Technical Yearbook 2023

DECEMBER

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The significance of the H 2 S-C6 pathway for volatile thiol production The significance of the H 2 S-C6 pathway was tested by monitoring the initial stages of fermentation using a defined synthetic grape medium and various yeast strains.

By Carien Coetzee

Introduction Volatile thiols (3MH, 3MHA and 4MMP) are sulphur containing compounds responsible for potent tropical fruit aroma in especially Sauvignon blanc wine, where levels can reach far beyond the sensory perception threshold. In the grapes, 3MH, 3MHA and 4MMP are not present in their free aromatic form and the aromatic compounds are formed during fermentation. For many years, researchers have been trying to establish the source of volatile thiols and the mechanisms involved in the formation thereof. Conjugated precursors Certain non-aromatic, non-volatile conjugated precursors are present in the grape juice. The yeast transforms these precursors during alcoholic fermentation to release the free and aromatic volatile thiols from the conjugated compound. 3MHA does not have a conjugated precursor in the grape. It is formed during fermentation through the esterification

of 3MH with acetic acid, a reaction regulated by specific enzymes produced by the yeast. Although the conjugated precursor’s involvement in the formation of volatile thiols has been established, several studies determined that these precursors only account for a small fraction of the total amount of thiols present in wine. Moreover, no direct correlation between precursor concentration in the juice and the amount of free volatile thiols in the resulting wines has been observed. H 2 S-C6 pathway Another possible source of volatile thiols is the formation through a mechanism where a sulphur-containing compound reacts with a six-carbon (C6) compound to produce the volatile thiols. Previous studies have shown that early H 2 S production by the yeast in the presence of unsaturated C6 compounds (such as hexenal) induces direct yeast-mediated formation of volatile thiols. This specific

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TECHNICAL YEARBOOK 2023