Acid is one of the five flavors. The taste is sour, sweet, bitter, spicy and salty. There are sour, bitter, sweet, spicy and salty in the five elements of Chinese medicine.
The acidic seasoning used by ancient people about 6,000 years ago is the natural plum fruit.
"Shang Shu • Telling Life" records: "If you are a peacemaker, you are only salt plum." This means that if you cook "hehe" (dish name), you must use salt and plum pulp.
The plum pulp is made by using the plums and then taking the juice.
Therefore, the pre-Qin ancient books commonly used the phrase "salt and salty plum acid".