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UBC Theses and Dissertations

Production of acetic acid by Saccharomyces cerevisiae during icewine fermentations Erasmus, Daniel Jacobus


The metabolic pathways in S. cerevisiae during fermentation of Icewine grape must have not been studied and the transcriptional response of this yeast to sugar stress has not been investigated. Global transcriptional analyses showed that the transcription of 589 genes in S. cerevisiae was affected more than two-fold in grape juice containing 40% (w/v) sugars. High sugar stress up-regulated the glycolytic and pentose phosphate pathway genes. Gene expression profiles indicate that the oxidative and non-oxidative branches of the pentose phosphate pathway were up-regulated and might be used to shunt more glucose-6-phosphate and fructose-6-phosphate from the glycolytic pathway into the pentose phosphate pathway, respectively. Structural genes involved in the synthesis of acetic acid from acetaldehyde, and succinic acid from glutamate, were also up-regulated. Genes involved in the de novo biosynthesis of purines, pyrimidines, histidine and lysine were down-regulated by sugar stress. Wine yeast produce acetic acid as a by-product of the hyper osmotic stress response caused by high sugar concentrations (>35 % w/v) in grape must. Seven commercially available wine yeast strains were compared for Icewine production. Yeast strains were evaluated for acetic acid and glycerol formation, fermentation rates and sensory characteristics. Fermentation rates and acetic acid and glycerol formation were significantly different (p<0.05) and linked to the yeast strain. Icewines produced with the different yeast strains showed significant differences for overall quality, perceived sulphur-like aroma and colour (p<0.05). ST, N96 and EC1118 were identified as the most suitable yeast strains for the production of Icewine. S. cerevisiae strain ST produced the lowest amounts of acetic acid and glycerol, while strain VIN7 produced the highest amounts of acetic acid and glycerol. Global gene expression analysis revealed that genes involved in glycerol and acetic acid formation were expressed at higher levels in VIN7 than in ST. PCR analyses showed that approximately 30 kb on the left arm of chromosome XV, close to the telomere, is absent in VIN7. Among the genes absent is YOL159C, a gene whose deletion causes osmosensitivity and an increase in Tyl retrotranspositions, which may cause genetic instability.

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