Perhaps the single most impactful and devastating series of events from the 2017 Napa vintage were October’s wine country fires. As the final presenter of the 2017 Napa Vintage Report, Dr. Eric Hervé, PhD. of ETS Laboratories, presented on his latest work, on smoke taint studies carried out by the ETS team since the California fires of 2008.
Research on smoke taint in grapes began in both Australia and British Columbia after a series of fires impacted the 2003 vintage. The research done from 2003 to 2007 revealed that volatile smoke compounds were indeed absorbed by grapes and leaves, and eventually became in a large part bound to sugars (they become “glycosylated”). The main issues with glycosylated compounds is that they are odorless and easily evade sensory detection. During fermentation, however, they are partially hydrolyzed (freed from their sugar moiety), which can cause smoke taint to appear in wine. For this reason, the industry needed markers for grapes that could be analyzed in the lab to estimate the risk of final wine smoke taint. These marker compounds that can be analyzed, by labs such as ETS Laboratories, are guaiacol and 4-methylguaiacol. To the experience of ETS Laboratories, reportable levels of free guaiacol in most grapes indicate some absorption of smoke volatile compounds, with higher levels meaning an increased risk of smoke taint materializing in wine.
Analyses run after fires in California during the 2015 vintage confirmed the empirical guidelines for interpreting grape tests outlined in 2008, and revealed guaiacol concentrations in final wines that were 3 to 5 times higher than the concentration measured in the original grapes.
Results from the 2018 vintage showed that, probably due to the late timing of the fires and less glycosylation happening inside senescent grapes, final concentrations of free guaiacol in wines were only on average 1.6 times higher than the concentrations measured in grapes at harvest. In other words, the risk of smoke taint appearing in wine appeared lower than would be the case when fire events happen earlier in the season.
With regard to mitigation of smoke taint, Dr. Hervé highlighted that data showed that washing the grapes and reducing the maceration time for red wines did not help reduce smoke taint. For future events involving smoke, two actions to try and mitigate smoke effects are to reduce the number of leaves that make it through sorting, and keep grapes separated into small lots through fermentation (if possible). Unfortunately, once smoke taint appears in wine, there is at this point no perfect remediation option available.