Abstract
When wine grapes are exposed to smoke, there is a risk that the resulting wines may possess smoky, ashy, or burnt aromas, a wine flaw known as smoke taint. Smoke taint occurs when the volatile phenols (VPs) largely responsible for the aroma of smoke are transformed in grape into a range of glycosides that are imperceptible by smell. The majority of VP-glycosides described to date are disaccharides possessing a reducing β-d-glucopyranosyl moiety. Here, a two-part experiment was performed to (1) assess the stability of 11 synthesized VP-glycosides towards general acid-catalyzed hydrolysis during aging, and (2) to examine whether yeast strains differed in their capacity to produce free VPs both from these model glycosides as well as from grapes that had been deliberately exposed to smoke. When fortified into both model and real wine matrices at 200 ng/g, all VP-disaccharides were stable over 12 weeks, while (42–50 ng/g) increases in free 4-ethylphenol and p-cresol were detected when these were added to wine as their monoglucosides. Guaiacol and phenol were the most abundantly produced VPs during fermentation, whether originating from natural VP-precursors in smoked-exposed Pinot Noir must, or due to fortification with synthetic VP-glycosides. Significant yeast strain-specific differences in glycolytic activities were observed for phenyl-β-d-glycopyranoside, with two strains (RC212 and BM45) being unable to hydrolyze this model VP, albeit both were active on the guaiacyl analogue. Thus, differences in Saccharomyces cerevisiae β-glucosidase activity appear to be influenced by the VP moiety.
Highlights
Smoke taint is an increasingly common wine fault that has, and will likely continue to have, serious economic consequences for the global wine industry, in fire-prone grape-growing and wine-producing regions
There are confounding variables in this type of experiment that preclude a systematic evaluation of volatile phenols (VPs)-glycoside stability (e.g., microbial vs. chemical hydrolysis, oxidative loses, or colloid association(s))
The model wine fortifications served as a simplified control system that permitted the exclusion of confounding variables that could exist in real wine including, but not limited to, co-association with other wine components such that the VP-glycosides are not detected using conventional analytical techniques
Summary
Smoke taint is an increasingly common wine fault that has, and will likely continue to have, serious economic consequences for the global wine industry, in fire-prone grape-growing and wine-producing regions. Guaiacyl-glycosides appeared to increase over the same time period—an observation that complicates the elucidation of the origins of the free VPs. Interestingly, guaiacyl-glycosides appeared to increase over the same time period—an observation that complicates the elucidation of the origins of the free VPs These data would suggest that the selection of yeast strains that are poorly able to hydrolyze VP-glycosides, in particular VP-glucosides, during primary fermentation would be a possible way to keep free VP levels low in wines produced from smoke-exposed grapes in which subsequent hydrolysis during aging is expected to be minimal. Control and smoke-exposed Pinot Noir grapes collected from the same vineyard were used, with the fortification of each set of samples with VP-glycosides permitting a comparison of the release of VPs from a pool naturally stored within grape tissues and verified glycoconjugates
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