Abstract
The effect of ultrasound (20 kHz, 153 μm) on the prefermentation extraction mechanisms in Sauvignon Blanc grapes was studied, focusing on 3-mercaptohexan-1-ol (3MH) and 4-mercapto-4-methyl-pentan-2-one (4MMP) precursors linked to glutathione (GSH) and cysteine (Cys). The treatment determined a positive extraction trend between the duration (untreated, 3 and 5 min) and the conductivity or the concentration of catechins and total phenols, significantly differentiated after 5 min. Nevertheless, the concentration of the thiol precursors in grape juice not only remained undifferentiated, but that of 3-S-glutathionyl mercaptohexan-1-ol showed a negative trend with the treatment time applied (168 ± 43, 156 ± 36, and 149 ± 32 μg/L, respectively, for control, 3 and 5 min). The divergence on the effect between families of compounds suggests an interaction between the sonication treatment and thiol precursor molecules. In order to evaluate the possible degradation properly, ultrasound was applied in a model solution spiked with 3MH and 4MMP precursors, reproducing the conditions of grapes. Except for Cys-3MH, the mean concentration (n = 5) for the rest of the precursors was significantly lower in treated samples, predominantly in those linked to glutathione (~−22% and ~18% for GSH-3MH and GSH-4MMP) rather than to cysteine (~−6%~−8% for Cys-3MH and Cys-4MMP). The degradation of precursors was associated with a significant increase of 3MH and 4MMP. The formation of volatile thiols following sonication is interesting from a technological point of view, as they are key aroma compounds of wine and potentially exploitable in the wine industry through specific vinification protocols.
Highlights
Thiols are molecules featuring a carbon bond to a sulfydryl group
We wanted to investigate whether US could be useful in speeding up and enhancing the extraction of thiol precursors from white grape skins, based on the results reported by Ferraretto and Celotti [22] who applied this technology in red winemaking
Sonication did not cause significant differences in the concentration of any of the thiol precursors and GSH-3MH showed a surprising negative trend unlike the results reported for conductivity, catechins, and total phenols
Summary
Thiols are molecules featuring a carbon bond to a sulfydryl group. The so-called polyfunctional thiols represent an important subclass in winemaking and juice production because of their extremely high aromatic power. 4-mercapto-4-methylpentan-2-one (4MMP), 3-mercaptohexan-1-ol (3MH), and 3-mercaptohexil acetate (3MHA) should be highlighted as the compounds with the greatest impact, due to their concentration in wine, odour activity values, and pleasing aroma [1]. Black currant or passion fruit, these cannot be usually detected during grape tasting. The odoriferous molecules are present in grape berries as nonvolatile precursors (Figure 1) and are linked to some amino acids or di- and tri- peptides, e.g., cysteine or glutathione, through a sulfur bond [1].
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