WINETECH Technical Yearbook 2020

Volatile sulphur compounds

The goal of this study was to set up a method for the determination of reductive sulphur compounds in order to offer an additional sensitive and reliable tool for investigations. This will allow not only the usual positive aromas to be evaluated (esters, higher alcohols, terpenes and thiols), but also some of the negative ones in order to obtain a more complex and complete picture of the phenomena occurring from the winemaking stages up to the consumption of the wine. INTRODUCTION The possible sources of reductive sulphur compounds (RSC) are numerous and varied and can occur at several stages during the winemaking process or storage of bottled wine (Kinzurik et al ., 2015). The relationships between the factors that affect the formation of RSC are complex and worth investigating. From an analytical point of view, a limited number of methods is equivalent to limited knowledge of the subject matter. RSC in wine are divided into ‘light’ (boiling point [b.p.] <90°C) and ‘heavy’ (b.p. >90°C) compounds, indicating the difficulty of using a relevant common sampling and enrichment technique (Nguyen et al ., 2007; Butzke & Park, 2011). The additional difficulties to overcome are related to the required sensitivity of the analytical technique and to the instability and differences in volatility of RSC.

Several analytical approaches have been employed to quantify sulphur (S) volatiles in wine. Based on the state-of-the-art instrumentation available to us through the collaboration with the Central Analytical Facility of Stellenbosch University, we optimised and implemented a method based on headspace sampling coupled with solid-phase microextraction (HS-SPME), combined with gas chromatography (GC) followed by tandem mass spectrometry (MS/MS), a more performant variant of the single MS. This setup offers the increased selectivity and sensitivity required for the RSC. This was coupled with a chemically sophisticated, but low-tech type of device for the direct measurement of H 2 S. MEASURING H 2 S S and S-containing compounds react selectively with a variety of reagents. This chemical property can be used for the determination of S-containing compounds. For example, the affinity of S for Mercury (Hg) was used in the original method for the analysis of thiols in wines. Some of the reactions produce coloured compounds and these can be used to monitor and measure S compounds. Some reagents are selective to only one compound, like H 2 S. A commercial device that can be used for this purpose has been available for a couple of years (Butzke & Park, 2011). Detector tubes are thin glass tubes with calibration

JANUARY/FEBRUARY 2020

ASTRID BUICA, SEBASTIAN VANNEVEL & WESSEL DU TOIT: Department of Viticulture and Oenology, Stellenbosch University, Stellenbosch KEYWORDS: Reductive sulphur compounds (RSC), wine, gas chromatography-tandem mass spectrometry (GC-MS/MS), colorimetric reactions

WINETECH TECHNICAL YEARBOOK 2020 93