Flavonoids are defense compounds that the grapevine synthesizes mainly in response to UV stress and, like the other phenols, have important biological properties, since they possess high antioxidant activity that prevents the formation of free radicals. However, in the specific case of one of these flavonols, quercetin, it has been observed that it can give rise to a defect in wines by forming insoluble precipitates both during cellar storage and in the bottle. This phenomenon is more frequent in high quality wines rich in polyphenols.
On the other hand, a recent study by the University of California, Davis, has also shown that the quercetin present in red wines could interfere with the correct metabolism of alcohol and cause headaches in certain particularly sensitive subjects.
According to this study, the headaches and reddening of the face suffered by some wine consumers could be attributed to a dysfunctional variant of ALDH2, the enzyme that metabolizes acetaldehyde, which would lead to its accumulation. Red wine contains much higher levels of quercetin and its glycosides than white wine or other alcoholic beverages. This work has shown that quercetin-3-glucuronide, a metabolite of quercetin, inhibits ALDH2 with an IC50 of 9.6 µM (IC50 is a measure of the inhibitory capacity of a specific biological activity). Red wine consumption has been observed to produce comparable levels. Therefore, this work hypothesizes that quercetin-3-glucuronide, derived from the various forms of quercetin present in red wines, inhibits ALDH2, resulting in elevated acetaldehyde levels and the subsequent occurrence of headaches in susceptible subjects.
The authors believe that the key to the explanation of this age-old mystery has finally been found. The next step is to test it through clinical trials in people who develop these headaches.
Quercetin is produced by grapes in response to sunlight. If grapes are grown with highly exposed bunches, much higher levels of quercetin are obtained. In some cases, they can be four to five times higher. In view of the problems posed by the presence of quercetin in wines, several recent studies have attempted to study operational solutions in the vineyard and winery to reduce its presence or increase its stability.
A study carried out within the framework of the Vintegro project and recently published in this journal details the state of knowledge and presents the results of the trials carried out to reduce quercetin content (aglycone and glycoside) using different types of oenological products (fining agents, yeast derivatives, enzymes, polyphenols and other products), simulating winery practices and applying different bottling methods:
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