To prevent “egg head rot”, who is the “flower protector” of Songhua preserved eggs?

Songhua preserved eggs have a unique flavor and are a traditional food that is widely loved by the people. However, the traditional method requires the use of a heavy metal compound - lead oxide. Of course, current methods have long abandoned lead, but we need to understand what role "lead" plays in the production process of preserved eggs, so that we can understand the effectiveness of the substitutes used now.

Why is lead added to the traditional Songhua preserved egg pickling process? That's because lead can protect the preserved eggs from alkali damage. Usually, the formation of Songhua preserved eggs will go through four processes in a strong alkaline environment: clearing, coagulation, color change, and maturation. The role of lead is to protect the coagulation process from clearing.

The first step is to clear the egg white. As the surrounding alkaline substances continue to penetrate into the fresh egg through the egg pores, the protein in the egg denatures and the higher-order structure is destroyed, but the primary and secondary structures are not damaged for the time being. The change in the higher-order conformation of the protein causes the "little brother" (subunit) that forms the protein to separate, and the bound water that was previously trapped by "hand in hand" (hydrogen bonds, disulfide bonds) is released, and a large number of "water-intolerant" groups (hydrophobic groups) on the outside of the subunit are exposed. The free water in the egg white increases, and the thick egg white gradually turns into a dilute solution, which is called clearing. The second step is coagulation. As the pickling progresses, the denaturation of the protein intensifies, and the alkali solution further destroys the structure of the subunit. The "little brother" subunit (peptide chain) contains a large number of hydrophilic groups, and the water absorption is enhanced. A large number of hydrophilic groups on the surface of the peptide chain inside the subunit capture the free water again through hydrogen bonds, and each peptide chain forms hydrogen bonds with water molecules, thereby aggregating and cross-linking with each other to form a mesh gel structure. The macroscopic phenomenon seen is the coagulation of protein to form a gel. If a large amount of alkali solution continues to penetrate after solidification, it will cause the peptide chains to continue to decompose into small molecules, which can no longer bind water molecules, causing the gel to become clear again and become a "rotten egg", which is alkali damage [1].

So who is the “flower protector” of Songhua preserved egg gel? In traditional processes, it is the heavy metal “lead” that we are all afraid of. Usually, metal compounds are added to the ingredients of preserved eggs at the same time as the strong alkaline reagent. The traditional process is to add lead oxide. Protein will produce hydrogen sulfide in a strong alkaline environment, and hydrogen sulfide will migrate outward to the preserved egg shell [2]. Lead ions react with hydrogen sulfide to form lead sulfide precipitation particles, which block the pores of the egg shell and prevent excessive alkali from penetrating and destroying the preserved egg gel and causing alkaline damage.

However, the toxicity of lead cannot be ignored. Therefore, in the current process of pickling preserved eggs, people will choose safer zinc sulfate and copper sulfate instead of lead oxide. This can not only prevent the preserved eggs from being pickled into "rotten eggs", but also avoid the toxicity of lead. You no longer have to be afraid of lead when eating preserved eggs!

[1] Cheng Jing, Li Lichan, Chen Xu, et al. Mechanism of gel and flavor formation during preserved egg processing[J]. Food Industry Science and Technology, 2024, 45(5): 357-366.

[2] Sun Xinsheng, Yang Linghan. Research progress on the production technology and active ingredients of preserved eggs[J]. Anhui Agricultural Science Bulletin, 2023, 16:140-143.

Written by: Mr. Zou's Science Garden, Peng Dan, Inspection Technician, and Zou Jingbo, Chief Technician