The cooking of fruit and vegetables always results in some loss of vitamins: water-soluble vitamins may be leached into the cooking water and some vitamins are destroyed when exposed to air. The loss of water-soluble vitamins can be very large if the vegetables are completely covered in water while being cooked - they may lose up to 80 percent of their vitamin C.
Steaming reduces this loss, but the destruction of vitamin C may be increased. Microwaving and pressure cooking both help to reduce the loss of essential vitamins, as less water is used and the cooking time is shorter.
** Asian Recipes