Mung bean soup reacts with some iron vessels and it will turn red because the prepared mung bean soup oxidizes and discolors the iron vessels. Ferrous and trivalent iron solutions are red, so mung bean soup cooked in iron containers will definitely turn red.

This has a lot to do with the quality of the water used. The Food and Nutrition Laboratory of China Agricultural University used tap water, mineral water, purified water and distilled water to make green bean soup. The results show that the color of green bean soup varies from one water quality to another. Distilled water is the greenest and will remain the same for a long time. The second type is purified water and mineral water. The color of green bean soup cooked in tap water changed the fastest. Obviously, it darkens almost every minute after coming into contact with air and quickly turns red. Therefore, it is best to use purified water when cooking green bean soup. What I boil in purified water will be green. Today I boiled it in tap water and it would be red and it would not taste as good as if I had boiled it in pure water. First of all, when green beans are boiled in cold water they turn red, and when the water is boiled the green beans turn green. My practical experience is that green beans are always boiled directly in cold water, but when boiled in plain water they must be green. Second, cooking time affects the color. My experience is that I rarely count the time. I just turn off the cooking of green beans by observation, but plain boiled green beans are still green, so it has nothing to do with time.

The fact is that green bean soup, whether red or green, is normal. The reason why green beans turn into red soup is because the substances in the green beans have changed. Of course, even if it turns into red green bean soup, there is no harm in drinking it. After the green beans are cooked, the phenolic substances in the green beans will react with the oxygen in the air, and the phenolic substances will turn into quinones, so we can see that the color of the green bean soup is red. The cooked soup is still green, which means there is no reaction. Green bean soup is green as long as it is not exposed to oxygen or less exposed, but over time it will turn red.

However, when cooked under normal conditions, both red and green soups are nutritious and harmless when they turn red. It can be consumed without fear.