Eggs that have hard yolks and can bounce like rubber after being boiled have appeared on the market in several regions across China, leading a food safety watchdog in Shanghai to launch a probe, officials said on Friday.
The investigation was launched after some consumers said they bought the 'artificial' eggs made by traders seeking profits, the China Daily said.
But food experts said the eggs were likely to be natural. Their unusual characteristics could be explained by high levels of a compound called gossypol.
When egg-laying hens eat gossypol-enriched feed, it binds to the protein in egg yolks.
Gossypol normally exists in the residue of cotton seeds added to chicken feed as an extra protein source.
Experts say large doses of the compound suppress sperm activity. Gossypol has been used in male contraceptive pills in a couple of countries.