Physiological Features of Saccharomyces cerevisiae and Alternative Wine Yeast Species in Relation to Alcohol Level Reduction in Wine

Resumen

One of the major problems of the wine industry in warm climate countries is the increasing alcohol content in wines, experienced during the last decades; which is the result of increasing sugar content in grapes at harvest time. This problem is mainly related with global climate change, but it is also connected to the changing preferences of consumers for full-bodied wines and strong aroma. However, due to health and road safety considerations, as well as to tax policies in some importing countries, the market is also demanding for wines with lower ethanol content. There are many points in the vine growing and winemaking workflow that can be targeted to reduce the alcohol content of the final wine. In this thesis I focused on the fermentation step, in which sugars are converted into ethanol by the activity of yeasts, mainly Saccharomyces cerevisiae. Previous work in this research group centered on respiration as the most promising yeast metabolic pathway that would have to be increased in order to divert carbon flow from ethanol production. Considering the Crabtree features of this yeast species, the use of non-Saccharomyces species was required. One major problem found to implement this approach at the industrial level was acetic acid production by S. cerevisiae, which is greatly enhanced in the presence of oxygen. Since this species is usually present in grape must, even in the absence of inoculation, and tend to dominate after some hours of fermentation, providing oxygen to natural grape juice brings about the risk of excess volatile acidity of the final wines, which would preclude commercialization despite any eventual success concerning alcohol reduction. In order to advance in the development of efficient yeast strains and fermentation procedures aiming to alcohol reduction in wine, while avoiding the drawbacks related with acetic acid production, my PhD work targeted both S. cerevisiae and non-Saccharomyces species. The focus of my work on non-Saccharomyces yeast strains was on understanding the physiology of aerobic growth on grape must for these species, including factors that affect alcohol and acetate yields, and the impact of these growth conditions at the transcriptome level. In the case of S. cerevisiae, I tried to understand by a combination of computational biology and genetic engineering approaches the genetic determinants of excess acetate production when cultures are aerated. The results obtained indicate that environmental factors that can be easily manipulated during wine fermentation have a huge impact on the yields of acetic acid and alcohol for all yeast species tested. In addition, I was able to identify several genes whose deletion results in reducing the problem of acetic acid production by aerated cultures of S. cerevisiae. These results will serve to guide the development of fermentation procedures using some non-Saccharomyces species, aiming to alcohol level reduction by respiration. On the other side, the information is serving to develop non-GMO S. cerevisiae derivatives that are improved for acetic acid production (reduced yield) and can be combined with non-Saccharomyces yeasts during the aerated step or used as pure cultures for alcohol level reduction.