A new study provides practical guidelines for using biomarkers to detect “smoky flavor” in grapes and wine affected by wildfire smoke.

As the wildfire season in the Western United States grows longer and stronger, it is hurting the wine industry due to the impact of wildfire smoke on grape quality. Volatile compounds in smoke impart an unpleasant “smoky taste” to wines made from affected grapes.

A new study by scientists at the University of California, Santa Cruz provides valuable data and guidance on using analytical chemistry to identify smoke-affected grapes and wines. The study is based on an analysis of more than 200 grape and wine samples from 21 California and Oregon wine regions.

Lead author Phil Crews, Distinguished Professor of Chemistry at the University of California, Santa Cruz, is also a winemaker and owner of a small winery (Pelican Ranch Winery). The extent of the smoke smell problem came to his attention after the 2018 Mendocino fire, he said, when major wineries began to turn away grapes from the affected region, and Crews was hired as a consultant by law firms representing wine professionals.

Earlier at the Australian Wine Research Institute (AWRI), researchers had already done work to identify compounds that could be found in affected grapes and wine and use them as “biomarkers” for smoke odor. Given these data, Crews tried to apply Australian methods to grapes and wines from California and Oregon.

“This research is very valuable — it could save countless dollars and is becoming more and more relevant in our world of drought and climate change,” said Eleni Papadakis, wine consultant from Portland, Oregon.

“I think I speak for the entire wine community when I express my excitement and appreciation for the compelling data and evidence-based advice that Prof. Crews and his team have provided in this groundbreaking work,” she added.

Crews’ approach emphasizes the direct measurement of smoke compounds in the form they are stored in grapes. Previous studies have linked smoke odor to volatile phenols present in smoke from burning vegetation. These compounds are absorbed through the skin of ripening grapes and accumulate in the berry, where they bind with sugars to form non-volatile compounds called phenolic diglycosides.

In bound form, phenolic compounds cannot be smelled or tasted, but unpalatable free phenols can be released by enzymes, either during wine fermentation or in the mouth by enzymes or bacteria present in saliva.

“We found that phenolic diglycosides are stable in Cabernet Sauvignon during bottle aging, but then during tasting, malodorous monomers are released in the mouth,” Crews said.

It’s important to measure the directly bound phenolic diglycosides, Crews says. These large compounds are not easily detected by standard methods used to analyze aroma and flavor compounds in wine (gas chromatography/mass spectrometry or GC/MS), but can be measured using more sophisticated methods (ultra high performance liquid chromatography or quantitative mass spectrometry).

The new study presents some of the first quantitative measurements of phenolic diglycosides in premium grapes and wines from California and Oregon, including eight different varieties harvested between 2017 and 2021.

The analysis focused on six biomarkers recommended by the AWRI as representative of compounds associated with smoke odor. Representative biomarkers are needed to make testing practical, as wood smoke contains hundreds of volatile compounds. However, new results have shown that the two AWRI biomarkers are useless, and Crews recommends replacing them with others.

“There are still major gaps in our understanding of these compounds, so more research is needed. But now people can use these procedures to look at a bottle of wine or a batch of grapes and determine if it might be affected by the smell of smoke,” the chemist explained.