The USDA Agriculture Research Service recently released a guide to flavonoid contents in various foods. From their press release:
"The flavonoids are the largest group of plant chemicals now widely studied by the scientific community because of their purported health benefits. Dietary flavonoids fall mainly into five subclasses and are found in certain teas, wines, fruits, vegetables, nuts, seeds, roots--and even chocolate. In addition to antioxidative effects, certain flavonoids are reported to have antimicrobial and possibly anticarcinogenic and cardioprotective effects. Food flavonoids include, for example, anthocyanidins in blueberries and cherries; catechins in tea, red wine and apples; and quercetin in onions."
You can download a pdf with more on flavonoids in tea and other substances from their website.
Another exciting development they recently completed on is a method of accurately measuring phenolic compounds in plants. They "identified nearly 60 phenolic components in Ginkgo biloba leaves, including many that had never before been detected in the popular herb. They also used the unique profiling method to differentiate phenolics in more than 360 other foods, such as Mexican oregano, Fuji apple peel, soybean seed, broccoli, dry beans, tea and coffee." I hope they publish a database on this new (and potentially very useful) information as well.
On a related note... I really believe in the consumption of whole, natural foods rather than the use of specific extracts from foods that are shown to be beneficial. All too often, people assume that because a group of foods shows a benefit and shares a component, that the component is the reason for the benefit. This type of illogical argument is, of course, easily derailed by the point that correlation is not causation. Who knows if one particular element is the cause? It could be any number of combinations of compounds. However, if we know that a food provides a benefit, then why not simply consume the food? This is something I have believed for some time now, but it is very clearly argued in "In Defense of Food: The Eater's Manifesto" by Michael Pollan (author of "The Omnivore's Dilemma"). If you're interested in that kind of thing, check it out.