Organic acid metabolism in Saccharomyces cerevisiae : genetic and metabolic regulation

[摘要] ENGLISH ABSTRACT: Organic acids are major contributors to the organoleptic properties of wine. Each acid indeedcontributes to the overall acidity of the product, which is an essential feature of wine quality. Inaddition, and an aspect that has been neglected in many evaluations in the past, each acid alsoimparts its own sensory characteristic to the wine. Changes in organic acid profiles therefore definerelevant sensory features of wine beyond the general perception of acidity.The main objective of this study was to investigate how different yeast strains and a number ofenvironmental factors (such as aeration, initial pH, temperature and sugar content) influence theorganic acid levels in fermenting musts at three critical physiological stages (exponential, earlystationary and late stationary phase). Five commercial wine yeast strains (VIN13, EC1118, BM45,285 and DV10) were selected and these strains were subjected to two widely differing fermentationconditions. The data showed significant variation in organic acid concentrations in the final productdepending on the yeast strain, and a more multifactorial experimental design was adopted toinvestigate the impact of environmental parameters. The impact on both grape-derived (tartaric,citric and malic acid) and fermentation-derived (succinic, acetic and pyruvic acid) acids wasevaluated. Condition-dependent shifts in the production of specific organic acids were observed.The multifactorial experimental design evaluated environmental parameters that can be at leastpartially controlled or managed in the cellar. The influence of individual and /or combinatorialfactors such as temperature, pH and sugar content of the must were also shown to affect acidprofiles of the synthetic wines.A further goal of this project was to identify genes that are involved in organic acid metabolism.Transcriptome data of the five yeast strains was analyzed in order to identify genes which showeddifferential expression between strains and/or time points paralleled by differences in organic acidsfor the same comparisons. A correlation model was constructed for genes identified in this mannerand model predictions were compared/aligned to observed changes in acid levels in response todeletion of the target genes. This approach provided some predictive capacity for modelling theimpact of target genes on acid levels. Although some predictions based on gene expression to acidcorrelations were not validated experimentally, the analysis as a whole provided new insights intoorganic acid evolution mechanisms of different strains at different stages of fermentation.Overall, the use of a multifactorial experimental design in the current study confirmed existingknowledge and sheds new light on factors which, either on their own or in combination with otherfactors, impact on individual organic acids in wine. As a practical outcome, the data can serve forthe development of guidelines for winemakers with regard to strain selection and management offermentation parameters in order to better control wine acidity and wine organic acid profiles.

[发布机构] Stellenbosch University