WINETECH Technical Yearbook 2020

The reasons for sluggish or stuck fer­ mentat ions have histor ical ly been attributed to viticultural or cellar aspects. Viticultural aspects include crop size, sugar concentration of the grapes, yeast nutrient concentration and spray residues, while the choice of yeast strain, yeast rehydration process, fermentation temperature, juice clarity, nitrogen concentration of the juice, inhibiting compounds and physical factors are determined by cellar management. More recent research however, emphasise the importance of the glucose-fructose ratio of the juice. According to the historical follow-up of sluggish or stuck fermentations, the determining factor in 90 or even higher percentage of the cases where it occurred was the glucose-fructose ratio in the juice being less than 1. The fructose concentration was consequently higher than the glucose concentration of the juice. This is intelligible, because the general wine yeast, Saccharomyces cerevisiae , is a glucophilic yeast, which prefers glucose more than other sugars. Many people believed that Saccharomyces bayanus strains will solve the problem, which is actually wrong. By using DNA-technology for the taxonomic classification of yeasts, it was found that most yeast species or strains are a mixture of various strains with one or two dominant parents. This confirmed that yeast strains, which were generally

known as Saccharomyces bayanus strains in the wine industry, were actually wrongly identified. It was actually Saccharomyces cerevisiae strains of the “Pris de Mousse” family. This conclusion was supported by the apparent discovery of a Saccharomyces bayanus strain in a pathogenic isolation. Further research did however find that it was actually Saccharomyces eubayanus. Only a few of these strains, which are cold tolerant and non-fructophilic, were isolated and even less are commercially available. During alcoholic fermentation the break­ down rate of glucose is mostly faster, which leads to an increase of the fructose- glucose ratio. When this happens, it can lead to sluggish or stuck fermentations. Saccharomyces cerevisiae strains are mostly used in wine cellars, but strains differ regarding their fructophilic or glucophilic ability. As a result of the emphasis on especially fruit-driven wines with softer tannins, the alcohol concentration of wines increased over recent years. Alcohol concentrations of 14-14.5% per volume are common and concentrations of 15% also occur. More emphasis was consequently placed on the connection between the physiological ripeness and sugar concentration of grapes. In regions with a warmer climate, physiological ripeness usually implicates higher sugar concentrations. The influence of physiological ripeness is however more

Fructophilic yeasts and yeast strain choice

Dried pure culture yeast.

MARCH 2020

CHARL THERON: Private consultant KEYWORDS: Alcoholic fermentation, glucose-fructose ratio

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