A popular restaurant has apologised to a mother and daughter who noticed that their tongues had turned black after eating a hot pot.

The amusing video of the mother and daughter picking out their blackened tongues happened on June 13 at a restaurant in Hangzhou in Zhejiang province, China.

However, they were less amused when friends suggested that it was a sign of 'nitrite poisoning' and that the relatives go to the hospital for a check-up.

In pickled food, a certain amount of nitrite can keep food fresh, not easy to spoil, and also make it bright in colour. Nitrite is used when pickling kimchi and curing bacon. A certain amount of nitrite is also produced in the human body.

However, experts at the Toxicology Department of Hunan Provincial Institute of Occupational Disease Prevention and Treatment quickly dismissed the concern, saying that if this was indeed nitrite poisoning there would have been other symptoms including blue skin and other discomfort in the rest of the body.

The spokesman noted that although the tongues of the mother and daughter in the video were black, the skin colour of their faces was normal, and they did not report having headaches or dizziness, so the initial judgment was that they were not suffering from nitrite poisoning.

Nevertheless, the hotpot eatery paid for the two to be checked at the hospital and also announced an inspection to find out why their tongues had turned black.