Simultaneous utilization of galactose and glucose by Saccharomyces cerevisiae mutant strain for ethanol production

Red algal biomass is a promising alternative feedstock for bioethanol production, due to several advantages including high carbohydrate content, growth rate, ethanol yield, and CO₂ fixation ability. However, it has been known that most yeast strains can not utilize galactose, the major sugar of red algae, as efficiently it can utilize glucose. The authors report a novel ethanogenic strain capable of fermenting galactose, Saccharomyces cerevisiae. This mutant yeast strain exhibited exceptional fermentative performance on galactose and a mixture of galactose and glucose. At 120g/L of initial galactose concentration, ethanol concentration reached 6.9% (v/v) within 36h with 88.3% of theoretical ethanol yield (0.51g ethanol/g galactose). The ethanol concentration and yield were higher than that for glucose at the same initial concentration. In a mixed sugar (galactose+glucose) condition, the existence of glucose retarded galactose utilization however, 120g/L of the mixed sugar was completely consumed within 60h at any galactose concentration. The critical inhibitory levels of formic acid, levulinic acid and 5-hydroxymethylfurfural (5-HMF) on ethanol fermentation were 0.5, 2.0, and 10.0g/L; respectively. From this result, the ethanol fermentation efficiency of the novel S. cerevisiae strain using the galactose base of red algae was superior to the fermentation efficiency when using the wild type strain, and the novel strain was found to have resistance to the major inhibitors generated during the saccharification process.