Technical Yearbook 2023

pathway of thiol production is disputed, mainly because the C6 precursors are rapidly converted to less reactive compounds within a few hours after yeast inoculation (before H 2 S formation), likely rendering it unavailable for the addition reaction. However, the early production of H 2 S by the yeast, together with the decline in C6 compounds and the possible formation and release of volatile thiols during these initial stages of fermentation, has only recently been scientifically investigated by New Zealand researchers. In this study, the significance of the H 2 S-C6 pathway was tested by monitoring the initial stages of fermentation using a synthetic grape medium with known composition and various laboratory and commercial yeast strains. Materials and methods Laboratory scale fermentation was performed by inoculating 100 mL of synthetic grape medium (supplemented with 1.5 mg/L of hexenal) with 11 selected yeast strains. After 12 hours of incubation at 25°C and agitation, the fermentation was stopped by removing the yeast cells through centrifugation, and the remaining media (slightly fermented juice) was retrieved for testing. Results Natural H 2 S production by the yeast during the initial 12 hours Results showed the early production of H 2 S by the yeast, as well as significant variability in the H 2 S-producing potential of the selected yeast strains, with some yeast producing more H 2 S than others during these initial stages of fermentation. C6 decrease during the initial 12 hours The decrease of unsaturated hexenal and hexenol during the initial 12 hours after initiation of fermentation was monitored. Results showed that all the yeast strains tested could metabolise hexenal, and the concentration of the C6 compounds significantly decreased in the presence of yeast. The production of volatile thiols during the initial 12 hours Despite the natural formation of H 2 S by the yeast and the consumption of the C6 compound, no 3MH or 3MHA was detected during the initial 12 hours after the initiation of fermentation. It is unsure if the concentration of H 2 S formation under the conditions of the study was sufficient to initiate thiol production and/or if the amount of volatile thiols produced was too low to be able to be detected analytically. Other than that, the volatile thiol concentration

in the slightly fermented synthetic must was measured, therefore excluding intracellular volatile thiol content. It is possible that the yeast still needs to release the volatile thiols from the cell. The measurement of intracellular volatile thiols poses many challenges and might not be possible to conduct at this stage. The production of volatile thiols during the initial 12 hours when an exogenous source of H 2 S was provided The early addition of exogenous H 2 S (in the form of NaSH) resulted in the production of 3MH (but not 3MHA), suggesting that sufficient H 2 S concentrations must be present to obtain thiols through the H 2 S-C6 pathway. Conclusion Even though the yeast naturally produced H 2 S and rapidly consumed the C6 compounds during the early stages of fermentation, no volatile thiols were detected in the medium. It could be that the concentration of endogenously formed H 2 S is not high enough to contribute substantially to free volatile thiols in a synthetic grape medium, especially considering that the addition of NaSH (which provided an exogenous source of H 2 S) did lead to increases in 3MH. Other than that, it is unclear if the yeast would have started producing and excreting the thiols if the fermentation continued. Challenges regarding the measurement of low concentrations of thiols could also contribute to further difficulties in determining the significance of this mechanism in volatile thiol formation. Therefore, the contribution of the H 2 S-C6 mechanism to the final concentration of volatile thiols in wines remains unclear, and further work is needed. Abstract The study aimed to test the significance of the H 2 S-C6 pathway in yeast for volatile thiol production. The study found that even though the yeast naturally produced H 2 S and rapidly consumed the C6 compounds during the early stages of fermentation, no volatile thiols were detected in the medium. However, the addition of NaSH (which provided an exogenous source of H 2 S) led to measurable increases in 3MH. The contribution and significance of the H 2 S-C6 pathway in the formation of volatile thiols (when no exogenous source of H 2 S was provided) remains unclear and needs further investigation. Abbreviations 4MMP: 4-mercapto-4-methylpentan-2-one; 3MH: 3-mercaptohexan-1-ol; 3MHA: 3-mercaptohexyl acetate; hexenal: (E)-hex-2-enal; hexenol: (E)-hex-2-en-1-ol. 

References https://www.wineland.co.za/the-significance-of-the-h2s-c6-pathway-for-volatile-thiol-production/

For more information, contact Carien Coetzee at carien@basicwine.co.za.

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TECHNICAL YEARBOOK 2023