Effect of Cooking methods on retention of vitamins and minerals in vegetables

Cooking affects the quality and bioactivity of vegetables. The effect was investigated of cooking conditions on the quality of Chinese kale subjected to either conventional boiling and steaming at 97 ± 2 °C or microwave boiling and steaming at 900 W. Cooking by either conventional or microwave conditions increased (P ≤ 0.05) the moisture content from 90.45% to 91.59–93.45%, but reduced the fiber content from 35.92 to 15.21–32.49% db and ash from 18.93% to 15.17–18.54% db. Either conditions could enhance the total phenolic content (TPC) from 1605.21 to 1634.69–2013.57 mg GAE/100 g db and antioxidant activities of Chinese kale based on 1,1-diphenyl-2-picrylhydrazyl (DPPH) and 2,2-azino-bis-3 ethylbenzothiazoline-6-sulphonic acid (ABTS) assays (P ≤ 0.05). Extending the cooking time for 1–2 min increased the TPC and scavenging activity based on DPPH assay (P ≤ 0.05). Steaming (615.75–794.72 mg TE/100 g db) by both conventional and microwave conditions produced higher scavenging activity based on DPPH assay than boiling (540.39–688.07 mg TE/100 g db). The maximum antioxidant activities based on DPPH (794.72 mg TE/100 g db) and ABTS (1592.72 mg TE/100 g db) assays were obtained from microwave steaming at 900 W for 4 min. Both conventional and microwave cooking conditions reduced the lightness from 63.03 to 46.84–51.03 and hardness from 42.27 N to 24.38–38.11 N, but increased the greenness of the Chinese kale from (-)7.11 to (-)11.37–(-)12.98. Therefore, microwave steaming could be recommended for cooking greenish vegetables due to the health benefit from the increased antioxidant activity, convenience and similar cooked food composition, color and texture compared to conventional cooking.