Phenolic acid contents of kale (Brassica oleraceae L. var. acephala DC.) extracts and their antioxidant and antibacterial activities

Nine phenolic acids were identified and quantified by HPLC–MS in leaves and 10 in seeds of kale (black cabbage). The free, ester (methanol-soluble), glycoside and ester-bound (methanol-insoluble) phenolic acid contents of the leaves were 487, 532, 4989 and 6402 ng/g fresh weight, respectively. Ferulic and caffeic acids (total contents; 4269 and 4887 ng/g, respectively) were the most abundant. The seed contents of these fractions were 1993, 1477, 1231 and 4909 ng/g dry weight (DW), respectively, and sinapic acid was the most abundant (5037 ng/g DW). The fractions’ total phenolic contents, determined colorimetrically, were highly correlated with their DPPH scavenging capacity, and in antimicrobial activity assays, with nine test organisms representing a wide array of taxa, all of the fractions were effective against Staphylococcus aureus, Enterococcus faecalis, Bacillus subtilis and (most strongly) Moraxella catarrhalis. Antimicrobial and antioxidant activities of kale phenolics in free and conjugated forms are discussed.