The bicarbonate of soda turns the water slightly alkaline, which helps to preserve the chlorophyll in green vegetables. It also breaks down the tougher outer layer of beans and peas to make them more tender. However, this method can result in losing some nutrients in the peas so, if you are cooking green vegetables to use cold in a salad, it is best to plunge them into iced water as soon as they are cooked. This will stop the cooking process and stop the vegetables turning a dull gree colour.