On a pleasant evening in October 2017, winemaker Alisa Jacobson and her friends picked the last of the Cabernet grapes at her friends’ winery in southern Napa. They were taking advantage of the cool evening temperatures, which help reduce damage to the fruit. Later that night, the pleasant evening air turned acrid—Jacobson was surprised to smell smoke wafting through her bedroom window despite only seeing wildfires on her radar to the far north, in Santa Rosa, CA.
Smoke from wildfires can make the skies an eerie yellow, as seen here over a Napa Valley, CA, vineyard in September 2020. The smoke’s effects on winery grapes continue to perplex growers, winemakers, and researchers. Image credit: Science Source/Peter Menzel.
A new fire had flared up—the largest in California’s history. Dubbed the Tubbs fire, it spread from Napa County to Santa Rosa, jumped across US Highway 101 in the middle of the night, and burned nearly 37,000 acres before it could be contained, killing 22 people and destroying more than 5,000 buildings, half of them homes. Winemakers like Jacobson, who is vice president for winemaking at Joel Gott Wines, felt the fire’s impact not only in the looming threat to their homes but to their livelihoods as well. Months after evacuation orders were lifted, the region inched back to normalcy. Then winemakers began to detect smoke in their wine.
Essence of Ash
Grapes exposed to smoke absorb chemicals that can alter—and sometimes ruin—the taste and smell of resultant wines. This so-called “smoke taint” has become a growing concern for the industry. Chemicals in smoke obscure wine’s flavor and fragrance, Jacobson says. “You can smell the taint on the aroma, like a campfire, and when you taste it it’s like an ashtray,” she says. “It lingers for minutes after you spit or swallow.”
But not all wines are so severely affected. Grape growers, winemakers, and researchers have been surprised by the complex, lasting effects of wildfire smoke. The vagaries of plant physiology and microbial fermentation and the influence of the wind and other elements all add up to make taint an unpredictable and elusive phenomenon. Precisely when smoke exposure will ruin a batch of wine is uncertain. “Just because you have smoke exposure in your vineyard doesn’t mean you’ll taste it in the wine,” says enologist Elizabeth Tomasini of Oregon State University in Corvallis. “Two wines that have the same amount of smoke compounds might taste very different. It’s tricky.”
Since 2017, nearly every year has brought record-breaking fires to the western United States. In 2019, Jacobson and other industry representatives teamed up with Tomasini and other researchers to form the West Coast Smoke Exposure Task Force. The group aims to develop tools to test, treat, and ideally prevent smoke taint from ruining wines. Perhaps the biggest surprise thus far has been the complicated nature of that taint. Trusted winemaking techniques, such as filtration or using egg whites to remove unwanted chemicals, have failed to remove traces of wildfires. “There are lots of techniques for other problems in the winery, so I thought we’d be able to fix it,” Jacobson recalls. “It was a surprise that smoke taint was so difficult to remove.”
A Problematic Pairing
Australian researchers have been trying to solve the problem of taint for more than a decade. On a visit to a winery approximately 15 years ago, enologist Kerry Wilkinson at the University of Adelaide in Australia first heard from winemakers that their wines tasted ashy after fruit had been exposed to smoke from a nearby prescribed burning. “Some told us the grapes themselves seemed fine, and the juice didn’t smell or taste of smoke,” Wilkinson recalls. “But once they began to ferment it, this distinctive odor and ashy finish emerged.”
Wilkinson had studied other kinds of burnt odors in wine, ones deliberately introduced by aging wine in toasted oak barrels. The process of toasting breaks a woody polymer named lignin into volatile compounds. Fermentation reactions tack sugars onto those aromatic chemicals to convert them into soluble forms known as glycosides, which create spicy clove- or vanilla-like notes in finished wine. “Knowing that chemistry raised the idea in my mind that something similar was happening with wildfire smoke compounds,” Wilkinson says.
