Why are preserved eggs so black

Hubei livestock and poultry manufacturers introduced that the traditional preserved eggs were made from duck eggs, mixed with lime (calcium oxide), plant ash, tea powder, salt, yellow lead powder, rice husk and other ingredients, and then wrapped and pickled. The production process of modern preserved eggs has gradually evolved into caustic soda, copper sulfate and soaking process.
But as for the black of preserved eggs, it is normal. In the process of salting, preserved eggs undergo a process of protein decomposition and reorganization, and dark substances are produced due to browning, but no harmful substances are produced. As long as the lead content of preserved eggs is not more than 0.5mg/kg, they can be marked as "lead-free". In addition, if this amount is met, moderate consumption will not affect your health.
There are many black spots on the eggshell, and the lead content is high
The black spots on the eggshell are actually the deposits after the chemical reaction of "processing aids". For example, if preserved eggs are made of copper sulfate, black copper sulfide deposits will be formed after reaction, and there will be black spots on the eggshell. In fact, black spots are affected by many factors such as production temperature and curing time. So we can't simply say that there are many black spots on the eggshell, that is, the lead content exceeds the standard.
What are the differences between duck eggs and preserved eggs
Reduced vitamin B content in preserved eggs
B vitamins are acidophilic and alkaliophilic. Both vitamin B1 and B2 are significantly reduced after the formation of songhua eggs, whether in eggs or duck eggs.
The sodium content of preserved eggs increases
The sodium content of preserved eggs is 542mg/100g, almost five times that of fresh duck eggs.
Because the saltiness of preserved eggs is not obvious, a large amount of soy sauce and salt are often added when cooking, and more sodium will be added.
People who like to eat preserved eggs will find that preserved eggs have two colors, one is yellow and the other is black. Preserved eggs with different colors are only made of different raw materials. Black preserved eggs are made of duck eggs and yellow preserved eggs are made of eggs. The taste and price of the two kinds of preserved eggs are also different. Relatively speaking, the price of black preserved eggs is higher than that of yellow preserved eggs, because the price of duck eggs is higher than that of eggs. In addition, the black preserved egg has a stronger fishy smell. It is recommended to use yellow preserved egg when salad preserved egg, because the taste is lighter and it tastes refreshing. When making porridge, you can choose black preserved eggs because the taste is heavier and the porridge is thicker.