Technical Yearbook 2023

DECEMBER

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Factors that affect alcoholic fermentation By Charl Theron

The alcoholic fermentation of grape juice is affected by the yeast strain, sugar concentration, alcohol concentration, fermentation temperature, presence of oxygen, concentration of assimilable nitrogen, vitamins and minerals, survival factors, preservatives, carbon dioxide and volatile acidity. Yeast suppliers usually have a variety of yeast strains, which are selected to meet the conditions and needs of the cellar. Yeast strains The action of yeasts is partly determined by their genetic makeup, which depends on the species and strain. Yeast strains differ in their fermentation kinetics, nitrogen requirements, ethanol resistance, temperature tolerance, as well as glucose and fructose consumption. These differences are more evident under stress conditions, which are determined by the adaptability of yeast strains. Yeast strains are selected for commercialisation based on the aromas and flavours they produce or enhance in wines, their temperature and alcohol tolerances, their nutrient demands and formation of volatile acidity, their suitability to ferment in barrels and their sensitivity to sulphur dioxide (SO 2 ). The sugar concentration of the grape juice Yeasts are living organisms. Due to differences in genetic

makeup, there are limitations to what some of them can and cannot do. The amount of sugar in grape juice determines the alcohol formed during alcoholic fermentation. If the sugar concentration of the juice is very high, it can result in high alcohol concentrations, which can slow down the fermentation. As a rule, many yeasts struggle to ferment 26°B to dryness under winemaking conditions, but exceptions do occur. Glucose and fructose are the two main sugars found in grape juice. Yeasts differ in their preference to ferment glucose and fructose. Yeasts that prefer glucose are called glucophilic yeasts, and those that prefer fructose are called fructophilic yeasts. Saccharomyces cerevisiae strains are glucophilic yeasts, while Starmerella bacillaris strains can be fructophilic. Fructose tastes sweeter than glucose, and if the fermentation stops before all the sugars have been fermented, it can taste different depending on whether residual fructose or glucose is still in the wine. Fructose consumption can be promoted by good yeast nutrition management during fermentation. Alcohol Alcohol has an inhibitory influence on alcoholic fermentation, but yeast strains differ in alcohol sensitivity. The inhibitory effect increases with increasing alcohol concentration. However, alcohol concentrations of up to 17% can be obtained with specific yeast strains. Fermentation temperature The optimum temperature of yeasts is approximately 30°C. Temperatures lower and higher than this affect yeasts. When making white wine, fermentation is usually done at lower temperatures to limit the loss of aroma and flavour. If the temperature drops too low, the fermentation can become stuck. Fermentation of white wines in stainless steel tanks

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TECHNICAL YEARBOOK 2023