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

• Published in: Food chemistry 2013-06, Vol.138 (2-3), p.857-865
• Main Authors: Fiol, Michaela, Weckmüller, Annika, Neugart, Susanne, Schreiner, Monika, Rohn, Sascha, Krumbein, Angelika, Kroh, Lothar W.
• Format: Article
• Language: English
• Subjects: Antioxidants - analysis; Automation; Biological and medical sciences; Boiling; Brassica - chemistry; Chromatography, High Pressure Liquid - instrumentation; Chromatography, High Pressure Liquid - methods; Cooking; Curly kale; Degradation reaction; EPR spectrometry; Flavonoids - analysis; Flavonol glycosides; Folin-Ciocalteu assay; Food industries; Fundamental and applied biological sciences. Psychology; General aspects; Hot Temperature; Methods of analysis, processing and quality control, regulation, standards; Online-TEAC; Phenols; Thermal stability
• ISSN: 0308-8146; 1873-7072
• DOI: 10.1016/j.foodchem.2012.10.101

► Three kale cultivars were subjected to thermal treatment and investigated. ► During boiling, total phenolic content and total antioxidant activity remain unchanged. ► HPLC-online-TEAC revealed distinct changes in composition and activity of the flavonols. ► Degradation products compensate the loss of the antioxidant activity of the original compounds. 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 4h 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.