Alterations in Yeast Species Composition of Uninoculated Wine Ferments by the Addition of Sulphur Dioxide

Uninoculated wine fermentations are conducted by a consortium of wine yeast and bacteria that establish themselves either from the grape surface or from the winery environment. Of the additives that are commonly used by winemakers, sulphur dioxide (SO2) represents the main antimicrobial preservative and its use can have drastic effects on the microbial composition of the fermentation. To investigate the effect of SO2 on the resident yeast community of uninoculated ferments, Chardonnay grape juice from 2018 and 2019 was treated with a variety of SO2 concentrations ranging up to 100 mg/L and was then allowed to undergo fermentation, with the yeast community structure being assessed via high-throughput meta-barcoding (phylotyping). While the addition of SO2 was shown to select against the presence of many species of non-Saccharomyces yeasts, there was a clear and increasing selection for the species Hanseniaspora osmophila as concentrations of SO2 rose above 40 mg/L in fermentations from both vintages. Chemical analysis of the wines resulting from these treatments showed significant increases in acetate esters, and specifically the desirable aroma compound 2-phenylethyl acetate, that accompanied the increase in abundance of H. osmophila. The ability to modulate the yeast community structure of an uninoculated ferment and the resulting chemical composition of the final wine, as demonstrated in this study, represents an important tool for winemakers to begin to be able to influence the organoleptic profile of uninoculated wines

Temporal Comparison of Microbial Community Structure in an Australian Winery

Most modern fermented foods and beverages are produced in fit-for-purpose facilities which are designed to ensure not only a reliable product, but also one safe for consumption. Despite careful hygiene, microorganisms can colonise these facilities and establish resident populations that can potentially contribute to the fermentation process. Although some microorganisms may not negatively affect the final product, spoilage microorganisms can be detrimental for quality, generating substantial economic losses. Here, amplicon-based phylotyping was used to map microbial communities within an Australian winery, before, during and after the 2020 vintage. Resident bacterial and yeast populations were shown to change over time, with both relative abundance and location within the winery varying according to sampling date. The bacterial family Micrococcaceae, and the genera Sphingomonas and Brevundimonas were the most abundant bacterial taxonomies, while Naganishia, Pyrenochaeta and Didymella were the most abundant fungal genera. Mapping the spatial distributions of the microbial populations identified the main locations that harboured these resident microorganisms, that include known wine spoilage yeasts and bacteria. Wine spoilage microorganisms, including the genefugura Lactobacillus, Acetobacter, Gluconobacter and Brettanomyces showed very low relative abundance and were found only in a couple of locations within the winery. Microbial populations detected in this facility were also compared to the resident microbiota identified in other fermented food facilities, revealing that microbial population structures may reflect the nature of the product created in each facility

Effects of Yeast Strain and Juice Nitrogen Status on Glutathione Utilisation during Fermentation of Model Media

Background and Aims. An OIV resolution provides guidelines on using glutathione as a prefermentation additive when the amount of yeast assimilable nitrogen (YAN) of a juice or must is adequate, to avoid the metabolism of glutathione by the yeast. The effect of YAN concentration on glutathione metabolism by yeast had not been determined. This study explored whether nitrogen management could be used to control glutathione consumption during fermentation. Methods and Results. An HPLC-UV method was developed to quantify reduced L-glutathione (GSH) and oxidised glutathione (GSSG) and used to monitor yeast GSH metabolism during alcoholic fermentation with two yeast strains (AWRI 1688 and AWRI 2861). The addition of GSH had no impact on the fermentation rate of the chemically defined medium, even in a limited YAN environment; however, a decrease in glutathione concentration occurred regardless of YAN concentration. The effect of GSH on volatile sulfur compound formation was yeast strain-dependent. Conclusions. Increasing the YAN status of a chemically defined medium led to a decrease in GSH consumption during fermentation, but the loss of GSH could not be prevented entirely, even with a low initial GSH concentration and high initial YAN. Significance of the Study. In the presence of higher concentrations of GSH during fermentation, there is a risk of forming undesirable fermentative sulfur compounds that are not mitigated through nitrogen supplementation. Thus, it seems unlikely that an argument could be made for the inclusion of GSH in relevant food standards codes as a wine additive especially if a lack of GSH metabolism was a criterion