Their colour seems to be indicative of their ORAC value, a measure of antioxidant activity, according to researchers.
"Beans are really loaded with antioxidant compounds. We didn't know they were that potent until now. In general, darker coloured seed coats were associated with higher levels of flavonoids, and therefore higher antioxidant activity," said Clifford Beninger, a former researcher for the USDA's Sugarbeet and Bean research unit.
The researchers tested the antioxidant activity of flavonoids, or plant pigment, found in the skin of 12 common varieties of dry beans. Findings suggest that one class of compounds in particular, anthocyanins, were the most active antioxidants in the legume.
Based on a previously published study of the anthocyanin content of black beans, Beninger found that the levels of anthocyanins per 100 gram serving size of black beans was about 10 times the amount of overall antioxidants in an equivalent serving size of oranges, and similar to the amount found in an equivalent serving size of grapes, apples and cranberries.
Human studies are still needed to confirm the link between bean antioxidants and health and until then, no one knows how many beans one must eat to obtain maximum health benefits. But the finding adds antioxidants to a growing list of healthy chemicals found in the popular legume, which is also rich in protein, carbohydrates, folate, calcium and fibre.
Antioxidants destroy free radicals, which are highly active chemicals whose excess has been linked to heart disease, cancer and aging. Black beans came out on top, having more antioxidant activity, gram for gram, than other beans, followed by red, brown, yellow and white beans, in that order.
Although only dry beans were used in the study, frozen or canned beans are thought to have similar antioxidant activity.
Full findings of the study are due to be published final issue of the Journal of Agricultural and Food Chemistry this year.