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

Abstract

When bushfires occur near grape growing regions, vineyards can be exposed to smoke, and depending on the timing and duration of grapevine smoke exposure, fruit can become tainted. Smoke-derived volatile compounds, including volatile phenols, can impart unpleasant smoky, ashy characters to wines made from smoke-affected grapes, leading to substantial revenue losses where wines are perceivably tainted. This study investigated the potential for post-harvest ozone treatment of smoke-affected grapes to mitigate the intensity of smoke taint in wine. Merlot grapevines were exposed to smoke at ~7 days post-veraison and at harvest grapes were treated with 1 or 3 ppm of gaseous ozone (for 24 or 12 h, respectively), prior to winemaking. The concentrations of smoke taint marker compounds (i.e., free and glycosylated volatile phenols) were measured in grapes and wines to determine to what extent ozonation could mitigate the effects of grapevine exposure to smoke. The 24 h 1 ppm ozone treatment not only gave significantly lower volatile phenol and volatile phenol glycoside concentrations but also diminished the sensory perception of smoke taint in wine. Post-harvest smoke and ozone treatment of grapes suggests that ozone works more effectively when smoke-derived volatile phenols are in their free (aglycone) form, rather than glycosylated forms. Nevertheless, the collective results demonstrate the efficacy of post-harvest ozone treatment as a strategy for mitigation of smoke taint in wine.

Keywords: amelioration; glycoconjugates; rate-all-that-apply; smoke taint; volatile phenols; wine.

Figure 1

Sensory profiles of wines made from control and smoke-affected grapes, with and without post-harvest ozone treatment (at 1 ppm for 24 h or 3 ppm for 12 h); A = aroma; F = flavor; AT = aftertaste. Values are mean ratings of one blended wine per treatment, presented to 50 judges; * indicates statistical significance (p = 0.05, one-way ANOVA). Smoke exposure occurred at approximately 7 days post-veraison.

Figure 2

Concentrations (µg/L) of (a,b) guaiacol, (c,d) phenol and (e,f) syringol in smoke-affected grapes, with and without ozone treatment (at (a,c,e) 1 or (b,d,f) 3 ppm for 6, 12 or 24 h).Values are means of three replicates (n = 3). Different letters indicate statistical significance (p = 0.05, one-way ANOVA). Post-harvest smoke treatments were applied on consecutive days, but smoke density was lower on the second day of treatment due to increased wind.

Figure 3

Total tannin and anthocyanin concentrations (mg/g) in smoke-affected grapes, with and without post-harvest ozone treatment (at (a,b) 1 or (c,d) 3 ppm for 6, 12 or 24 h).Values are means of three replicates (n = 3). Different letters indicate statistical significance (p = 0.05, one-way ANOVA); ns = not significant. Post-harvest smoke treatments were applied on consecutive days, but smoke density was lower on the second day of treatment due to increased wind.