Much information has been published on the physiological properties of resveratrol, the phenolic compound found in wine that has been associated with the prevention of cardiovascular disease and possibly certain cancers. But few if any studies have investigated the impact of various common fining agents in removing resveratrol from red wine.
Now, Nuria N. Tobella, a graduate student in enology at the University of California, Davis, has conducted research on that topic under the supervision of her advisor Andrew Waterhouse, professor of enology, and presented her findings to the American Society for Enology & Viticulture.
Phenols and polyphenols are complex substances found primarily in grapeskins and seeds, that give wines their characteristic aromas, flavors and mouthfeel. Because red wines tend to have longer skin contact and maceration times during the vinification process, they are higher in phenolic compounds, including resveratrol, than white wines. Phenolics are also anti-oxidants: They can protect wines from the harmful effects of air by slowing down the rate at which oxygen converts ethanol into sherry-like acetaldehydes, and they can also inhibit the growth of the bacteria that cause wine to turn into acetic acid, or vinegar.
Resveratrol, long a traditional East Asian medicine derived from certain plants, was first discovered in wine in 1992. It is found in two forms, or isomers: trans-resveratrol and cis-reserveratrol. Leroy Creasy, professor of pomology at Cornell University and the discoverer of trans-resveratrol in wine, calls the two isomers "mirror-images of each other."
The trans-isomer has long been studied in medicine; when it was found in red wine, it caused a flurry of excitement due to suggestions that it could be the ingredient in red wines responsible for the so-called "French Paradox" health phenomenon. However, trans-resveratrol is present in wines in very small amounts, and some early research suggested the quantities were not high enough to make resveratrol as protective of the heart even as common aspirin.
But it was the identification of the cis-isomer in 1993 that caused the Canadian researchers who found it to write that "current estimates of the beneficial health properties of red wine attributed to resveratrol will have to be revised sharply upwards" because the cis- isomer greatly increased the total amount of resveratrol found in red wine by as much as 400 percent.
In addition, either the trans- or cis- isomer may have a glucose, or plant sugar, molecule attached to it. If it does, it is called a piceid isomer, or the glycoside of resveratrol. Much work remains to be done to determine which of the four forms of resveratrol does exactly what in the human body, and how.
But it is known that resveratrol's glycoside isomers are extracted more at the beginning of fermentation because they are more hydrosoluble [soluble in water], while resveratrol itself is extracted more at the end of fermentation, due to prolonged skin contact.
Over the past two or three years, scientists around the world have increased the tempo of their experiments with resveratrol. Among them has been Waterhouse, who is studying resveratrol's ability to reduce the so-called "bad" cholesterol from blood, which could have enormous implications for the prevention of cardiac disease.
But it also has been determined that the precise amounts of the four kinds of resveratrol found in wines vary widely, depending on the varietal, wine region, and even the weather in different vintages. Pinot Noir and Mourvedre, for example, have been found to have (in a 5 percent solution added to distilled water) high levels of resveratrol. Vinification techniques such as cap management, yeast strain used, and the introduction of a malolactic fermentation, are known to affect the amount of resveratrol found in wine. But one of the final steps of vinification, fining, has not been looked at until now, "and that's where we started this study," Tobella says.
Tobella fined Pinot Noir and Merlot with six common fining agents: PVPP (polyvinylpolypyrolidone, added to distilled water), gelatin (in a 3 percent solution, added to distilled water and heated to 140 degrees F), egg albumin (in a 10 percent solution and refrigerated), carbon, bentonite (in a 5 percent slurry), and agar (in a 1 percent solution).
Each fining agent was pipetted into a test tube containing 25 ml of wine. The test tube was vortexed to mix the contents and then transferred to a 25 ml vial. The vial was completely filled-that is, it contained no headspace-to prevent oxidation, and also was protected from the light. The wine sample settled overnight in the cellar at 12 degrees C. The following day, the wine was decanted and filtered, and HPLC (High-Pressure Liquid Chromatography) was used to determine the cis- and trans-resveratrol and piceid isomer concentrations in the samples.
Tobella's results showed that PVPP and carbon were most effective in removing trans- and cis-resveratrol as well as trans- and cis-piceid. At the highest levels of carbon, the average residual resveratrol isomers were about 55 percent, and 80 percent for piceid. PVPP had an even greater absorption of resveratrol isomers (37 percent remaining) and a similar effect on piceid isomers (82 percent remaining). Although all four forms of resveratrol were filtered out, both agents showed a higher affinity for trans-resveratrol than for the others.
Gelatin, egg albumen, agar and bentonite, on the other hand, showed no significant effect on any of the four compounds and their concentrations "remained essentially intact," Tobella wrote.
Asked if the study had any practical implications for winemakers, Tobella said that because carbon and PVPP "are not used very much in red wines, the main conclusion is that fining doesn't have an effect [on removing resveratrol] in typical production."
If the cardioprotective properties of resveratrol
are definitively established, and then allowed by authorities
to be promoted by wineries on labels, in brochures, or elsewhere,
vintners can at least be assured that if they fine their red wines
with gelatin, egg albumen, agar or bentonite, they need not fear
removing something that, someday, could be a powerful marketing
tool.