BEIJING, Apr 26 (AP) -- More than 50 children were poisoned by a kindergarten breakfast in central China, state media said Thursday, in the latest case highlighting problems in the country's food supply chain.

Xinhua News Agency said the children were hospitalized after eating breakfast Wednesday at a private kindergarten in Zhengzhou city in Henan province.

Thirty have been released and the others remain under observation.

Xinhua said doctors believe soybean milk given to the children was not boiled properly.

Mass poisonings are common in China, which has been struggling to improve a dismal food safety record. Manufacturers often mislabel food products or add illegal substances to them. Cooks routinely disregard hygiene rules.

Last week there were three cases in various parts of the country involving about 280 people.

In the latest incident, the person in charge of the private kindergarten and three cooks who prepared the breakfast have been detained by police, Xinhua said.