Biomarkers unveiled: novel method for smoke taint detection in grapes and wine

As the summer heat rises, the frequency of wildfires escalates, leaving a detrimental impact on the global wine industry. Volatile compounds present in the smoke from these fires can be absorbed by grapes, yielding a distinctly unpleasant flavor in wines made from such fruit, a phenomenon known as "smoke taint."

A recent study spearheaded by scientists at the University of California, Santa Cruz provides valuable data and guidelines on utilizing analytical chemistry to identify grapes and wines affected by smoke taint. The study, published on March 3, 2023, in the Journal of Natural Products, draws from the analysis of over 200 grape and wine samples from 21 viticultural regions in California and Oregon.

The study's principal investigator, Phil Crews, a distinguished research professor of chemistry at the University of California, Santa Cruz, and owner and winemaker of the boutique winery, Pelican Ranch, was motivated to delve into the matter after the 2018 Mendocino Complex fire.

Crews lamented, "I discovered that the appropriate analytical data were not being provided to determine if grapes or wines were affected by smoke." His expertise was sought by law firms representing wine professionals after large wineries began refusing grapes from the fire-affected region.

The most enlightening research on the issue, Crews found, had been carried out at the Australian Wine Research Institute (AWRI). There, researchers had identified a set of compounds detectable in smoke-tainted grapes and wines, to be used as "biomarkers." Realizing that most American labs serving the wine industry were not conducting adequate measurements, Crews designed his study to apply the Australian methods to grapes and wines from California and Oregon.

Crews' approach emphasizes the direct measurement of smoke-derived compounds as they are stored in the grapes. Volatile phenols present in vegetation smoke are absorbed by the skin of ripening grapes and accumulate there, forming non-volatile compounds called phenolic diglucosides.

Standard methods used to analyze aroma and taste compounds in wine (gas chromatography/mass spectrometry, or GC/MS) do not easily detect these large compounds. They can, however, be measured using more sophisticated methods (ultra-high-performance liquid chromatography, or UHPLC, and quantitative mass spectrometry).

The new research offers some of the first quantitative measurements of phenolic diglucosides in premium grapes and wines from California and Oregon, encompassing eight different varieties harvested from 2017 to 2021. The results include reference data for normal grapes as well as grapes exposed to six different levels of natural forest fire smoke.

The analysis focused on six biomarkers recommended by the AWRI as representative of compounds associated with smoke taint. Representative biomarkers are needed to make the testing practical, as wood smoke contains hundreds of volatile compounds. Yet, new findings suggested two of AWRI's biomarkers were unhelpful, leading Crews to recommend replacing them with different biomarkers.

"There are still huge gaps in our understanding of these compounds, so more research is needed," stated Crews. "But people can use these procedures now to examine a bottle of wine or a batch of grapes and determine if they are likely to be affected by smoke taint."

At his research lab at the University of California, Santa Cruz, Crews focuses on the chemistry of tropical marine sponges and the identification of new compounds active against cancer and other diseases. For the smoke taint study, he collaborated with a commercial testing lab, SC Laboratories in Santa Cruz, which has the necessary staff and equipment for large-scale analysis.