Harmonious: A Quick Course in Wine Chemistry
What’s in the Glass? From phenols to flavonoids, these chemists break down the chemical science of winemaking.
Published June 1, 2001
By Fred Moreno and Jill Stolarik
Academy Contributors

The label on a bottle of Cabernet Sauvignon states, “contains 13% alcohol.” Other wines contain similar amounts—give or take X%. So, what’s in the other 87%? Andrew L. Waterhouse, Ph.D., a chemist from the Department of Viticulture and Enology at the University of California at Davis, gave a quick molecular tour of the inside of the flagon. In addition to water and ethanol (alcohol), the main components of wine are organic chemicals of the phenol family. Besides grapes, berries and other vegetables also contain phenols in varying amounts.
Phenols give the wine its “body” or astringent and bitter taste, as well as its color. Red wine contains more phenols than white wine. As wine ages, the phenols undergo chemical changes, with accompanying changes in color, such as the “browning” of white wine and the deepening color of red wine over time. Phenols are made of an aromatic ring (benzene and similar compounds) to which a hydroxyl group (OH) is attached. Phenols in wine are powerful antioxidants that destroy free radicals in the body, explained Lester Packer, Ph.D., from the Department of Molecular Pharmacology and Toxicology at the University of Southern California in Los Angeles.
Don’t Peel Me A Grape
There are two types of phenols in wine: “flavonoids” and “nonflavonoids.” The flavonoids are composed of three benzene rings, and react readily, binding to other molecules. What these “other molecules” are determines the type of the flavonoid—and there are between 6,000 and 8,000 different species of flavonoids. A group of flavonoids, called the flavon-3-ols, have been well characterized in wine.
Flavon-3-ols are usually concentrated in grape seeds, stems and skin. When these parts of the grape are left in for as long as possible during the wine-making process, more flavon-3-ols end up in the resulting wine than if the seeds, stems and skin are removed earlier. One particular type of flavon-3-ol, “hydrolyzing tannin,” comes from the oak barrels in which the wine was fermented. Again, the longer the wine stays in these barrels, the more hydrolyzing tannin ends up in the final product.
The non-flavonoids in wine comprise many classes of chemicals including hydroxycinnamates, benzoates, and stilbenes. Much ado has been made in the media about the health benefits of a particular kind of stilbene, called, “resveratrol,” which is unique to grapes and is not found in other fruits or vegetables. Red wine contains more resveratrol than white wine or grape juice, though the fermentation process removes most of it from fresh grapes, where it is concentrated in the skin.
All Wines Are Not Created Equal Speaking on the diversity of wine, Fulvio Mattivi, a physician from the Agricultural Institute of San Michele in Italy, says that the relative amounts of the different flavonoids and non-flavonoids vary from one brand and type of wine to another. Even wine made from the same type of grape, raised by the same grower, in the same field, using the same method of processing, varies tremendously from one year to the next.
Aged Wine Loses Its Vitality
Dr. Waterhouse agrees. “You can’t just get a bottle of wine, study it and say, ‘This re p resents all wine,’” he says. For example, expensive wines, made from grapes that received generous amounts of sunlight, may contain four times as many flavonols as cheaper wines. Other phenols in wines can differ from each other by as much as a factor of five, due to differences in soil, growing conditions, type of grape, and processing methods.
The concentration and molecular structure of the phenols changes as grapes are processed into wine, and as new wine ages into old wine. Oxidation (the addition of an oxygen atom to a molecule) takes place, and the resulting products of the various oxidized phenols then interact with each other. Resveratrol loses glucose of its molecule. The oak barrel tannins break down and release ellagic acid. After a few months, the hydroxycinnamates begin to break down. The flavon-3-ols form bonds with other molecules and change into a form that is not found in the fresh grape.
The good news about all these chemical changes is that as wine ages, the number of anti-oxidizing molecules in it actually increases. Antioxidants purge the body of free radicals and are good for us.
The Bad News
The bad news is that, despite a rise in the increasing number of anti-oxidizing molecules over time, their bioavailability decreases. The anti-oxidizing molecules that are formed during aging are much larger than the original phenols and thus are not as well absorbed in the alimentary canal. After one or two years of aging, the amount of anthocyanin, for example, has decreased to about one-sixth of the original level. After five to ten years of aging, wine has lost most of the smaller, and presumably health-friendly, phenols. “When we look at very old wine, the only component that stands out is gallic acid,” says Waterhouse.
So, ironically, a $X bottle of 3-year-old wine is healthier drink than a $X bottle of 25-year old. (Of course, it may not taste as good!)
Also read: Bottoms Up: The Science of Alcohol