Technical Yearbook 2024

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Yeast cell density and the effect on volatile thiol formation By Carien Coetzee

Introduction The use of pure yeast cultures (for example, active dried yeast) to perform alcoholic fermentation has become popular in the wine industry mainly due to its ease of use and the fact that it facilitates more reliable, predictable and timely wine fermentations. Pure yeast cultures can also impart specific organoleptic properties to a wine depending on the genetic makeup of the yeast in combination with the fermentation parameters such as fermentation temperature and the concentration and type of nutrition available to the fermenting cells.

One fermentation parameter often overlooked is the yeast inoculum size, as different yeast suppliers usually specify inoculum rates for their products. Following the manufacturer’s recommended rates usually ensures a viable cell population of more than 5 x 10 6 viable cells/mL, which is, in most cases, needed for timely and successful fermentations. Usually, the proper rehydration of 25 g active dry yeast per hectolitre of juice (25 g/hL) is required to achieve the desired viable cells/mL. This is based on the active dry yeast containing 2 x 10 10 cells/g. In practice, there are rarely intentional deviations from the

Abstract A recent study investigated the effect of varying the yeast population at the time of inoculation and how it affects the overall fermentation with a special focus on the formation and release of volatile thiols. Results showed that an increased yeast inoculum could result in wines with elevated concentrations of 3MH and 4MMP.

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TECHNICAL YEARBOOK 2024