Why does the egg shell turn brown or green?

□ Science Times reporter Shi Qi

Recently, a film and television actor mentioned in an interview that when he was filming in another place, he heard that the local green-shelled eggs were "very nutritious", so he went to eat them. Unexpectedly, he vomited and had diarrhea after eating them. Are green-shelled eggs more nutritious or have they gone bad?

Yun Wuxin, a popular science writer on food health, said: "The color of the eggshell is determined by the hen's genes and has nothing to do with nutritional value." It turns out that the "coloring" of the eggshell depends on the pigmentation deposited in the hen's body. If there is no pigmentation, the eggshell will appear white, which is its main component - calcium carbonate. If there is protoporphyrin pigment deposition, the eggshell will be brown, which is commonly known as a red shell. If there is biliverdin deposition, the hen will lay eggs with blue eggshells. If there is both protoporphyrin and biliverdin deposition, the eggshell will appear green.

"Even if the eggs are laid by the same hen, the nutritional composition of different eggs is not exactly the same. It is impossible to make valuable inferences about the nutritional composition of eggs based on the color of the eggshell," said Yun Wuxin.

The color of the egg yolk has nothing to do with nutrition. If the hen eats more lutein, the egg yolk will be yellow, and if the hen eats more astaxanthin, the egg yolk will be red. From the statistical data, the only information that the eggshell color can reveal is that darker eggshells may be thicker and less likely to break during transportation.

As for the controversy over green-shelled eggs causing food poisoning, Yun Wuxin said that the egg white and yolk in green-shelled eggs are the same as those in ordinary eggs, but the person involved mentioned that "the egg white is also green", which means that the eggs are likely to have gone bad. "In addition, it may not be due to the eggs, but to other foods eaten during that period," Yun Wuxin said.