Browsing by Author "Fairbairn, Samantha"

Now showing 1 - 3 of 3
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    Defining the chemical features of wine perception
    (Stellenbosch : Stellenbosch University, 2018-03) Fairbairn, Samantha; Bauer, Florian; Da Silva Ferreira, A. C.; Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology. Institute for Wine Biotechnology.
    ENGLISH ABSTRACT: All wines evoke a product recognition, regardless of quality and cultivar, but what is the origin of this feature? The prevalence of this wine concept suggests that its formation occurs independent of the varietal, and ageing-related aromas, and is therefore potentially a function of yeast metabolism. Yeast utilise the nutrients present in grape must to produce biomass, and metabolites which ultimately signify the conversion of grape juice to wine. Consequently, the nutrient composition is highly influential on the aromatic outcomes of alcoholic fermentation. Synthetic grape must is widely used to evaluate all facets of the fermentation process but there remains much to learn. In this study, the impact of two nutrients, namely, amino acids and anaerobic factors, were evaluated with regard to their impact on yeast growth and aroma production under fermentative conditions. This work also examines the extent to which yeast de novo metabolism, both primary and secondary metabolism, contributes to the formation of the wine-like feature. In a single amino acid context, a linear relationship was apparent between the amino acid concentration and the production of their associated volatile products. This relationship was evaluated in more complex amino acid mixtures and as expected, this linear relationship was lost. Nonetheless, a significant degree of responsiveness between the amino acid and its catabolites remained. The impact of sterol (plant or yeast derived) or unsaturated fatty acid treatments, individually, as well as in combinations, were compared for their contributions to biomass formation and aroma production. Sterols had a greater impact on biomass development, as the fermentations treated with only unsaturated fatty acids displayed a poorer response. Moreover, they differently impacted aroma production. The unsaturated fatty acid lowered the production of acetate esters, medium chain fatty acids and their esters, whereas sterol supplementation generally bolstered the production of all compounds measured. This work highlights the importance of anaerobic factor management during winemaking. Although these nutrients certainly impact wine aroma, this study also sought to examine the degree to which these nutrients contribute to wine (product) recognition. Using a novel fermentation-based approach, Saccharomyces cerevisiae converted a synthetic grape must into a wine-like product. These synthetic products underwent sensory evaluations to rate the product’s resemblance to wine as well as to describe the aroma. This sensory data was used as a decision-making tool to decide upon treatments to be studied in subsequent fermentations. Ultimately, a wine-like character was created by altering the anaerobic factor composition of a synthetic grape must. The use of this synthetic grape must would allow for the more meaningful sensory characterisation of these synthetic products, in addition to providing a wine-like matrix used to evaluate the sensory implications of wine odorants.
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    The impact of single amino acids on growth and volatile aroma production by Saccharomyces cerevisiae strains
    (Frontiers Media, 2017) Fairbairn, Samantha; McKinnon, Alexander; Musarurwa, Hannibal T.; Ferreira, Antonio C.; Bauer, Florian; Lanciotti, Rosalba
    Nitrogen availability and utilization by Saccharomyces cerevisiae significantly influence fermentation kinetics and the production of volatile compounds important for wine aroma. Amino acids are the most important nitrogen source and have been classified based on how well they support growth. This study evaluated the effect of single amino acids on growth kinetics and major volatile production of two phenotypically different commercial wine yeast strains in synthetic grape must. Four growth parameters, lag phase, maximum growth rate, total biomass formation and time to complete fermentation were evaluated. In contrast with previous findings, in fermentative conditions, phenylalanine and valine supported growth well and asparagine supported it poorly. The four parameters showed good correlations for most amino acid treatments, with some notable exceptions. Single amino acid treatments resulted in the predictable production of aromatic compounds, with a linear correlation between amino acid concentration and the concentration of aromatic compounds that are directly derived from these amino acids. With the increased complexity of nitrogen sources, linear correlations were lost and aroma production became unpredictable. However, even in complex medium minor changes in amino acid concentration continued to directly impact the formation of aromatic compounds, suggesting that the relative concentration of individual amino acids remains a predictor of aromatic outputs, independently of the complexity of metabolic interactions between carbon and nitrogen metabolism and between amino acid degradation and utilization pathways.
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    Stress, fermentation performance and aroma production by yeast
    (Stellenbosch : Stellenbosch University, 2012-03) Fairbairn, Samantha; Smith, Anita; Bauer, Florian; Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology. Institute for Wine Biotechnology.
    ENGLISH ABSTRACT: Yeast strains contend with numerous stresses during winemaking. An inability to perceive and initiate the physiological changes needed to adapt to stress, has been linked to slow or incomplete (residual sugar > 4 g/L) fermentations. Wine yeast strains differ in genotype; this is manifested as differences in their stress tolerance, and fermentation performance. The first goal of this study was to evaluate how the initial sugar (200 or 240 g/L) and nitrogen (50, 100, 250, or 400 mg/L) content, and the fermentation temperature (15°C or 20°C) affected the fermentation performance of 17 commercial wine yeast strains. Fermentation performance was evaluated based on the fermentation kinetics (lag phase, maximum fermentation rate and total weight loss by CO2 evolution), residual sugar content and yeast dry weight. The results demonstrate that the fermentation performances of commercial yeast cultures are significantly and differently affected by initial nitrogen and sugar levels, as well as the fermentation temperature. Additionally, excess nitrogen had a negative impact on the fermentation kinetics and sugar consumption. Nitrogen deficiency is a common cause of slow and incomplete fermentations, as it affects yeast growth and thus fermentation rates. Nitrogen supplements are routinely added at the onset of fermentation, reducing the risk of problematic fermentations. Therefore characterising the fermentative ability of a strain over a range of oenologically relevant conditions, could aid winemakers in selecting a yeast strain capable of fermenting a grape must (of known sugar and nitrogen levels) to completion at the desired fermentation temperature. Investigations on fermentation related stress generally focus on its influence on fermentation rate and sugar consumption. However, from a winemaking perspective, the strain’s ability to produce the desired volatile aroma compounds is equally important. Yet, literature provides little insight into the influence stress has on the volatile aroma profile; this is surprising as wine aroma is closely linked to wine quality and consumer liking. The final goal of this study was to evaluate changes to the volatile aroma profiles produced by five commercial yeast strains, in response to hyperosmotic and temperature stress. The concentrations of the aroma compounds were quantified using a gas chromatograph coupled to a flame ionization detector. The results show that hyperosmotic and temperature stress caused significant changes in the levels of a number of aroma compounds. Furthermore, the changes observed differed among the evaluated strains, as well as for the fermentation stress treatments studied. Future aims should be directed towards the potential application of yeast strain selection as a means to avoid problematic fermentations in grape must; in addition to the further characterisation of the relationship between stress and the resultant volatile aroma profile in wine.