Effects of different cooking methods on physicochemical and bioactive compounds of selected green vegetables in northeastern region, Bangladesh

This study evaluated the effects of three cooking methods, including boiling (100^o C, 5 min), steaming (7.5 min), and microwave (900 W, 1 min) on physicochemical characteristics and bioactive compounds of three widely consumed winter vegetables in Bangladesh, such as green bean (Lablab purpureus), cabbage (Brassicaoleracea var. capitata), and mustard leaf (Brassica juncea). The ascorbic acid (AA) content was decreased significantly after all cooking treatments. In general, the AA retained by different vegetables was highest after microwave cooking and lowest after boiling. Cooked vegetables contained more β-carotene than fresh vegetables, and maximum retention was observed after steaming. However, total flavonoid contents were decreased slightly for all treatments. With the exception of cabbage, both steaming and microwave cooking enhanced significantly (P<0.05) the polyphenol content (TPC) in all vegetables. The highest TPC was found in the green bean during boiling (72.5 mg GAE/100g), and the lowest was found in cabbage with steaming (35.58 mg GAE/100g). There is a significant increase in antioxidant activity after boiling and steaming, except microwave cooking of cabbage and mustard leaf. However, the lightness (L*) of all vegetables decreased for each cooking method, but the greenness intensity increased after microwave cooking. Overall, cooking can alter the food's physical and nutritional content but depend on vegetables and the method of cooking.