As wildfires continue throughout California’s wine regions, thoughts obviously turn first to the risks to human safety. Indeed, dozens have died since the year’s first fires in the spring, and as the fires continue and destroy thousands of acres of property, the disaster has been humanitarian above all.
But there is also the cost to businesses in the area to take into account. Particular in their sensitivity to fire are California’s wine regions, and for a surprising reason—while a winery may be fortunate to be bypassed by the direct damage of heat and fire, the exposure to smoke presents a particular threat to the finished product, which may only be detected after production has finished. Whole batches of wine may be produced without knowing whether the end product will be damaged by smoke, a result that only reveals itself after proper fermentation.
In a coincidentally well-timed paper published by the Journal of Agricultural and Food Chemistry in September, University of Munich Professor Wilfried Schwab and a team drawn from the Professorship for the Biotechnology of Natural Products explored the source of “ash” or “ashtray” flavors in wine exposed to forest fires.
“The smoke volatiles (phenolics) are absorbed while the grape berries are still attached to the plant,” Dr. Schwab explains to Food Quality & Safety. “In the grapes, the phenolics are transformed to non-volatile metabolites (a sugar molecule is attached). During fermentation, the volatile phenolics are released again due to the activity of the yeast.”
The reason for this is that aromatic substances are attracted to sugar, which can retain them and release them later on through a process called glycosylation. Dr. Schwab’s team has revealed that an enzyme called glycosyltransferase binds aromatics to sugar molecules—which in the case of wine production, bonds the aroma of smoke to the sugar of the fruit and leaves of the grapevine. Glycotransferase is normally called upon to process a substance called resveratrol that exists in the grapevine, but the smoke-aroma molecules are unfortunately similar in their structure to resveratrol, so smoke aroma is processed in the place of that substance. However, there is no indicator of this damage in grapes on the vine. It is only after adding yeast that the sugar molecules finally separate once again, revealing the smoke odor.
However, it remains mysterious precisely what relationship exists between smoke damage to winery grapes and physical exposure to fires themselves.
“In our experience, close(r) proximity to a fire does not always automatically translate into an increased concentration of smoke taint compounds in grapes or wine,” notes Dr. Markus Hererich, Group Manager for Research at the Australian Wine Institute. “Similarly, smoke exposure as such may or may not lead to uptake of taint related volatile phenols (as smoke composition is variable and is a function of the type of fire, fuel, and environmental conditions such as UV/sun light and atmospheric chemistry which all influence the concentration of volatile phenols in smoke. In summary, production at risk is most likely a fraction of the grapes which yet have to be harvested from smoke exposed vineyards.”
Dr. Herderich says that while his expertise is not in California grapes, it is reasonable to presume that vines and grapes close to the California fires are responding to smoke exposure through the uptake and glycosylation of volatile (aromatic) phenols.
Unfortunately, Dr. Schwab says, “The producers can probably not avoid the uptake of the smoke chemicals into the cells (unless they can stop the fires), but they could choose yeast strains which are unable or at least show a reduced activity to release the smoke volatiles from the non-volatile metabolites.”