Glycerol overproduction by engineered Saccharomyces cerevisiae wine yeast strains leads to substantial changes in by-product formation and to a stimulation of fermentation rate in stationary phase

Six commercial wine yeast strains and three nonindustrial strains (two labo ratory strains and one haploid strain derived from a wine yeast strain) wer e engineered to produce large amounts of glycerol with a lower ethanol yiel d. Overexpression of the GPD1 gene, encoding a glycerol-3-phosphate dehydro genase, resulted in a 1.5- to 2.5-fold increase in glycerol production and a slight decrease in ethanol formation under conditions simulating wine fer mentation, All the strains overexpressing GPD1 produced a larger amount of succinate and acetate, with marked differences in the level of these compou nds between industrial and nonindustrial engineered strains. Acetoin and 2, 3-butanediol formation was enhanced with significant variation between stra ins and in relation to the level of glycerol produced. Wine strains overpro ducing glycerol at moderate levels (12 to 18 g/liter) reduced acetoin almos t completely to 2,3-butanediol. A lower biomass concentration was attained by GPD1-overexpressing strains, probably due to high acetaldehyde productio n during the growth phase, Despite the reduction in cell numbers, complete sugar exhaustion was achieved during fermentation in a sugar-rich medium. S urprisingly, the engineered wine yeast strains exhibited a significant incr ease in the fermentation rate in the stationary phase, which reduced the ti me of fermentation.