Polyphenols In Grapes And Wine

Polyphenols or phenolic compounds found in the plant kingdom account for anywhere from 4000-8000 individual compounds (Fero-Luzzi and Serafini, 1995; Dresoti, 1996; King and Young, 1999; Croft, 1998; Bravo, 1998). These compounds are the secondary by-products of plant metabolism and have often evolved to help protect the plant from environmental stressors and predators. Consequently, in addition to grapes and wine, polyphenols are found in many commonly eaten fruits and vegetables, such as apples, grapefruit, and onions as well as teas (Fero-Luzzi and Serafini, 1995; Bravo, 1998).

The various polyphenolic substances in wine and grapes have been described over the years and efforts continue to better characterize their composition (Soleas and Goldberg, 1999; Hertog et al., 1993b,c; Lanningham-Foster et al., 1995). Overall, the total polyphenolic content of red wines has been estimated to be about 1200 mg/L (Soleas and Goldberg, 1999), while others have reported concentrations as high as 4000 mg/L, of which 500-900 mg/L come from the tannins (Dresoti, 2000). Part of the discrepancy reported may relate to different extraction techniques. In contrast, the polyphenolic content of white wine is about 200-300 mg/L (Frankel et al., 1995). It is also reported that the total flavonoid content of red wine can be 10-to 20-fold higher than in white wine (Soleas and Goldberg, 1999). Grape juice has about one half the flavonoid content of red wine by volume (Constant, 1997). The stilbene, resveratrol, is present mainly in grape skins, where concentrations are estimated to be 50-100 |J.g/g (Chun et al., 1999). Thus, resveratrol is found primarily in red wines, with concentrations around 1 mg/L (Brouillard et al., 1997). The concentrations of select flavonoids and resveratrol in wine have been summarized in a more comprehensive review (Dubick and Omaye, 2001).

It is also realized that polyphenols in aged wines may be more polymerized than those in young wines or grape juices (Bravo, 1998; Brouillard et al, 1997; Muller and Fugelsang, 1997). In addition, the amount of flavonoids in wine can be influenced by several factors, including temperature, sulfite and ethanol concentrations, the type of fermentation vessel, pH, and yeast strain, as well as the type of insecticides or herbicides used on the grapes (Muller and Fugelsang, 1997; Das et al., 1999).

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