Doctoral Degrees (Institute for Wine Biotechnology)

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Browsing Doctoral Degrees (Institute for Wine Biotechnology) by browse.metadata.advisor "Rossouw, D."

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    Organic acid metabolism in Saccharomyces cerevisiae : genetic and metabolic regulation
    (Stellenbosch : Stellenbosch University, 2016-03) Chidi, Boredi Silas; Bauer, Florian; Rossouw, D.; Stellenbosch University. Faculty of Agrisciences. Dept. of Viticulture and Oenology. Institute for Wine Biotechnology.
    ENGLISH ABSTRACT: Organic acids are major contributors to the organoleptic properties of wine. Each acid indeed contributes to the overall acidity of the product, which is an essential feature of wine quality. In addition, and an aspect that has been neglected in many evaluations in the past, each acid also imparts its own sensory characteristic to the wine. Changes in organic acid profiles therefore define relevant 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 of environmental factors (such as aeration, initial pH, temperature and sugar content) influence the organic acid levels in fermenting musts at three critical physiological stages (exponential, early stationary 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 fermentation conditions. The data showed significant variation in organic acid concentrations in the final product depending on the yeast strain, and a more multifactorial experimental design was adopted to investigate 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 was evaluated. Condition-dependent shifts in the production of specific organic acids were observed. The multifactorial experimental design evaluated environmental parameters that can be at least partially controlled or managed in the cellar. The influence of individual and /or combinatorial factors such as temperature, pH and sugar content of the must were also shown to affect acid profiles 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 showed differential expression between strains and/or time points paralleled by differences in organic acids for the same comparisons. A correlation model was constructed for genes identified in this manner and model predictions were compared/aligned to observed changes in acid levels in response to deletion of the target genes. This approach provided some predictive capacity for modelling the impact of target genes on acid levels. Although some predictions based on gene expression to acid correlations were not validated experimentally, the analysis as a whole provided new insights into organic acid evolution mechanisms of different strains at different stages of fermentation. Overall, the use of a multifactorial experimental design in the current study confirmed existing knowledge and sheds new light on factors which, either on their own or in combination with other factors, impact on individual organic acids in wine. As a practical outcome, the data can serve for the development of guidelines for winemakers with regard to strain selection and management of fermentation parameters in order to better control wine acidity and wine organic acid profiles.