Uptake and Glycosylation of Smoke-Derived Volatile Phenols by Cabernet Sauvignon Grapes and Their Subsequent Fate during Winemaking

Abstract

Wine made from grapes exposed to bushfire smoke can exhibit unpleasant smoky, ashy characters, which have been attributed to the presence of smoke-derived volatile phenols, in free or glycosylated forms. Here we report the uptake and glycosylation of volatile phenols by grapes following exposure of Cabernet Sauvignon vines to smoke, and their fate during winemaking. A significant delay was observed in the conversion of volatile phenols to their corresponding glycoconjugates, which suggests sequestration, the presence of intermediates within the glycosylation pathway and/or other volatile phenol storage forms. This finding has implications for industry in terms of detecting smoke-affected grapes following vineyard smoke exposure. The potential for an in-canopy sprinkler system to mitigate the uptake of smoke-derived volatile phenols by grapes, by spraying grapevines with water during smoke exposure, was also evaluated. While “misting” appeared to partially mitigate the uptake of volatile phenols by grapes during grapevine exposure to smoke, it did not readily influence the concentration of volatile phenols or the sensory perception of smoke taint in wine. Commercial sensors were used to monitor the concentration of smoke particulate matter (PM) during grapevine exposure to low and high density smoke. Similar PM profiles were observed, irrespective of smoke density, such that PM concentrations did not reflect the extent of smoke exposure by grapes or risk of taint in wine. The sensors could nevertheless be used to monitor the presence of smoke in vineyards during bushfires, and hence, the need for compositional analysis of grapes to quantify smoke taint marker compounds.

Potential Mitigation of Smoke Taint in Wines by Post-Harvest Ozone Treatment of Grapes

“…Low levels of VP glycosides (<25 µg/kg) were detected in control grapes, irrespective of any O 3 treatment, in agreement with previous studies which report VP glycosides as natural constituents of some grape varieties [ 11 , 13 , 41 ]. Grapevine exposure to smoke resulted in significantly higher levels of VP glycosides, particularly glycosides of guaiacol, phenol, cresols and syringol ( Table 1 ), for which pentose glucosides of guaiacol, phenol and cresol and the glucose glucoside (gentiobioside) of syringol were most abundant ( Table S1 ), again in agreement with previous research [ 18 , 42 , 43 ]. Statistically significant compositional differences were observed amongst smoke-affected grape samples.…”

“…However, it be noted that due to windy conditions being experienced on the second day of application, fruit was exposed to less dense smoke, such that VPs were higher in treated with 1 ppm of O3, than with 3 ppm of O3 (Figure 2, Table S6). Elevated con tions of VPs were nevertheless detected in grapes as a consequence of post-harvest exposure; guaiacol and syringol were the most abundant VPs present, and 4-methy Numerous studies have demonstrated that following their uptake from smoke into grapes, VPs accumulate in glycoconjugate forms due to rapid in vivo glycosylation [8,18,43,60,61]. As such, when post-harvest O 3 treatments were applied in the first trial, smoke-derived VPs were only present in glycoconjugate forms ( Table 1 and Table S1).…”

“…VPs are toxic for plant cells and are thought to be transformed into glycosidic forms to prevent cellular damage [ 15 , 16 ]. The glycosylation process has been shown to occur rapidly following smoke exposure [ 17 , 18 ] and while VP glycoconjugates are odorless and flavorless, they can be metabolized during fermentation [ 11 , 13 ] and by human salivary enzymes [ 19 ], resulting in the release of VPs and perception of the smoke-related aromas and flavors, and ashy aftertaste characteristic of smoke taint.…”

“…Smoke taint has become an issue of increasing concern for grapegrowers and winemakers around the world since it is difficult to produce high-quality wine from smoke-affected grapes [ 3 ]. Over the past decade, considerable research has been undertaken to understand the impacts of grapevine exposure to smoke, and to mitigate the sensory consequences of smoke taint in wine; from partial defoliation of grapevines [ 20 ] and washing smoke-affected fruit [ 3 , 14 , 18 ] in the vineyard, to limiting skin contact during fermentation [ 7 ], and treating smoke-tainted wine via reverse-osmosis and solid phase adsorption [ 21 ] or the addition of activated carbon [ 22 ] or cyclodextrins [ 23 ], in the winery. These methods are often time and resource intensive and/or offer limited efficacy (i.e., they typically remove VPs, but not VP glycosides).…”

Techniques for Mitigating the Effects of Smoke Taint While Maintaining Quality in Wine Production: A Review

“…Summary of the methods evaluated for prevention of smoke taint in the vineyard.Washing vines or grapes with water, aqueous ethanol, or milk after smoke exposure did not affect the guaiacol content of grapes or juice. Misting grapes during smoke exposure partially mitigated the uptake of volatile phenols by grapes but did not influence the perception of smoke taint in wine[29,40,55].…”