Wildfires are tainting wine grapes, and the industry is at risk

Smoke taint is the undesirable ashy, smoky aroma of wines produced from grapes that have been exposed to wildfire smoke while ripening. Smoke taint is the unpleasant ashy, smokey aroma that wines produce when grapes are exposed to wildfires while they are ripening.

British Columbia is already ahead of the 10-year average in terms of total hectares burnt and number forest fires. These fires are mostly located near the Okanagan Valley in Canada, a region that is known for its wine production.

Wildfires are a constant threat to Okanagan grape and wine producers.

I am an analytical chemist who has been researching how wildfire smoke affects wine from areas affected by forest fires.

Smoke taint is a stealthy danger

Smoking salmon or clothing can impart a wonderful aroma. Wines are not affected by smoke in the same manner.

Wine grapes readily absorb the compounds that produce smoky smells. However, once they have been absorbed, enzymes transform them into forms that cannot be detected by taste or smell. The problem is that the yeasts that are used to ferment wine can regenerate the original smoky smells.

The wine industry in B.C. is under threat from smoke. Most tainted wine smells bad and is unsellable. The worst part is that the odours are often not detected until wineries have spent money and time harvesting and fermenting tons of grapes, which may smell fine.

Willamette Valley Vineyards, Turner, Ore., has a lot of barrels of Chardonnay after a California winemaker canceled the contract in 2019 due to smoke-tainted wildfire wine. (AP Photo/Andrew Selsky)

The economic impact of smoke contamination is difficult to quantify but may be significant. The Australian wine industry, for example, has suffered losses up to $300 million Australian dollars due to bushfires smoke. This is equivalent to C$276 million. While some wine tainted by smoke might be sold in B.C. as novelty items, most of it is unusable and must be thrown out.

The Chemistry of Smoke Toxicity

Smoke’s aroma is largely due to a group of compounds called volatile phenols. Volatile compounds are easily odorized because they readily evaporate even at low temperatures. Volatile phenols can be smelled at concentrations as low as the low part-per billion range. This is equivalent to adding a teaspoon of salt to an Olympic-sized pool.

The process of glycosylation is used to detoxify volatile phenols when they enter ripening grapes and their leaves.

In California, ash-covered grapes are hanging in a vineyard that was covered by smoke caused by wildfires. (AP Photo/Eric Risberg)

These volatile phenolic glycosides are not flammable and do not have an aroma. There is evidence that bacteria are responsible for metabolizing them in the mouth. This may explain why people perceive a smokey taste or smell after drinking wine.

The real problem is when the yeast that ferments the grapes breaks down the volatile phenolic glycosides. The resulting smoky smells are regenerated in grapes, which previously smelled fine. It is interesting to note that grapevine smoke does not always result in a tainted product. Sometimes, wines aged in barrels are contaminated with desirable levels of volatile compounds. We have used sophisticated analytical instruments to solve these chemical mysteries.

Can smoke taint contaminate grapes before they are harvested?

Since 2015, my students at the University of British Columbia Okanagan Campus, located in Kelowna, have led Canadian research into smoke taint.

Prior to this research, most of it was conducted by Australian researchers. We reasoned, that while Australia and B.C. both face frequent wildfires we would find that each region would have its own unique chemical evidence of smoke-taint.

We developed a method for simulating the effects of a forest on B.C. We used ponderosa needles, bark, and soil organic matter to fuel our vineyards.

The tent is used to simulate forest fire smoke’s effects on grapes. (Matthew Noestheden), author provided

We reasoned by boiling grape juice with hydrochloric and chemically simulating the yeast-induced transformations that occur during winemaking, that we could predict quickly the concentration of volatile phenols following fermentation.

In less than an hour, our research revealed that volatile phenols derived from smoke enter grapes to be transformed into non-volatile form. Since they can only be detected after boiling or fermentation in acid, we know that grapes transform these smoke-derived volatiles phenols. They are also not detected as glycosides.

Winemakers can devise ways to remove volatile phenols from grape juice and wine by identifying how grapes trap them chemically. It is important to identify these volatile phenols because the yeasts can release them from unidentified forms.

A little prevention is worth a lot

We are still working to determine how volatile phenols in grapes alter the chemical composition of the grapes. However, we’ve started to use our tools to prevent smoke taint on vineyards.

pre-screening of approved agricultural sprays showed that one product intended for cherries significantly reduced volatile phenols levels in grapes exposed to smoke at harvest. A follow-up study in three Okanagan Vineyards failed to replicate the effects.

Wesley Zandberg, James Favell and their research assistants test possible protective sprays for table grapes outside of their lab at UBC’s Okanagan Campus. (Alexander Garner), author provided

We discovered that red table grapes are available all year round in Canadian grocery shops and mimic the changes in volatile phenols seen in grapes on-the vine. We will now be able screen dozens of variables in the laboratory, without being restricted by the Canadian grape growing season.

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