Thiol precursors in Catarratto Bianco Comune and Grillo grapes and effect of clarification conditions on the release of varietal thiols in wine

Background and Aims: Varietal thiols characterise the typical aroma of several white wines, such as Sauvignon Blanc. Their presence and sensory perception were suggested in two Sicilian grape cultivars, Catarratto Bianco Comune (CBC) and Grillo, though it has not been analytically proven. Methods and Results: Varietal thiol precursors and free varietal thiols were assessed in CBC and Grillo grapes, musts and wines by ultra-performance liquid chromatography/electrospray ionisation high-resolution MS. The isobaric compounds including S-3-(hexanal)-glutathione and S-3-(4-mercapto-4-methylpentan-2-one)-glutathione (GSH-4MMP) were distin- guished by comparing their accurate masses and high-resolution-MS/MS spectra with those of their synthetic standards. S-3-(Hexanal)-glutathione, S-3-(hexan-1-ol)-glutathione and S-3-(hexan-1-ol)-cysteine were found in grape, must and wine, whereas GSH-4MMP and its hydrolysed forms were not found. Their concentration decreased during winemaking, mostly after grape pressing. We compared the effect of clarification conditions based on the exposure of must to either air or CO2 on the concentration of thiol precursors and free thiols in wine; however, negligible differences were observed. The concentration of free thiols in the wines was found to be in the range 400–1100 ng/L and they were unaffected by the two clarification conditions tested. Conclusions: The isobaric S-3-(hexanal)-glutathione and GSH-4MMP were clearly distinguished for the first time by ultra- performance liquid chromatography/high-resolution MS through their retention times and MS spectra. These varietal thiols were revealed in CBC and Grillo wines for the first time. The air-free and air-exposed clarification treatments had little effect on the concentration of the varietal thiols. Significance of the Study: This study highlights the major impact of the varietal thiols (mainly 3-mercapto-hexan-1-ol and its acetate form) on the sensory properties of CBC and Grillo wines.