This is related to oxidation, but oxidation is also related to many conditions, such as using an iron pot to make it red, but using an casserole is gone. And old mung beans will turn red.

Open the lid and cook-red, oxidized.

Cooking with cover-green, oxygen-deficient, no oxidizing substances.

No matter how many mung beans are put in, when the mung bean soup is just boiled, turn off the fire and you will find that the mung bean soup you cooked is green. You put it in a bowl, and when it cools, you will find that the original green mung bean soup in your bowl turns red slowly with time. The mung bean soup in the pot is gradually turning red.

Nowadays, most people cook with rice cookers, but some people at home use iron spoons (my home is a two-piece set of long iron spoons, and every time I stir with the spoon that comes with the rice cooker, it will be fine, but it will turn red with metal).

Cooked for too long, it will turn red easily when boiled dry and added with water.