Raw mung beans put a long time is generally not red, unless spoiled, then the whole appearance is very obvious changes. Mung beans will only be boiled into mung bean soup may become red, this is because the skin of the mung bean contains a phenolic substance, when this chemical in the oxidation of oxidative polymerization occurs, will lead to a change in the color of mung bean soup. If the raw ones turn red, it is obvious that they are not edible.