Effect of Processing on Antioxidants and Their Activity in Dietary Fiber Powder from Cabbage Outer Leaves

It is well known that Brassica vegetables (e.g., cabbage) contain significant amounts of bioactive compounds with high antioxidant activities. Outer leaves of white cabbage (Brassica oleracea L. var. capitata), a typical by-product from a cabbage processing plant, have the potential of being transformed into dietary fiber powder with high levels of antioxidants. However, losses of health-beneficial bioactive compounds in cabbage leaves may occur during processing and should first be identified. This work was aimed at studying the effects of various processing steps, that is, sample preparation, which included sample slicing and blanching using either hot water or steam, as well as hot air drying, on the evolutions of various phytochemicals in cabbage outer leaves. The dominant antioxidants, that is, phenolics (measured in terms of the total phenolic content, TPC), vitamin C, -carotene, -tocopherol, and their total antioxidant activity (TAA), were assessed both during sample preparation and drying. The results showed that slicing prior to blanching led to higher losses of antioxidants during either hot water or steam blanching. Water blanching led to lower retention of water-soluble antioxidants; namely, TPC and vitamin C. No significant changes in the contents of -carotene and -tocopherol were observed after blanching. The results further illustrated that all antioxidants degraded significantly during drying. This resulted in approximately 50% retention in -carotene and less than 40% retention in TPC, vitamin C, and -tocopherol. The final powdery products exhibited a TAA, as assessed by 2,2-diphenyl-2-picryl-hydrazyl (DPPH) and -carotene bleaching assays, of approximately 70%.