1, air oxidation: boiled green bean soup put a long time, the color will become darker and darker, and even gradually become red, this is because the polyphenols in the mung beans are oxidized and converted to quinones, which produces a color change.
2, metal utensils: In addition, because of the role of flavonoids and metal ions in mung beans, but also the formation of darker complexes, so with the iron pot cooked out of the mung bean soup will become red or darker.
3, the water is too hard: water quality is also an important cause of discoloration, if the local water is hard, and more metal ions in the water, but also accelerate the oxidation of the phenolics of the mung bean itself and produce complexes, resulting in soup color red.
Tips for preventing mung bean soup from turning red:
1, use a pressure cooker to cook mung bean soup
Oxidation reaction requires the participation of oxygen, the use of a pressure cooker can be very good to isolate the oxygen outside the pot, to prevent the oxidation of phenolic substances in the mung bean polymerization to produce red substances. But such green bean soup needs to be consumed quickly, or when it is exposed to air, it will still undergo a slow oxidation reaction and eventually turn red.
2, add a little white vinegar or lemon juice to the water
Experimentation found that, with different pH water cooking green bean soup, the degree of discoloration will be different. Acidic tap water after cooking discoloration is very small, slightly alkaline tap water after cooking discoloration is very rapid. So adding white vinegar, citric acid and other acidic substances to the water can effectively stop the discoloration of green bean soup.
3, with pure water to cook green bean soup
Separately with tap water, mineral water, pure water to cook green bean soup, the result is the pure water cooked out of the color of the greenest, and the long-term does not change color. Tap water to cook green bean soup color changes the fastest, in contact with the air after almost every minute are significantly darker, and soon turned red