Browsing by Author "October, Francois Marc"

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    Effect of yeasts and oenological parameters on acetaldehyde production during alcoholic fermentation of South African grape musts
    (Stellenbosch : Stellenbosch University, 2020-03) October, Francois Marc; Van Jaarsveld, Francois; Rohwer, Johann; Du Toit, Wessel J.; Stellenbosch University. Faculty of Science. Dept. of Biochemistry.
    ENGLISH ABSTRACT: Acetaldehyde plays a role in the rate of fermentation and the quality of wine. High levels of acetaldehyde in fermenting juice may result in sluggish/stuck fermentations, and in wine, it may impart undesirable aromas usually associated with oxidative aromas. Depending on its levels, acetaldehyde has an effect on yeast metabolism and can therefore impact alcoholic fermentation. The overall aim of this project was to investigate the effect of yeasts and oenological parameters on acetaldehyde production, to better understand the impact of acetaldehyde on alcoholic fermentation and wine sensorial composition. Ten commercial Saccharomyces cerevisiae strains and 10 non-commercial non-Saccharomyces yeasts were evaluated. These yeasts were screened in a laboratory trial for their acetaldehydeproducing ability during alcoholic fermentation, and resulted in the selection of a high-, medium- and low-acetaldehyde producing yeasts. The selected yeasts were the S. cerevisiae yeasts NT50 (high), NT116 (medium) and VIN13 (low); and the non- Saccharomyces yeasts Torulaspora delbrueckii (high), Candida guilliermondii (medium) and Candida valida (low). The above-mentioned selection of Saccharomyces yeasts was used individually for vinification of grape must, as well as in all possible permutations with the non- Saccharomyces yeasts, and resultant wines analysed chemically and evaluated sensorially. The initial sensory results showed noticeable differences between treatments, in terms of aroma and sweetness. Statistical evaluation of the data from the screening and cellar trials showed that yeast strain and time of fermentation have an impact on levels of acetaldehyde. The ability of the yeast strains to produce acetaldehyde was affected differently by fermentation temperature during the screening trial. Wines coinoculated with non-Saccharomyces cerevisiae yeasts have lower levels of acetaldehyde than wines only inoculated with Saccharomyces cerevisiae yeasts as observed in the cellar trial. Sulphur dioxide (SO2) has a very high affinity for acetaldehyde, therefore the impact of various concentrations of SO2 on the levels of acetaldehyde in fermenting must was monitored in a second cellar trial. The resulting effects on fermentation and final wine quality were monitored. Although it is known that SO2 impacts wine quality, it was also found that the varying levels of SO2 have a direct effect on the acetaldehyde levels produced during fermentation. During a separate fermentation trial (laboratory-scale), using three Saccharomyces cerevisiae yeast strains, the total enzyme activity of alcohol dehydrogenase (ADH) was monitored. The ADH activity showed a similar trend to acetaldehyde concentration, where high enzyme activity of the Saccharomyces cerevisiae yeasts correlated with high acetaldehyde levels. In summary, there were significant differences in acetaldehyde levels between yeast strains tested in this study and the levels were within acceptable ranges normally found in wines. Higher acetaldehyde levels were found in wines inoculated with S. cerevisiae, exposed to high SO2 levels, and fermented at higher temperatures. There was a direct correlation between total ADH activity and total acetaldehyde production of Saccharomyces cerevisiae yeasts. To ensure lower levels of acetaldehyde in wine, winemakers should preferably coinoculate with low ADH activity Saccharomyces cerevisiae and non-Saccharomyces yeast strains, at low fermentation temperatures, while ensuring low levels of SO2 before fermentation.