Why is mung bean soup red

The turning red of mung bean soup is mainly related to the oxidation of polyphenols in mung bean skin. Prolonged cooking time, alkaline water quality, or contact with metal utensils may accelerate this reaction. The key factors affecting the color change of soup include the degree of bean skin damage, mineral content in water, heating temperature, and storage time. Green bean skin is rich in polyphenol oxidase, which promotes the combination of polyphenols and oxygen to form quinone compounds during prolonged high-temperature boiling, leading to the polymerization of red pigments. The water quality in northern regions is generally hard, with high levels of calcium and magnesium ions. Alkaline environments can significantly promote polyphenol oxidation reactions. When cooking in an iron pot, iron ions combine with tannic acid in mung beans to produce reddish brown substances. When the skin of fresh mung beans is intact and the cooking time is controlled within 20 minutes, the soup color usually remains emerald green. If distilled water or a small amount of lemon juice is added to adjust the pH, it can effectively inhibit oxidation reactions. Traditional clay pots or glassware are more conducive to maintaining stable soup color than metal cookware. The refrigerated mung bean soup will gradually turn orange red as the oxidation degree deepens with prolonged storage time.

It is recommended to use newly cooked mung beans from the same year for quick cooking. After the water boils, reduce the heat to maintain a slight boiling state to avoid excessive stirring that may cause damage to the bean skin. It can be paired with white ingredients such as lilies and lotus seeds to enhance the visual effect. When drinking, add rock sugar or honey for seasoning according to your constitution. People with gastrointestinal deficiency and cold should drink it warm, and diabetes patients should control the amount of sugar added. It is best to boil and drink green bean soup freshly in summer, as the nutritional value of overnight green bean soup will be significantly reduced.