Grape-derived polyfunctional thiols contribute positively to the aroma quality of wines, especially aromatic white wines. These thiols, such as 3-mercaptohexanol (3MH), 3-mercaptohexyl acetate (3MHA), and 4-mercapto-4-methylpentant-2-one (4MMP), have very low aroma thresholds (ng/L), but impart highly desirable aromas (e.g., grapefruit, passionfruit, blackcurrant) to finished wines. Recently, studies have shown that the application of urea and micronized sulfur to the foliar during growing season in Pennsylvania resulted in a significant increase (73-300%) in varietal polyfunctional thiol content in finished wines. Unfortunately, the repeated application of these treatments can be time-consuming and expensive. In the present study, liquid-chromatography with tandem mass spectroscopy was used to investigate the impact of additions of elemental sulfur (S₀), in combination with SO₂, and/or ascorbic acid to grape juice on the concentration of polyfunctional thiols in finished wines. This approach relies on the recently elucidated mechanism by which S₀ is biologically transformed (i.e., via yeast) to hydrogen sulfide during the primary fermentation, which is subsequently capable of reaction with juice-derived C6 alkenes (e.g., trans-2-hexanal, trans-2-hexanol, hexanol) via 1,4 Michael addition reactions to yield desirable polyfunctional thiols. These reactions have been shown to be effectively modulated through the addition of SO₂ (a multifunctional antioxidant capable of quenching thiols, scavenging hydrogen peroxide, inhibiting polyphenol oxidase, and quenching carbonyl-containing oxidation products) and ascorbic acid (a strong reducing agent and oxygen radical scavenger). We hope to determine if the S₀ addition technique is viable as a simple and cost-effective method of increasing the pool of desirable polyfunctional thiols in Pennsylvania wines.

Misha Kwasniewski

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