Icewine fermentation by Saccharomyces cervisiae : fundamental stress responses and comparative fermentation dynamics
Pigeau, Gary M.
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Icewine is a unique dessert wine fermented from the juice of grapes that have frozen naturally on the vine. Fermentations of 40 Brix (>400g/L soluble solids) juice typically used in Canada to produce Icewine are osmotically stressful, frequently sluggish, may take months to reach desired ethanol levels and are accompanied by high levels of glycerol and volatile acidity. Yeast placed under osmotic stress produce glycerol to restore cellular turgor and it has been suggested that acetate is produced to maintain redox balance. The primary objective of this project was to examine, at the molecular level, the cause of and metabolic pathway utilized by Saccharomyces cerevisiae wine yeast in the production of glycerol and acetic acid, the major contributor to volatile acidity. Northern analyses of glycerol-3-phosphate dehydrogenases (GPDl and GPD2) and aldehyde dehydrogenases (ALD2,ALD3,ALD4,ALD5 andALD6) were performed to ascertain their contributions to glycerol and acetate production respectively. It was determined that only GP Dl and ALD3 had transcriptional patterns which correlate to the elevated levels of glycerol and acetate found in Icewine. Acetaldehyde, a precursor to acetate, was found to stimulate only the ALD3 isoform, but increased expression of ALD3 alone does not cause increased acetate production. This indicates that upregulation of ALD3 is not sufficient to increase acetic acid in wine and requires additional responses found in cells lmder hyperosmotic stress. The secondary objective of this project was to provide the first comprehensive, descriptive analysis of the fermentation of Canadian Icewine. The effect of increasing juice soluble solids from 40 to 42, 44 and 46 Brix on wine yeast's ability to grow and ferment, with a focus on acetic acid production, titratable acidity changes and the maximum amount of sugar consumed by the yeast, was investigated. It appears from this study that yeast divert metabolic resources away from growth and towards combating osmotic stress since yeast growth is negatively correlated and glycerol and acetic acid production are positively correlated with Icewine juice concentration. Based on this work, it appears that the upper limit of Riesling Icewine juice concentration that can be fermented to produce an Icewine with 10% v/v ethanol is 42 Brix. These results are of importance to the commercial production of Icewines. The compositional profile of 346 Icewine juices collected in the Niagara Peninsula of southern Ontario is also reported. Malic acid was found to be a main contributor to juice titratable acidity, while the increase in titratable acidity of the finished wines can be accounted for by the acetic acid produced during fermentation. The large proportion of Icewine titratable acidity represented by acetic acid may require winemakers to further adjust the acidity of the wine to achieve the sugar to acid balance required in this wine style. The utilization of nitrogen sources during fermentation was also investigated to determine icewine juice induced osmotic stress affected nitrogen uptake.