According to my years of experience, there are three reasons. One: It is caused by improper use of iron pots. This is because the material composition of the iron pot is black, which is easy to oxidize when it comes into contact with air. "Oxidized substances" entering the soup will make the soup black, so the iron pot must be treated scientifically before use. Two: In addition to the pot, it is caused by the use of inferior "seasoning bags". Most of the current "Huainan beef soup" is not the traditional one. Many of them are bought directly from the market stalls (said to be the "beef soup set meal", but actually the "pot-stewed meal meal" of ordinary pot-stewed vegetables) or "shanzhai". Thirdly, in order to increase the taste of beef soup, additives such as "beef essence, delicious king, beef essence" are used to discolor the soup. Ordinary beef "seasoning package" is not effective, and it is difficult to make authentic and delicious taste. Some stores use these "additives" sold in the market to adjust the soup to narrow the taste difference with "authentic Huainan beef soup", but these "additives" often make beef soup black!