Influences of viticultural and oenological factors on changes incis‐andtrans‐resveratrol in commercial wines

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Abstract
During fermentation of black grape juice in the absence of skins, minor losses of trans‐resveratrol occur over a 63‐day period accompanied by a modest increase in concentrations of cis‐resveratrol which can only account for a tiny fraction of the latter seen in most red table wines. In the presence of skins, two extraction patterns were noted: a rapid increase in trans‐resveratrol over days 2–4 followed by a plateau and a delayed increase of cis‐resveratrol starting 1 day later, or a gradual increase in both isomers reaching a peak around day 8–10 followed by a decrease. A comparison of Ontario wines from the 1992 and 1993 vintages demonstrated marked variations in the resveratrol isomer content of Pinot Noir and Gamay Noir, but other cultivars such as Merlot and Cabernet Sauvignon showed little change. Many white grapes had higher skin concentrations of trans‐resveratrol than red grapes but cis‐resveratrol could not be detected in the skins of any grapes tested. Variations of four to six fold in skin concentrations were observed among the same cultivar grown in different districts of Niagara. Ageing in oak barrels and treatment with charcoal caused major losses of both isomers, but treatment with a 0.75 μm filter pad halved the trans‐resveratrol concentration without affecting that of cis‐resveratrol. Bentonite, gelatin, silica, egg white and diatomaceous earth had no effect on either isomer.