Vegetables contain a lot of VC, most of which belong to soluble VC. Contact with air will produce a chemical reaction, which will lead to a large loss of VC and a decline in nutritional value. Solution: when cooking, you can add a little oil to the water to cover the surface of the dish, so that VC will not be lost. You can also add a little alkaline water to the water to change the pH value of the water, and the cooked food will be brighter in color and will not change color for a long time.