter with 3L optimized molasses broth adjusted to an initial pH of 5.5 and fermentation controlled at 40 degrees C and 300rpm agitation resulted in 72.4g/L ethanol, 1.21g/L/h productivity and 0.36g/g yield at 60h. Strain DMKU 3-S087 improvement was performed by mutagenesis using ultraviolet radiation and ethyl methane sulfonate (EMS). Six EMS mutants produced higher ethanol (65.2 +/- 0.48-73.0 +/- 0.54g/L) in molasses broth containing 200g/L TFS and 1g/L (NH4)(2)SO4 by shake flask fermentation at 37 degrees C than the wild type (59.8 +/- 0.25g/L). Among these mutants, only mutant S087E100-265 produced higher ethanol (62.5 +/- 0.26g/L) than the wild type (59.5 +/- 0.02g/L) at 40 degrees C. In addition, mutant S087E100-265 showed better tolerance to high sugar concentration, furfural, hydroxymethylfurfural and acetic acid than the wild type.