1, the result of oxidative polymerization of phenols dissolved from mung bean skin in mung bean soup under the action of oxygen in the air. Therefore, it is easy to change color when cooking mung bean porridge with open cover, but it is not easy to change color when cooking mung bean porridge with pressure cooker.

2. Boiling mung bean soup is also related to water quality. The higher the alkali content of water, the heavier the red color. For example, the water in the north is hard, the nitrile analogy is more, and mung bean soup tends to turn red when cooked. If the local water quality is slightly poor, tap water is not needed. Mung beans can be boiled in pure water or boiled in boiling water, but tap water is not needed.

3. If the pot is covered, the mung bean soup may turn red if it is stuffy for too long.

4. If you use aged mung beans, the boiled mung bean soup must be red.

5. Too much water and too little mung bean can easily turn red.

6. The container for cooking mung bean soup is stained with oil, which is easy to cause the phenomenon of "red soup".

7. Cook mung bean soup in an iron pot, and ferric oxide is used to generate trivalent iron, so it is red.