Indeed, she found similar chemical processes at play in grapes themselves. Wildfires burn lignin in trees and produce volatile phenols that can be toxic to plants. To protect themselves, grapevines react by coupling these aromatic compounds to sugars. Once bound, these conjugated chemicals are soluble in water and can be metabolized or transported out of cells. The bound versions of smoke chemicals are no longer volatile, so they can be tough to detect via smell or taste—meaning a smoke-tainted grape may seem no different from one not exposed to smoke. But during fermentation, yeast enzymes can break these bonds, releasing the phenols once more and causing an ashy, smoky finish to wines.
The process of binding phenols to sugars is surprisingly quick—an hour or two of exposure can result in smoke taint, Wilkinson says. And smoke doesn’t just deposit a residue on the fruit’s surface. Even after the skins are removed, as in the process of making white wines, sugar-bound phenols in the flesh of grapes can prove problematic. Wilkinson and her student were the first to find that smoke could have a significant impact on the grapevines in the field and the wine they produced (1). Longer durations of smoke exposure also resulted in a heavier taint, they found.
Although the enzymes that bind smoke compounds to sugars are common across plant species, the problem of taint appears to be unique to grapes. It’s possible that the chemical reaction occurs differently in other plants, or that the fruits or vegetables are already harvested by the end of summer, when wildfires usually flare up. “No one’s ever said they’ve got smoke-tainted cauliflowers or oranges,” Wilkinson says. “We’ve done experiments applying smoke to strawberries and cherry tomatoes and haven’t seen the same taint.”
At a winery in West Kelowna, British Columbia, overlooking Lake Okanagan, analytical chemist Wesley Zandberg and his team study the effects of simulated wildfire smoke on Pinot Noir grapes. Image credit: Matthew Noestheden (photographer).
Exactly how and when the wildfire-induced chemicals infuse that noxious taint remains something of a mystery.
Wine contains hundreds of flavor-producing compounds that vary depending on grape varietals, soil, growing conditions such as warmth, the fermentation process, and more. These factors can introduce the same chemical markers currently associated with wine taint, particularly in wines such as Syrah or Shiraz. “These grapes naturally make very high levels of phenols,” says analytical chemist Wesley Zandberg at the University of British Columbia in Vancouver, Canada. “Wine experts might call the flavors peppery or spicy, and it’s considered an attractive feature in these varieties.”
Identifying the “normal” levels of these naturally occurring chemicals is critical to knowing whether levels are elevated after a fire. “We don’t have good baseline data,” Zandberg says. “What’s normal for each variety or region? Now, we’re looking at data to understand whether the levels are normal or elevated—and if it’s elevated, is it enough to cause a problem downstream when making wine?”
Wilkinson’s studies have identified certain markers of smoke taint, such as guaiacol and 4-methyl-guaiacol. But there’s no comprehensive list, she says. Because these compounds are also naturally present in untainted wines, it’s possible that smoke is problematic not because it adds volatile compounds but because increased amounts block other flavors, says Zandberg, who is studying the chemical changes in wine caused by smoke (2). “Maybe the smoke exposure does put smoky flavored compounds into the wine,” he says, “but maybe it also prevents the formation of beneficial flavors that would otherwise be present.”
Smoke markers may vary because of wildfires, too. Although all wood contains lignin, its structure can vary by species, so the tree type can affect the smoke’s chemistry. Moisture in the air, the temperature of a fire, wind direction, distance from a fire, and other factors influence whether a particular wildfire will result in smoke-tainted wine. In 2020, for instance, catastrophic fires swept across the winemaking regions in California and Oregon, and the smoke drifted far north into vineyards in British Columbia. But even in Napa, certain pockets remained unaffected, explains enologist and analytical chemist Anita Oberholster of the University of California, Davis. “Topography, wind direction and other factors all play a part,” she says. “It’s very difficult to predict.”
