Improvement of ethanol production from starch by recombinant yeast through manipulation of environmental factors

The production of ethanol from starch has been investigated in a genetically modified Saccharomyces cerevisiae strain, YPB-G, which secretes a bifunctional fusion protein that contains both the Bacillus subtilis α-amylase and the Aspergillus awamori glucoamylase activities. The effects of a number of environmental factors on starch degradation, ethanol production, and plasmid stability have been assessed in batch culture. These include initial glucose supply, colony selection methodology prior to inoculation, and medium formulation. Cultures containing 40 g/l starch were observed to degrade starch effectively and produce higher amounts of ethanol in shorter periods. The provision of glucose in the growth medium during the early phases of fermentation resulted in faster growth and higher ethanol productivities. YE-Salts medium was found to support plasmid-containing cells throughout the whole fermentation; only 15% of the recombinant cells had lost the plasmid content by the end of the fermentation of 120 h. Fed-batch cultures produced high yields of ethanol on starch (0.46 g ethanol/g substrate) through the longer production period.