Because food itself, or water and air all contain trace amounts of lead. Therefore, strictly speaking, there are no lead-free preserved eggs, only low-lead preserved eggs.
Lead-free preserved eggs can be eaten. It is not lead-free, but the content of lead is much lower after the improved technology. Although it has little impact on the health of adults, it is best for children to eat less or not.
In order to promote the solidification of protein, some heavy metals such as lead oxide or copper should be added to the traditional preserved eggs during the curing process. If they are eaten for a long time, the lead or copper in them will accumulate slowly and be harmful to health. Nowadays, the pickling process of preserved eggs has been improved, and lead oxide has been replaced by copper sulfate and zinc, hence the name "lead-free preserved eggs". It is best to add seasonings such as vinegar, ginger and garlic when preserved eggs, which can remove the alkaline taste of preserved eggs and have the function of sterilization and disinfection. Preserved eggs should not be stored for a long time. Preserved eggs stored for too long will evaporate too much water. Preserved eggs with evaporated water will be as hard as rubber and difficult to digest after eating.