Role of extraction procedures on the concentration of varietal thiol precursors in Grillo white grape must

Abstract

Background and Aims: Varietal thiol precursors (VTPs) decrease strongly after the pre-fermentative operations. The effect of grape must extraction steps on the concentration of 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) was investigated. Methods and Results: Must produced on a commercial scale was sampled throughout the pre-fermentation steps. Must was also prepared in the laboratory for assessing the role of copper, sulfite and air. In commercial must fractions, over 95% of G-3SHal was lost because of sulfite addition, while the concentration of G-3SH and Cys-3SH increased along with must yield, and it exceeded the concentration found in berries. No Cys-3SH was found in the intact grape; instead a trace concentration of G-SHal, and a high concentration of glutathione and even cysteine was determined when sulfite was added. A large amount of G-3SHal was formed when copper was depleted. Conclusions: Our data suggest that Cys-3SH and G-3SHal are promptly and mainly formed in grape must upon grape pressing. Sulfite and copper prevented the formation of VTPs, while settling of must led to a loss of VTPs. Significance of the Study: Varietal thiol precursors can be preserved through the proper management of pressing with particular attention to must contact with air, extraction conditions, contact time with solids and addition of sulfite.