The color change is mainly caused by the oxidation of phenols in mung bean skin. In water, the polyphenols in the epidermis are dissolved out, and are easily oxidized into red quinones when in contact with air. In addition, the pH of water will also affect the oxidation of phenols. The degree of discoloration in acidic water is small, and the discoloration in alkaline water will be very severe. To sum up, if the color of mung bean soup does not change much, the first method is to avoid contact with air as much as possible, and cooking in pressure cooker is much better than cooking; The second method is to add a little white vinegar or lemon juice to the water to make it acidic.

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