Why does mung bean porridge turn red?

Every summer, especially in midsummer, people like to cook mung bean porridge. That's because mung beans have the effects of quenching thirst, relieving heat, clearing away heat and detoxifying, and are a must-have consumer product for many families to cool off. In addition, mung beans are extremely rich in nutrients, making mung bean porridge a soup with high nutritional value. Mung beans are green, but mung bean porridge is often red. Why is this? Can you still drink the red mung bean porridge?

Why is mung bean soup red?

Turns red on contact with air

Exposure to air. When cooking mung bean soup, uncover the pot and stir it from time to time. This is the main reason why the mung bean soup turns red. This is because the polyphenols dissolved from the mung bean skin undergo oxidative polymerization under the action of oxygen in the air, thus forming red substances. Therefore, it is best to boil the mung bean soup to the bottom of the pot to avoid contact with air.

Turns red after cooking in an iron pan

The flavonoids in mung bean skins may form darker complexes after reacting with metal ions, causing the mung bean soup to change color. Although this reaction does not produce toxic substances, it may interfere with the antioxidant effect of mung beans and hinder the absorption of metal ions. Therefore, when cooking mung bean soup, using an iron pot is the least suitable, and a casserole is the most ideal.

Cook with honey to change color

When honey is added to the mung bean soup and cooked together, the minerals in the honey will form a complex with the polyphenols in the mung bean, and the colors are mostly sauce color, black, brown, dark green, etc., making the color of the mung bean soup dark and black. It is recommended not to cook the mung bean soup with honey, but you can add a little when eating.

Discoloration after boiling in alkaline water

Studies have found that the acidity and alkalinity of water can affect the color of mung bean soup. Acidic tap water changes color little when boiled, while slightly alkaline tap water changes color very quickly after boiling. The tap water in the north is mostly alkaline water, so the color of the mung bean soup changes very quickly when cooked. You can see an obvious color change within two minutes. It is recommended to use pure tap water to cook it.

Turns red after adding edible alkali

Mung beans contain a variety of B vitamins. Adding citric acid or salt to the mung bean soup can enhance the antioxidant effect. Some people add edible alkali to make the mung bean soup thicken quickly. However, the addition of edible alkali will reduce the antioxidant effect, and alkali will seriously destroy a variety of B vitamins. At the same time, the flavonoid antioxidant components in mung beans will be lost due to the addition of alkali, the structure will change, and the color will turn yellow.

Can I still drink mung bean soup after it turns red?

Mung bean soup can still be drunk after it turns red.

Mung bean soup can still be eaten after it turns red, and it does not have any toxic effects. This is because the anti-heat active ingredients of mung beans are mainly polyphenol antioxidants. If the mung bean soup turns red, the heat-relieving effect will not be good and the nutrients will not be as comprehensive, but it will not affect consumption.

How to cook mung bean soup without changing color

Cook with purified water

So, will the mung bean soup turn red when cooked? Actually not. The experiments we conducted have proved that mung bean soup can be kept for a long time without changing color. The secret lies in the fact that the water used to cook the beans is different.

When using tap water, mineral water, purified water and deionized water to cook mung bean soup respectively, the result is that deionized water has the greenest color and it remains unchanged for a long time. The color of mung bean soup cooked with tap water changes the fastest. It becomes noticeably darker almost every minute after coming into contact with air and soon turns red. Obviously, it is due to the different water quality.

We adjusted tap water to different pH values ​​to cook mung bean soup and found that the acidity and alkalinity can greatly affect the color change of the soup. Acidic tap water changes color very little after boiling, while slightly alkaline tap water changes color very quickly after boiling. The tap water in the north is mostly alkaline, so the color of the mung bean soup changes very quickly, and you can see a clear color change within two minutes. However, for deionized water, the color change is minimal, regardless of the pH.

In this way, you have a simple method to cook mung bean soup without changing its color - just cook it with pure water and there will be no problem.

Add some vinegar when cooking

If there is no purified water at home, it is not impossible to keep the green color of mung bean soup by cooking it with tap water. Just add half a spoonful of white vinegar or squeeze in half a spoonful of lemon juice when cooking the mung bean soup, and the mung bean soup will keep its green color for a longer time in the air.

The key is not to add too much, and don't let the water become sour, just adjust it to pH 6.

When cooking, it is best to prepare the water first, and then cook the mung bean soup, so that it will not change color easily. Even if stored for half an hour, it can still maintain its beautiful green color.

Cover the pot, cook quickly and drink quickly

When cooking mung bean soup, you should cover the pot to minimize the contact area with oxygen and reduce oxidation. Moreover, boiling water takes a shorter heating time than cold water and causes slightly less oxidation; cooking in a pressure cooker reduces exposure to oxygen, which is beneficial to maintaining the color of the soup. However, they will still change color quickly once the soup is scooped out.

At the same time, the cooking time should also be paid attention to. When cooking mung bean soup daily, it is difficult for them to maintain their yellow-green color because they are cooked for too long and are in sufficient contact with oxygen. You may as well take out the soup that has been boiling for less than 10 minutes and drink it as soon as possible, because at this time the color of the bean soup is emerald green, and the dissolved substances are mainly the active ingredients in the bean skins, and it has the lowest degree of oxidation and the strongest heat-clearing ability.

After taking out the soup, do not throw away the remaining mung beans. You can add boiling water to continue cooking them into mung bean paste, or add rice to make mung bean porridge.