Fresh smoke from a recent nearby fire poses the greatest risk to wines. Volatile phenols begin to decompose in the atmosphere within a day, reducing the risk of taint. Zandberg is currently comparing smoke-tainted wines and grapes from the United States with those in British Columbia to understand how the age of smoke and the distance it traveled altered how wines in both regions were affected.
In Oregon, Tomasini examines the sensory piece of this puzzle. If wines naturally contain the same phenols that smoke carries, is there a tipping point at which the concentration of these flavors makes the product unacceptable to a consumer? They are currently developing experiments that involve controlled taste tests of wines exposed to smoke and wine samples spiked with smoke markers to identify when an expert taster might deem a wine “tainted,” Tomasini says.
Their team couples this sensory analysis with chemical tests to pinpoint the molecules that create problematic flavors. Analyzing both the sensory aspects and chemistry together in this way is critical, Tomasini says, because the problems of tainted wine are as much about perception as chemistry. “It’s not just about smoke compounds, but everything else in there,” she says. “Wine is a beautiful combination of art and science.”
Screening for Solutions
An industry desperate for answers is hoping to find ways to predict when these factors converge. Contracts between growers and winemakers cover other kinds of problems such as fungal contamination, Jacobsen says. But there’s currently no way to know for sure whether a batch of grapes will yield tainted wine. As a result, growers might find over-cautious winemakers rejecting fruit because there was smoke in the area. Or winemakers might buy a batch of fruit and later find their wines unsellable.
“It’s not just about smoke compounds, but everything else in there. Wine is a beautiful combination of art and science.”
In 2020, another historic fire season started in mid-August, when most grape varieties had yet to be harvested. Oberholster was inundated with requests from winemakers who wanted help analyzing grapes and wine samples for evidence of smoke taint. “Given the timing and the severity of the fires,” she says, “the extent of the problem was just enormous.”
Even as fires raged nearby and the pandemic took hold, Oberholster and her colleagues analyzed thousands of samples by using gas chromatography and mass spectrometry, expensive and laborious methods that require 30 minutes to an hour to process a single sample. Oberholster also conducted dozens of informational sessions on Zoom to help those affected understand the problem and what they could try to mitigate taint. Many believe, for instance, that turning on sprinklers to wash the grapes during smoke exposure will help. But Wilkerson and others have found that dousing the grapes likely only makes a difference to ash deposits—not smoke taint. “These compounds get into the grapes so quickly that even washing during the exposure doesn’t help,” Wilkerson says.
Nor does removing smoke-exposed skins—and anyway it’s not a viable solution for red wines, where the skin imparts the drink’s ruby hues. Experiments with different yeast strains haven’t yielded much success either, Oberholster says. Some filtration techniques familiar to winemakers can help, including membrane filtration, reverse osmosis, and treatments involving activated charcoal or milk proteins, which are already used to remove bitter compounds from wine. But these treatments remove more than just smoke taint chemicals. “These methods work well if wines are not severely affected, but they lack specificity,” Oberholster says. “Even if you remove the smoky flavor, they reduce the overall quality of the wine.”
For now, the best solution for the industry remains a sensory one, according to Oberholster. She recommends that growers make small-scale batches of wine to test whether grapes will yield smoke taint—and use their results to inform discussions with winemakers on whether to harvest and approaches to mitigate the flavor of fire.
Wilkinson, Zandberg, and others are experimenting with coating compounds such as a clay named kaolin, which essentially coats grapes to help prevent smoke from penetrating skin. These are commonly used on cherries, apples, and other produce already, but the researchers have only had limited success with smoke-exposed grapes thus far.
Remedies will be essential, Jacobsen says. When wildfires loom, industry workers scramble to protect their lives and belongings; they often can’t reach their fields, let alone work on them, to protect or harvest grapes. “We have to figure out how to make wines without smoke taint,” she says, “But we also have to figure out how to live and work with the threat of wildfires.”