Evaluation of “Accentuated cut edges” technique on the release of varietal thiols and their precursors in Shiraz and Sauvignon blanc wine production

Abstract

Accentuated cut edges (ACE) is a novel grape crushing technique used sequentially after a conventional crusher to increase the extraction rate and content of polyphenolics, as shown for Pinot noir wine. This inspired us to apply the technique during Shiraz and Sauvignon blanc winemaking, primarily to assess its impact on the extraction of varietal thiol precursors in grape must/juice and formation of varietal thiols in the resultant wines. Other variables were jointly studied, including skin contact time and water addition to Shiraz grape must, and yeast strain and malolactic fermentation (MLF) for Sauvignon blanc. Varietal thiol precursors (3-S-glutathionylhexan-1-ol (GSH-3-SH), 3-S-cysteinylhexan-1-ol (Cys-3-SH)) and varietal thiols derivatised with 4,4′-dithiodipyridine were separately determined using high performance liquid chromatography tandem mass spectrometry. The concentrations of GSH-3-SH and Cys-3-SH in Shiraz grape must and varietal thiols (3-sulfanylhexan-1-ol (3-SH) and 3-sulfanylhexyl acetate (3-SHA)) in Shiraz wine were not significantly affected by grape crushing method, but a shorter skin contact time (3 days) during fermentation significantly increased 3-SH compared to a 6-day treatment. For Sauvignon blanc juice, the evolution profile of GSH-3-SH in the ACE treatment during cold maceration showed an increase from 313 µg/L at the beginning of maceration to the maximum content of 514 µg/L within 9 h, in stark comparison to the conventional crushing treatment, which initially contained 315 µg/L and reached a maximum of 382 µg/L at the end of the cold maceration period (21 h). The evolution profile of Cys-3-SH was similar to that of the GSH-3-SH, yielding 16 µg/L in ACE and 7 µg/L in conventional crushing at the end of maceration. Varietal thiols were determined in Sauvignon blanc wine, including 4-methyl-4-sulfanylpentan-2-one (4-MSP) and enantiomers of 3-SH and 3-SHA, with concentrations of 76–188 ng/L for 4-MSP, 456–864 ng/L and 434 850 ng/L for (3S)-3-SH and (3R)-3-SH, respectively, and 13–29 ng/L and 6–15 ng/L for (3S)-3-SHA and (3R)-3-SHA, respectively. Three-way analysis of variance revealed that their concentrations were significantly affected by the interaction effects of crushing method, yeast strain, and MLF, with ACE significantly increasing their concentrations compared to conventional crushing. Differences were also observed for yeast strain and MLF, with VIN13 yeast strain leading to greater amounts of 3-SH and 3-SHA enantiomers but less 4-MSP than Sauvy, whereas MLF treatment afforded higher amounts of 3-SH enantiomers and 4-MSP but lower levels of 3-SHA enantiomers than those without MLF. The molar conversion yield from the sum of GSH-3-SH and Cys-3-SH to the sum of 3-SH and 3-SHA was relatively low – ranging from 0.65% to 1.01% – and was significantly affected by two-way interaction effects, with VIN13, MLF, and ACE significantly increasing the conversion yield by up to 0.2%.