Retention of Ascorbic Acid and Major Mineral Contents in Water Spinach and Chinese Kale after Three Different Cooking Methods

ABSTRACT

Introduction: The water spinach (Ipomoea aquatica) and Chinese kale (Brassica oleracea L. var. alboglabra) are popular in Malaysia, relatively cheap and rich sources of vitamins, dietary fibre and minerals. A study was conducted to determine the retention of ascorbic acid (vitamin C) and major minerals (zinc, iron and copper) in water spinach and Chinese kale after boiling, stir-frying and steaming. Methods: Ascorbic acid was determined using potassium iodate titration method. Major mineral contents were determined using atomic absorption spectroscopy. Results:The results showed that stir-frying retained the highest ascorbic acid level in both water spinach (64.4%) and Chinese kale (85.9 %). Zinc showed maximum retention in boiled water spinach (103.6%) while stir-fried Chinese kale retained the highest content of zinc (88.8 %). The retention of iron level in water spinach ranged from 49.1 to 101.3% while that in Chinese kale was from 42.5 to 117.5%. The highest retention of copper was obtained in steamed water spinach (93.6%) and boiled Chinese kale (106.3%). Conclusion: Stir-frying is the best cooking method to retain ascorbic acid in water spinach and Chinese kale while zinc, iron, and copper retention in both vegetables varied depending on the type of cooking. These findings could be part of consumer education on methods of food preparation for optimum nutrient retention.