The Origin and Control of Hydrogen Sulfide during Fermentation of Grape Must

¹ Oude Meester Group, Limited, R.I.T. Centre, P.O. Box 456, Stellenbosch, 7600, Republic of South Africa.

Grapes of five white cultivars were crushed to provide a total of 104 samples of five cultivars. After centrifugation, several chemical analyses were performed on musts including an assessment of hydrogen sulfide (H₂S) development during fermentation. Statistical evaluation established significant correlations (0.001 level) of: free amino nitrogen (FAN) with H₂S and soluble solids; protein nitrogen with titratable acidity, turbidity, total nitrogen, and nonprotein nitrogen; ammonia nitrogen with soluble solids, titratable acidity, and pH; nonprotein nitrogen with turbidity and total nitrogen; and, finally, pH with titratable acidity. Considered the most important of these correlations was that between FAN and H₂S. Subsequent fermentations, using nonfoaming yeast and protease, were each characterized by several distinct consequences including the stimulation of H₂S. These experiments demonstrated that actively fermenting yeasts are capable of extracellular proteolytic activity and that the degradation of must proteins is implicated in the formation of H₂S.

Clarification of settled musts by centrifugation had a small but marked effect. Bentonite treatment and removal of the bentonite-protein sediment before fermentation substantially decreased the development of H₂S but adversely affected fermentation activity. By contrast, fermentations conducted in contact with bentonite stimulated both H₂S development and fermentation rates, as did the pasteurization of must. Enhancement of the assimilable nitrogen content of musts resulted in the most effective control of H₂S. Fermentation rates were accelerated, and, in some instances, suppression of H₂S was complete.

It thus appears that the formation of H₂S occurs indirectly as a result of the nitrogen demand of yeasts and not because of their sulfur requirements as such.

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