Adaptive Evolution of Commercial Wine Strains for Reduced Ethanol Production
The goal of this project is to use the process of adaptive evolution to generate strains with reduced levels of production of ethanol. A corollary of this grant is to use this analysis to understand the basis of the appearance of increased ethanol yields evident in some commercial wine strains. The presence of external aldehydes was used as the driving force to evolve wine yeast towards less efficient conversion of sugar carbon to ethanol. 5-hydroxymethlyfurfural and furfural, two aldehydes normally present in the winery environment, were selected for this study. Strains were screened for initial resistance to HMF and furfural and were categorized as resistant, partially resistant to one or both compounds and sensitive to one or both compounds. Seven strains were selected for the adaptive evolution study and grown for 250 generations in the presence of aldehydes. Strains evolved in one of three directions: no change in ethanol yield per input carbon (one strain), slight to modest increases in ethanol level (five strains) and a reduction in ethanol yield (one strain). Aldehydic stress is common in the winery environment and these results offer credence to the belief that more tolerant robust strains develop the capacity for higher ethanol yields. The only strain that showed reduced ethanol yields was a vineyard, not winery, isolate that had no inherent tolerance to HMF. Thus it is possible to use adaptive evolution in the presence of aldehydic stress to generate strains with a reduced tendency to produce ethanol. However many wine strains have naturally evolved tolerances to aldehydes and this seems to correlate with increased not decreased ethanol production. Under natural conditions the genetic tendency appears to be in the direction of reduction of biosynthetic activities that consume reducing power in favor of the reduction of catabolic and environmental aldehydes.