Effect of preliminary processing, method of drying and storage temperature on the level of antioxidants in kale (Brassica oleracea L. var. acephala) leaves

The levels of vitamin C, polyphenol constituents and Trolox equivalent antioxidant activity (TEAC) were analyzed in raw kale leaves, blanched leaves and dried leaves obtained using air and freeze-drying methods. 100 g of raw kale leaves contained 683 mg vitamin C and 2236 mg polyphenols (identified using the HPLC method); the level of antioxidative activity was 71 μM Trolox/1 g dry matter. Compared with the raw material, blanching before drying brought about significant decreases of 15% in vitamin C, 32% in polyphenols and13% in TEAC. After 12-month storage, air-dried material retained 30-37% polyphenols; 43-57% vitamin C; and 41-50% of the initial TEAC level; the corresponding values for freeze-dried material were 40-47%; 50-65% and 54-66% depending on the type of sample. Freeze-dried kale leaves contained higher levels of antioxidants than air-dried material: polyphenols, vitamin C and TEAC were respectively 36%, 15%and 33% higher.