Posted: Jun 04, 2018
St. Helena, Calif.—Since the 2017 fires swept across Northern California wine country, there’s been an overload of smoke taint talk. Can we detect smoke taint? Can we prevent smoke taint? Can we take taint out of smoke-exposed wines?
At the Innovation + Quality (IQ) conference held May 23 and 24 at the Silverado Resort and Spa in Napa, Calif., a panel of experts from across the globe gathered to discuss these questions in relation to their recent research.
Spoiler alert: there are still no definitive answers to any of these questions. But here is a summary of some of the studies being done and what we, as an industry, have learned thus far.
Dr. Kerry Wilkinson, associate professor in oenology at the University of Adelaide in Australia, kicked off the session with smoke taint 101:
During a fire, compounds called volatile phenols are released. These volatile phenols permeate the grape skins, bond with the sugars inside the skins and form molecules called glycosides. Note: volatile phenols of smoke tainted grapes are located in the skin and do not penetrate to the pulp.
The glycoside bond makes the phenols non-volatile, meaning unpleasant aromas associated with smoke are not detectable. However, the issue winemakers have is that once grapes go through maceration and fermentation, acidity breaks apart the glycosides, releasing the sugar-phenol bond and making phenols volatile again.
Wilkinson’s research focused on the difference in smoke taint effects in seven different varieties: Chardonnay, Sauvignon Blanc, Pinot Blanc, Pinot Noir, Shiraz and Cabernet Sauvignon. It was a controlled experiment with three rows of vines for each variety placed in a tent with smoke and three rows of vines of each variety acting as a control group in a tent without smoke. “It was a massive experiment,” said Wilkinson referring to the sheer labor of the project. “We hope to never have to repeat that process.”
Wilkinson exposed the grapes to smoke at seven days post-veraison when, according to the Wilkinson, the berries are most susceptible to the permeation of the volatile phenols. Wilkinson’s teamThey harvested each row of vines at maturity and immediately looked at the fruits’ composition. Looking at the precursor compounds, Wilkinson noticed there were big differences between the smoke and control groups. As expected, the glycoside compounds were much more prevalent in the grapes exposed to the smoke than those in the control.
The exception was Shiraz, which as Wilkinson said “had a whole heap [of glycosidesl] in the control. “So we know that Shiraz produces these [glycosides] compounds on its own. It has nothing to do with smoke; it just produces glycol in its precursor form as part of what it does naturally.”
When she produced wine with the grapes, Wilkinson said she was able to see that the level of glycosides was still quite low in the control wine (except for the Shiraz) but much higher in the smoke-exposed wine. “But the levels we see in the grapes don’t necessarily predict the levels in the resulting wines,” she said.
When she measured the volatile phenols in the wine to see how much of those compounds were released (because that’s what affects the aromas and the flavor qualities), the red varieties had much higher instances of volatile phenols than the whites. This, Wilkinson concluded, is an effect of the skin contact during fermentation.
“But then we saw some varietal differences,” said Wilkinson, “It looks like some varieties are more susceptible than others.”
Of the whites, the Pinot Gris had much higher levels of taint, both chemically and from a sensory perspective than did the other white grape varieties. Cabernet was the most tainted of the reds.
Why? According to Wilkinson, it could be the thickness of the grape skin, it could be the skin to pulp ratio, or some other physical property of the grape. “I’m trying to look for another project to look at that in more detail,” said Wilkinson. “I’ve been working on this for more than 10 years and we don’t have all the answers.... As we answer one question we end up asking another two.”
Effect of fire source Dr. Tom Collins, assistant professor of grape and wine chemistry at Washington State University in Pullman, Wash., presented his research on how the source of smoke influences the smoke taint composition in wine grapes. “We see two kinds of smoke events in Washington: rangeland and Ccascade forest fires,” he said.
Collins measured the chemical composition of the smoke from 15 different rangeland plant varieties known to grow along vineyard sites and compared that to the smoke composition of western red cedar and Douglas fir bark mulch found in Washington forests. The chemical compositions varied across the burnt plant life, as he suspected, but it was the rangeland plants that provided the highest amount and most variety of volatile phenols.
To conduct the smoke taint trial, Collins used Merlot and Cabernet grapevines just past veraison. He covered four rows of 30 vines of each variety in temporary hoop-houses, exposing two rows of each variety to smoke for a 38-hour time period, treating the Merlot with rangeland smoke and the Cabernet to forest mulch smoke.
“Do the different compositions affect the perception of wine?” asked Collins. “You’ll have to taste to find out.”
Collins presented both his Merlot and Cabernet for comparison. While both samples presented obvious instances of smoke taint it was, in fact, the Merlot that emitted the most sensory phenolic affects. Where the nose of the Cabernet was almost muted completely, presenting ash-like flavors on the palate only, the Merlot had the stereotypical “ash tray” scent and a more dominant burning sensation in the mouth and throat. But, according to Collins, analysis of the exact volatile and non-volatile composition is ongoing.
The technical tastings at IQ were from trials on minimizing smoke taint sensory perception in wines. These included decreased fermentation temperatures and oak regimen (conducted at the University of California, Davis, research and teaching wWinery), flash détente (conducted by the Carneros Vintners) and varied maceration time (conducted by A to Z Wineworks). While all trials displayed some minimizing eaffect onto the tainted wines, experts across the board agreed that none of them are a real cure for the latent phenolic compounds that reside inside smoke tainted wine.
One of the latest innovative tools, Purovino, uses ozone — a high concentration of oxygen on the skin of the grapes — to remove volatile phenols from tainted fruit. According to Professor Fabio Mencarelli, professor of innovative technologies for food handling and enology at the University of Tuscia in Viterbo, Italy, the ozone treatment strips the phenolic compounds to their most basic, atomic forms, rendering them completely inert. Thus, volatile phenols can no longer influence the grapes’ chemical composition.
During the trial tasting, Robert Rex of Deerfield Ranch presented his ozone-treated Cabernet Sauvignon. While the treated wine had no perceived smoke eaffects, Rex said that he felt the treatment also took away any sense of terroir. As the ozone treatment strips away at the smoke taint phenols, so, too, does it strip away at the phenols innate in the grapes’ natural composition. “When I conducted the taste test with other winemakers, they actually prefer the untreated wine. It’s more complex,” he said.
An inconclusive conclusion
Dr. Anita Oberholster, assistant extension specialist in enology at the University of California, Davis, in Davis, Calif., summed up what the wine industrywe knows about smoke taint into five key points:
• There is no smoke taint carry-over from one season to the next.
• Grapes are most susceptible to smoke taint during veraison.
• Because the volatile phenols permeate the grape skins, washing grapes does not work.
• Predicting smoke taint effects in wine is still an ongoing process.
• There is currently no 100% solution to “curing” smoke taint.
The IQ conference is organized by Wine Business Monthly magazine, which is part of the Wine Communications Group that also owns Wines & Vines.
By Stacy Briscoe
May 25, 2018
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