Influence of pre-fermentative steps on varietal thiol precursors

Abstract

The content of 3-sulfanyl-1-hexanol and its acetate ester in wine is affected by a number of factors, including the concentration of its precursors S-3-(hexan-1-ol)-L-glutathione (G-3SH), S-3-(hexan-1-ol)-L-cysteine (Cys-3SH) and S-3-(hexanal)-glutathione (G-3SHal) in the grape must. The role of grape must extraction steps on the content of volatile thiol precursors (VTP) in must fractions was investigated.

Grillo grape must samples were drawn along with the grape must extraction process in a winery under either air-exposed or air-free conditions, as well as under laboratory conditions and their VTP and glutathione (GSH) content was assessed by UPLC-HRMS. The roles of copper ions and sulfite were also investigated.

Under industrial conditions, more than 95% of the grape G-3SHal was lost following to the grape crushing due to sulphite addition. The content G-3SH and Cys-3SH increased with the must yield, specially under air-exposed conditions, while the GSH level decreased. Under laboratory conditions, trace amount of 3-SHal was obtained when air-free condition was applied or sulfite was added, instead milligrams per litre levels were obtained if air-exposed condition was applied, especially (14.5 mg/L) when copper sequestering salts were added. Negligible amounts of GSH (as well as grape reaction product) were detected in all the laboratory-made samples except when sulphite was added (GSH=33 mg/L).

The data strongly suggest that G-3SHal in grape must is mainly produced along with must extraction following to the binding of GSH to (E)-2-hexenal which is readily reduced to G-3SH. Sulfite addition strongly prevents the VTP formation, as well as copper ions. Therefore, grape must extraction must be considered among the main factors affecting the VTP content in grape must.