The reason why mung bean syrup may appear black can be explained from two aspects: ingredients and production steps.

In terms of composition:

1. If the water used is not good, it may contain some metal ions, which may react with polyphenols in mung beans to produce black precipitate, which will lead to the blackening of mung bean soup.

2. If an iron pan is used, the mung bean skin contains a substance called tannin, which easily reacts with iron to generate black iron tannic acid, which leads to the blackening of mung bean soup.

From the point of view of manufacturing steps:

1. Quick cooking: In order to improve the efficiency of cooking mung bean soup, some people may use a pressure cooker or cook it quickly in a short time. This practice may lead to the destruction of polyphenols in mung beans, increase their solubility, and finally basically all of them will be converted into black precipitates.

2. Excessive boiling: If the mung bean soup is heated continuously and boiled excessively, it may lead to the release of polyphenols or the blackening of mung bean soup.

To avoid these problems, you can try the following methods:

1. Choose pure boiled mung bean soup to reduce metal ions in water.

2. Avoid using iron pots, and choose casserole, glass pot or ceramic pot.

3. Don't overheat the mung bean soup to avoid cooking for a long time.

4. If the black precipitate has appeared, you can try to remove it with a sieve, or heat it to boiling again, and then cool it quickly to make part of the precipitate return to the solution.