Impact of elemental sulfur (S0) residues in Sauvignon blanc juice on the formation of the varietal thiols 3-mercapto hexanol and 3-mercaptohexyl acetate

Abstract

Elemental sulfur is a fungicide used by grape growers to control the development of powdery mildew, caused by the fungus Erysiphe necator. This compound is effective, cheap and has a low toxicity with no withholding period recommended. However, high levels of S0 residues in the harvested grapes can lead to the formation of reductive sulfur compounds that can impart taints and faults to the wine. Hydrogen sulphide (H2S) is a very volatile and unpleasant sulfur compound which formation is connected to high residues of S0 in juice (10 – 100 mg/L). These residues can be minimized with pressing and clarification of the juice prior to fermentation, but may increase during prolonged maceration. At the same time, H2S can play a role on the formation of the important varietal thiols 3-mercapto hexanol (3MH) and 4-mercapto-4-methylpentan-2-one (4MMP) as the direct sulfur donor to E-2-hexenal or mesityl oxide, respectively. Sauvignon blanc juices from three different locations was obtained at a commercial winery in Marlborough, New Zealand. One sample (A) was collected from the receival bin and pressed to obtain 25 L of juice. Two other samples (B and C) were collected from the commercial pressing operation. The samples were cold settled, racked to glass bottles (700 mL of juice), and then 0, 2, 10 or 50 mg/L of a wettable elemental sulfur compound was added. The fermentation was carried out using Saccharomyces cerevisiae (EC1118) at 15°C. The juices showed quite different potential to produce 3MH and 3MHA, and without any added sulfur, juice A produced a high amount of 3MH (6,000 ng/L), while juices B and C showed signs of oxidation and little 3MH was formed (< 600 ng/L). The addition of 50 mg/L of elemental sulfur caused a 1.7-fold increment in 3MH for juice A. For juice B detectable levels of 3MH and 3MHA were only observed for the extreme addition of 50 mg/L S0, which led to a 20-fold increase in 3MH production for juice C. Even though the results showed a clear relation between S0 in juice and varietal thiols in wine, the deliberate increase in the fungicide use close to harvest needs to be carefully managed, as levels of unwanted reductive sulfur compounds including H2S, methanethiol and carbon disulfide in the final wine were found to increase with the higher elemental sulfur additions.