Abstract: Free and bound phenolic compounds were extracted chemically from kidney bean seed coats with seven different colors. The total phenolics, total flavonoids, total anthocyanins and phenolic composition of the phenolic extracts were analyzed and their antioxidant activities were determined by 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging, 2,2’-azinobis (3-ethylbenzothi azoline-6-sulfonic acid) ammonium salt (ABTS) radical scavenging and ferric reducing assays. The results showed that the contents of total phenolics, total flavonoids and total anthocyanins in the seven samples ranged from 0.76 to 48.73 mg/g, from 2.93 to 201.2 mg/g, and from 0 to 2.57 mg/g dry mass (DW), respectively. The seed coats of black kidney bean had the highest contents of total phenolics and total anthocyanins, while total flavonoids content was the highest in the seed coats of red kidney bean. Gallic acid, chlorogenic acid, catechin, 3,4-dihydroxybenzoic acid and hydroxybenzoic acid were the main phenolic compounds in the seed coats of kidney beans, and quercetin only existed in the seed coats of red kidney bean (48.64 μg/g). The seed coats of white kidney bean contained the highest content of coumaric acid (65.40 μg/g). The seed coats of black kidney bean possessed the strongest antioxidant capacity with DPPH and ABTS cation radical scavenging capacities of 156.0 mg/g and 260.8 mg/g and ferric reducing power of 214.7 mg/g, respectively. The content of total phenols in the seed coats of kidney beans was significantly correlated with antioxidant activities (P < 0.01). The seed coats of black kidney bean was rich in phenolic compounds and exhibited antioxidant activities, making it a potential ingredient for functional foods.

Key words: kidney beans, phenolic substances, antioxidant activity