Since the fires began to spread in Northern California, the famous wine country has been devastated as the flames continue to threaten the areas around Napa and Sonoma. Many known wineries, hotels, and restaurants have already suffered severe damage, while many won’t know the full extent of damage until after the evacuation order has lifted. Several fires have ravaged more than 50,000 acres of land in multiple wine producing counties.
The fires have not only affected physical areas, but also possibly this year’s grape harvest.
The smoke from wildfires can cause something called “smoke taint”. This is due to volatile phenols that are found in wildfire smoke. The vines can absorb these compounds, and transfer them to the grapes, causing unwanted flavors in the wine. According to ETS Laboratories, who partner with winemakers to help apply science to produce better wine, these off-flavors can be described as “smokey”, “bacon”, “campfire”, and “ashtray”, and can have long lasting and lingering taste on the palate even after the wine is swallowed or spit out.
Smoke taint became a real concern for California vineyards after the wildfires of summer 2008, so ETS developed an analytical tool to screen grapes for the risk of smoke taint. According to their site, “the analysis measures trace levels of free guaiacol and 4-methylguaiacol in whole berries.”
Having knowledge of the levels of the two main volatile phenols in smoke, guaiacol and 4-methylguaiacol, in the berries, allows for the winemakers to assess the risk of smoke taint and adjust the course of action for their wines, if needed.
However, Northern California wines may have gotten lucky this time, because an estimated 90% of grapes had been harvested just before the start of the fires.
According to Napa Valley Vintners, a nonprofit trade association, “it is too soon to determine the economic impact of this disaster, what is known is of the grapes remaining on the vine when the fires began, most were thick-skinned Cabernet Sauvignon. Wineries that could safely access their vineyards continued to pick their grapes. For those grapes picked after the fires started, winemakers are diligently inspecting them and having wines made from the laboratory-tested grapes for the possible effects of smoke.”
Depending on volatile phenolic compounds (VPs) present, wine can possess negative organoleptic properties, with sensory descriptors, such as the off-putting flavors mentioned earlier. The evaluation of smoke exposure is a significant quality control issue for wineries as the concentration of VPs may increase during fermentation or aging.
The compounds outlined in Figure 1 demonstrate that the VPs associated with smoke exposure are structurally similar, with several existing as positional isomers. This structural similarity can make their chromatographic resolution challenging. The VPs discussed are suitably volatile, such that they are amenable to GC separation. It was hypothesized that the use of hydroxyl groups around a polar WAX-type stationary phase would provide an advantageous refinement in positional isomer resolution when compared to more traditional bonded silica GC phases.
The polyethylene glycol Zebron-WAX GC phase resolved all critical pairs of VPs, including the very challenging cresol positional isomers (Figure 2. A). Chromatographic resolution is required for accurate quantitation of all three cresol isobars, as their structural similarity results in identical product ions when analyzed by GC-MS/MS.
Similar chromatographic resolution of the cresols was not feasible in a reasonable time scale using a 5%-phenyl GC phase, despite using a 60m column (Figure 2. B). In fact, resolution of m/p-cresol was never observed on the 5%-phenyl column, despite running extended gradient and isothermal methods. The obtained resolution of m/p-cresol on the Zebron-WAX column will facilitate the accurate characterization of VPs in smoke exposed grapes (Figure 3).
As further climate research models predict increasing frequency of brush fires, it is critical to expand our current understanding of this economically important phenomenon. Areas prone to these fires are doing their due diligence to ensure that their products are of highest quality. This will hopefully aid the development of more accurate models of predicting wine quality issues when using smoke-exposed grapes and may also inform remedial and preventative strategies.