Science of Resveratrol

Resveratrol (3,5,4'-trihydroxystilbene) is a polyphenolic phytoalexin. It is a stilbenoid, a derivate of stilbene, and is produced in plants with the help of the enzyme stilbene synthase.

It exists as two structural isomers: cis- (Z) and trans- (E), with the trans-isomer shown in the image. Trans-resveratrol can undergo isomerisation to the cis- form when heated or exposed to ultraviolet irradiation.[citation needed]

The amount of resveratrol in food substances varies greatly. Ordinary non-muscadine Red wine contains between 0.2 and 5.8 mg/L, depending on the grape variety, whilst white wine has much less - the reason being that red wine is fermented with the skins, allowing the wine to absorb the resveratrol, whereas white wine is fermented after the skin has been removed. Wines produced from muscadine grapes, however, both red and white, may contain more than 40 mg/L.[2]. [1]

Fresh grape skin contains about 50 to 100 micrograms of resveratrol per gram.

In grapes, resveratrol is found primarily in the skin and seeds. This is particularly true for muscadine grapes, whose skin and seeds have about one hundred times the concentration as the pulp. The amount found in grape skins also varies with the grape cultivar, its geographic origin, and exposure to fungal infection. The amount of fermentation time a wine spends in contact with grape skins is an important determinant of its resveratrol content

The trans-resveratrol concentration in 40 Tuscan wines ranged from 0.3 to 2.1 mg/L in the 32 red wines and had a maximum of 0.1 mg/L in the 8 white wines tested. Both the cis- and trans- isomers of resveratrol were detected in all tested samples. Cis-resveratrol levels were comparable to those of the trans-isomer. They ranged from 0.5 mg/L to 1.9 mg/L in red wines and had a maximum of 0.2 mg/L in white wines.

Reports suggest that some aspect of the wine making process converts piceid to resveratrol in wine, as wine seems to have twice the average resveratrol concentration of the equivalent commercial juices.

"All of the muscadine table wines sampled had greater trans and cis resveratrol concentrations than any other wines sampled. The muscadine table wines varied between 9.2 and 31.9 mg/L cis resveratrol and between 4.9 and 13.4 mg/L trans resveratrol."