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, Calif.

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 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 also to their livelihoods. 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 “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.

“Just because you have smoke exposure in your vineyard doesn’t mean you’ll taste it in the wine,” says enologist Elizabeth Tomasino of Oregon State University in Corvallis. 

Since 2017, nearly every year has brought record-breaking fires to the western United States — with the current wildfire season in the West on pace to be possibly the worst yet. In 2019, Jacobson and other industry representatives teamed up with Tomasino 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 says. “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.

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. 

Complex chemistry

How and when the wildfire-induced chemicals infuse that noxious taint remains something of a mystery.

It does appear that 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. Analytical chemist Wesley Zandberg, at the University of British Columbia in Vancouver, is 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, Tomasino 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? Tomasino and colleagues are 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.”

“It’s not just about smoke compounds, but everything else in there,” she adds. “Wine is a beautiful combination of art and science.”

Screening for solutions

Remedies are hard to come by. Right now, there’s no way to know for sure whether a batch of grapes will yield tainted wine. As a result, growers might find overcautious winemakers rejecting fruit because there was smoke in the area. Or winemakers might buy a batch of fruit and later find their wines unsellable — potentially leading to disputes.

In 2020, even as fires raged nearby and the pandemic took hold, enologist and analytical chemist Anita Oberholster of the University of California at Davis 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. Many believe, for instance, that turning on sprinklers to wash the grapes during smoke exposure will help — it doesn’t. “These compounds get into the grapes so quickly,” 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, Oberholster says. She recommends that growers make small-scale batches of wine to test whether grapes will yield smoke taint.

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 the skin. These are commonly used on cherries, apples and other produce already, but the researchers have had only 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.”

A longer version of this article originally appeared in the Front Matter section of the Proceedings of the National Academy of Sciences.