Cocoa powder and baking chocolate deliver the highest levels of the antioxidant resveratrol.  This study was conducted among 19 cocoa-containing and chocolate products on the U.S. market and published by the American Chemical Society. Scientists found trans-resveratrol levels were highest in cocoa powders.

Abstract:Dietary resveratrol has been implicated in the health benefits associated with grapes and red wine, more specifically with potential benefits for metabolic syndrome, energy use, and increased endurance. Levels of trans-resveratrol and its glucoside, trans-piceid, were determined in 19 commercially available cocoa-containing and chocolate products from the U.S. market. Amounts of trans-resveratrol and trans-piceid were closely correlated with the amount of nonfat cocoa solids in the cocoa-containing products. Among these products, trans-resveratrol levels were highest in cocoa powders, followed by unsweetened baking chocolates, dark chocolates and milk chocolates. These cocoa-containing and chocolate products have about 3−5 times more trans-piceid than trans-resveratrol. Levels of trans-piceid were highest in the cocoa powders, followed by unsweetened baking chocolates, dark chocolates and milk chocolates. On an equal weight basis, cocoa powder had about half as much trans-resveratrol as the average California red wine. On a per serving basis, cocoa-containing and chocolate products had less trans-resveratrol than red wine and grape juice but more than roasted peanuts. Overall, these cocoa-containing and chocolate products rank second after red wines and grape juice in foods with the highest levels of total trans-resveratrol in the diet.

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