Thermal-induced changes of kale’s antioxidant activity analyzed by HPLC–UV/Vis-online-TEAC detection

Generally, boiling of vegetables is assumed leading to lower nutritional values because of leaching effects and activity loss of bioactive compounds. Kale (Brassica oleracea var. sabellica) reveals a great diversity of flavonoids, which have been shown to be good antioxidants. As Brassica vegetables are mainly consumed cooked, the influence of boiling on kale’s flavonoids and their antioxidant activity was investigated. Therefore, three kale cultivars were cooked at 100 °C for 2 and 4 h prior to analysis. The total phenolic content (TPC) and the total antioxidant activity (TEAC assay and EPR spectrometry) of each cultivar were determined and revealed no change, independent of cooking time, although kale samples visually altered. Using the HPLC–UV/Vis-online-TEAC approach, distinct changes in composition and antioxidant activity of the flavonoids were detectable. Thus, it was observable, that the antioxidant activities of the reaction products compensated the “loss” of the antioxidant activity of the original compounds of the uncooked material.