This is because of the variety of lettuce, which belongs to purple leaf lettuce. Purple leaf lettuce is brightly colored and has higher nutritional value than ordinary leaf lettuce and is favored by people. Its nutritional value cannot be ignored. Because it is rich in antioxidants, vitamins, fiber and other elements, regular consumption can help cleanse the liver and promote gallbladder. The purple pigment of the leaves of this variety is easy to disappear under high temperature, but the chlorophyll contained in it is not easy to disappear, so it turns green after being cooked.