The Chemical Reaction of Glutathione and trans-2-Hexenal in Grape Juice Media To Form Wine Aroma Precursors: The Impact of pH, Temperature, and Sulfur Dioxide

Abstract

The aldehyde 3-S-glutathionylhexanal is an intermediate which is produced during the formation of the wine aroma precursor 3-S-glutathionylhexanol, after the reaction of glutathione with trans-2-hexenal. This study was conducted to assess whether the chemical, as opposed to the enzymatic, production of 3-S-glutathionylhexanal could occur at a significant rate in grape juice. LC-MS/MS was used in low- and high-resolution modes, in combination with functional group derivatization, to identify and quantitate products. In comparison to cysteine, glutathione was found to induce less cyclized products on reaction with trans-2-alkanals and the glutathione-derived products were more reactive to hydrogen sulfite. The zero-order rates for 3-S-glutathionylhexanal formation in model grape juice were 1.08 ± 0.08 and 0.45 ± 0.05 mg/(L·day) glutathione equivalents at 25 and 13 °C, respectively, and the reaction rate increased 3-fold by increasing the pH from 3.2 to 3.8. 3-S-Glutathionylhexanal was detected in all five white grape juices examined. The concentration of the aldehyde could be increased by up to 10-fold after being released from hydrogen sulfite, demonstrating a potentially novel source for the production of varietal thiol aroma compounds in wine.

New Precursors to 3-Sulfanylhexan-1-ol? Investigating the Keto–Enol Tautomerism of 3-S-Glutathionylhexanal

“…1 H and 13 C NMR were in agreement with that reported in the literature [ 8 , 10 ]. Where tautomers are distinguishable by NMR, the enol form is indicated by *.…”

“…Sauvignon Blanc, imparting characteristic grapefruit and passionfruit aromas [ 1 , 2 , 3 , 4 , 5 , 6 , 7 ]. Despite the prominence of these odorants in the finished wines, these compounds are not found in appreciable levels in the grape berry or pressed juice and are produced almost exclusively during fermentation [ 8 , 9 ]. Currently only ~50% of the 3SH produced through fermentation can be explained through previously published pathways, allowing scope for more work to further enhance our understanding of the production of this impact odorant [ 9 ].…”

“…One of the initial precursor compounds to 3SH produced in grapes and grape juice is 3- S -glutathionylhexanal (glut-3SH-al), formed by the Michael addition of glutathione to the α,β-unsaturated aldehyde, E -2-hexenal, upon the breakdown of fatty acids resulting from cell damage [ 8 ]. This detoxification process occurs both chemically and enzymatically and the aldehyde conjugate is subsequently transformed further into other precursors of 3SH and, through alcoholic fermentation by yeast, to 3SH [ 8 , 11 , 12 ].…”

“…Even once the conjugate with glutathione has been formed, the aldehyde functionality retains this reactivity. When glut-3SH-al was first identified in grape juice, it was identified in conjunction with the bisulfite adduct, glut-3SH-SO 3 , formed by the addition of a bisulfite ion to the aldehyde group [ 8 , 12 ].…”

“…The conversion of glut-3SH-al to 3SH is not yet well understood, with this precursor contributing to only 0.4% of total 3SH produced during fermentation in previous work [ 12 ]. It has been suggested that the bound form of glut-3SH-al as glut-3SH-SO 3 impedes downstream metabolism to 3SH, resulting in the low conversion rates and forming an untapped sink of potential 3SH in the finished wine [ 8 ].…”