WINETECH Technical Yearbook 2020

were very similar and Aureobasidium pullulans and Cladosporium dominated with a small amount of Hanseniaspora. Aspergillus flavus and Penicillium species, which are both potential micotoxin producers, occurred in all cases in very low populations. During alcoholic fermentation, the population within each sulphur dioxide level was relatively stable with Saccharomyces cerevisiae dominating from the beginning until the end. Considerable differences however occurred between the different levels. Hanseniaspora species maintained a 20% share in the population during fermentation of the 0 mg/L sulphur dioxide level. This diminished as the sulphur dioxide levels increased, which can be attributed to sulphite sensitivity, because the final alcohol concentration at all three levels was similar. Non- Saccharomyces yeasts, which originate from the vineyard, including Hanseniaspora and Aureobasidium

pullulans , can promote grape cultivar aromas by producing enzymes which react with precursor compounds. Hanseniaspora can for example form compounds like acetoin (buttery) and ethyl acetate (fruity at low concentrations). Saccharomyces uvarum , which can contribute to wine quality through the formation of glycerol, also occurred in low populations. Various new commercial forms of it have been developed over recent years. This species produces low alcohol concentrations. As a result of climate changes, which result in grapes with higher sugar concentrations, it can possibly be utilised to limit such higher alcohol concentrations. Saccharomyces cerevisiae was the dominant species during the fermentation of all the treatments. Traditionally it was accepted that yeast which occur in the vineyard dominates spontaneous fermentations, without the addition of sulphur. Saccharomyces cerevisiae yeasts were however dominant, having the highest

cellar population which was built up over years. Each of the three different treatments had four dominant strains during the total alcoholic fermentation. One of them occurred in all three treatments. It was the strain, which was used in the cellar for inoculated fermentations. The domination of Saccharomyces cerevisiae strains already occurred in the beginning of the alcoholic fermentation. Seeing that different strains thereof can form flavour-active compounds like iso-butanol, acetaldehyde, n-propanol and isoamyl alcohol, the relatively high populations during fermentation are significant. Eighteen sensory characteristics were evaluated by the panel. Two of these, namely the citrus and pome fruit character, differed significantly. Both are associated with cool-climate wines like Pinot gris. Wines of the 0 mg/L treatment displayed the most citrus character, while the pome fruit character occurred the most

in the 0 and 20 mg/L treatments. The 40 mg/L treatment displayed the least of both characters. Significant other flavour differences were possibly limited, because new barrels were used for the research. The difference in the fruity character of the different treatments can possibly be attributed to the formation of different flavours by yeasts at low sulphur dioxide additions or the neutralisation thereof by sulphur dioxide at higher levels (Morgan

& Durall, 2020). REFERENCE

Morgan, S.C. & Durall, D.M. 2020. How sulphur dioxide addition at crush can impact wine yeasts, bacteria and sensory attributes. Wine Business Monthly , August 2020: 50-55.

For more information, contact Charl Theron at vinofino@mweb.co.za.

WINETECH TECHNICAL YEARBOOK 2020 177