Egg lovers may have noticed a strange greenish layer surrounding the yolk of tea eggs, which also carries an odd odor. What exactly is this substance? Is it toxic and unsafe to eat?

Eggs contain methionine. Extended boiling breaks down this amino acid to produce sulfides. These sulfides react with iron ions inside the egg, forming dark ferrous sulfide — this is the scientific reason for the green ring on egg yolks. This phenomenon is even more common in salted duck eggs, and ferrous sulfide also forms in preserved eggs, though it is hard to spot due to their naturally dark color.

Do these dark green sulfides harm human health? First of all, only trace amounts exist on the egg yolk. Under gastric acid, they may break down into hydrogen sulfide and ferrous chloride. If no reaction occurs with gastric acid, ferrous sulfide is water-insoluble and will not take part in bodily metabolism at all. Furthermore, these tiny sulfide deposits do not hinder protein digestion in any way.

More importantly, methionine (also known as methionine) is not exclusive to eggs; it is a widespread amino acid in all kinds of food. Cystine and cysteine can also release sulfides. Taken together, nearly all foods contain these three amino acids. It is impossible to stop eating simply over this trace compound.

In addition, human digestion relies on microbial fermentation, and hydrogen sulfide is a normal byproduct — it is even one of the compounds responsible for the unpleasant smell of flatulence. The hydrogen sulfide naturally produced in intestines far outweighs the tiny amount of insoluble ferrous sulfide on egg yolks. There is absolutely no need to discard tea eggs just because of the green discoloration.

If you dislike the green ring, shorten the boiling time. Steep the eggs in tea brine overnight, and they will still absorb rich tea flavor fully.