Insight on Lugana flavor with a new LC-MS method for the detection of polyfunctional thiols

Abstract

The analysis of polyfunctional thiols in wine is challenging due to their low abundance and instability within a complex matrix. However, volatile thiols are highly aroma-active, making their accurate quantification in wine at low concentrations crucial [1].

This work aimed to develop and validate a modern, high-throughput analytical method for detecting the most important thiols reported in literature. This led to a rapid preparative process involving the derivatization of a low amount of wine, followed by Solid Phase Extraction (SPE) to yield a purified and concentrated sample. 2-Phenyl-1,2-benzisoselenazol-3(2H)-one, known as “Ebselen,”, was chosen for derivatization due to its high reactivity and selectivity with mercapto functional groups, its retention on a reverse phase column, and its ability to ionize readily in both polarities. This method quantifies the three main varietal thiols, and 12 other compounds related to less-represented aromas or off-flavors.

We present a case study of 41 Lugana wines from various wineries. The most studied varietal thiols 3-mercaptohexan-1-ol (3MH), 3-mercaptohexan-1-ol acetate (3MHA), and 4-mercapto-4-methylpentan-2-one (4MMP) were analyzed using the QuEChERS method of Carlin et al. [2] and compared with the new SPE method validated in this study.

In addition to the already cited varietal thiols, the method allowed straightforward quantification of other important mercapto compounds, including ethyl-2-mercaptopropionate (fruity), ethyl-3-mercaptopropionate, 2-furanmethanethiol, and benzyl mercaptan (burnt, roasted). Other compounds contributing vegetal notes included 3-mercapto-2-methylbutanol and 3-mercapto-3-methylbutanol, while citrus and grapefruit aromas were linked to 4-mercapto-4-methylpentan-2-ol, 3-mercaptopentanol, and 3-mercapto-1-heptanol. Additionally, off-flavors like ethanethiol and 3-mercapto-2-methylpropan-1-ol and 3-methyl-2-buten-1-thiol were included.

All analytes demonstrated good linearity, with determination coefficients > 0.99 and recovery rates between 70% and 130%, and half reached a limit of quantification of 1 ng/L. 3MH and 3MHA were present in all analyzed wines, while high 4MMP was unexpectedly found in five wines, raising questions about its origin. Ethyl-3-mercaptoproprionate, 2-furanmethanethiol, and benzyl mercaptan occasionally exceeded the perception threshold, contributing empyreumatic flavors to some Lugana wines, whereas the other analytes remained below perception threshold.

References

[1] Rauhut D. and Kiene. F. (2019). Red Wine Technology, 273–282.

[2] Carlin S., Piergiovanni M., Pittari E., Lisanti M.T., Moio L., Piombino P., Marangon M., Curioni A., Rolle L., Rìo Segade S., Versari A., Ricci A., Parpinello G.P., Luzzini G., Ugliano M., Perenzoni D., Vrhovsek U., Mattivi F. Food Research International (2022), 157, 111404.