Mung bean soup becomes red because over boiled mung beans, skin polyphenols will be dissolved in the mung bean soup, with the storage time, under the action of oxygen will gradually change color. From green to brownish yellow, and then gradually turn brown.

Water quality also affects the color of green bean soup, generally cooked in the north of the green bean soup is red, the south of the green bean soup is green, the difference between the two water quality is the main reason for the difference in the color of the soup.

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An expert once experimented with acidic tap water boiled mung bean soup discoloration is very small, while the slightly alkaline tap water boiled The color change is very rapid. The water in the north is alkaline and the water in the south is acidic. This is why the northern version of green bean soup is mostly red.

The cookware used to make green bean soup also plays a role, using a casserole or other airtight non-ferrous cookware to cook out of the green bean soup because of oxidation is small, but also can keep the green state for a longer period of time.