FLAVONOIDS


Flavonoids are naturally occuring phenolic compounds in plants. The antioxidative effect of flavonoids had long been recognized. They have been reported to inhibit lipid peroxidation, to scavenge free radicals and active oxygen, to chelate iron ions, and to inactivate lipoxygenase. Catechin and quercetin, flavonoid pyrocatechol derivatives, possess high antioxidant activity and have been used to stabilize lard. Some flavonoid pyrogallol derivatives, such as tea tannins, also possess antioxidant activity.

Polyphenols are the most abundant group of compounds in fresh tea leaves and are found in green and black tea beverages at 30-42% and 3-10% of the total dry matter, respectively. It has been reported that the average intake of all flavonoids was 23 mg/day, and the most important source of flavonoids was tea (48% of total intake). All tea extracts (green tea, pouchong tea, oolong tea, and black tea) exhibited marked antioxidant activity, reducing power, and scavenging effects on active oxygen and free radicals.

However, plant phenolics have sometimes been found to show pro-oxidant properties. Several flavonoids have been shown to autoxidize and generate reactive oxygen species, such as hydrogen peroxide. They are also capable of reducing Fe3+ to Fe2+ and resulting in the formation of hydroxyl radicals by reacting Fe2+ with H2O2.

Coffee and chocolate products contain a range of polyphenolic antioxidants known as flavonoids, which may have health benefits such as prevention of heart disease. In terms of polyphenol content, it was found that coffees are lower than teas and wines, but higher than just about anything else you can think of in beverages (like fruit juices). Because Americans drink a lot of coffee, it represents their top source of antioxidants from food. It was found that polyphenol levels in milk chocolate, dark chocolate, and cocoa powder are extremely high on a weight basis. A 40 g milk chocolate bar contains more than 300mg of polyphenols; dark chocolate has more than double that; and cocoa powder has about four times that.

The amount of polyphenols in a serving of dark chocolate is comparable to that in a cup of black tea and higher than in a glass of red wine (things we normally think of as great sources of antioxidants). So chocolate is extremely high in polyphenols compared to other foods.