Boiled in pure water, it is not easy to change color. China Agricultural University Food Nutrition Laboratory boiled mung bean soup with tap water, mineral water, purified water and distilled water. It was found that the color of mung bean soup cooked with different water quality was different. Distilled water is the greenest color and remains unchanged for a long time. Followed by pure water and mineral water. Mung bean soup cooked in tap water has the fastest discoloration. After being exposed to the air, it will obviously darken almost every minute and turn red soon. Therefore, it is best to boil mung bean soup with pure water.

A few drops of lemon juice is helpful for mung bean soup. It was found that the pH value also greatly affected the discoloration effect of the soup. Tap water in the north is alkaline water, which changes color very quickly after boiling mung bean soup. After pouring out the mung bean soup, you can see the obvious color change within two minutes. However, after adding vinegar or lemon juice to tap water, the color of the boiled mung bean soup changed little. Therefore, if you cook mung bean soup with tap water, you can add half a spoonful of white vinegar or squeeze a few drops of lemon juice in the water, but be careful not to add too much, otherwise the soup will turn sour and affect the taste.

Boil, add water and cook for 8 minutes. The method of cooking mung bean soup is to boil water first, then add mung beans, continue to cook for 8~ 10 minutes on low heat, and then pour out the mung bean soup. At this time, because the color of the soup is turquoise, the dissolved substances are mainly the effective components in the bean skin, with the lowest oxidation degree and the strongest heat-clearing ability. After the soup is poured out, the remaining beans can be boiled into green bean paste or rice into mung bean porridge.