BEIJING - No pathogenic bacteria were detected in milk that sent young students to a hospital for treatment in a city of Northwest China's Shaanxi province, local food safety officials said on Tuesday.

On the morning of April 22, as many as 251 children at the Yuhe Town Central Primary School, in Yulin city's Yuyang district, fell ill after drinking school milk produced by the Mengniu Dairy Group.

By Saturday, the students had been discharged from the hospital.

The milk was purchased by the school and then distributed to the students in the morning.

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According to test results, it met China's national standards for food safety and no pathogens were detected in the milk or the students' vomit and feces.

The tests were conducted by Yulin's Center for Disease Control and Prevention.

The report said the symptoms were likely the result of the way the students had drunk the milk that morning. "After drinking the cold milk with an empty belly, some students experienced upper abdominal pain, nausea, vomiting and other symptoms of gastritis," it said.

Zhao Yuanhua, vice-president of the Mengniu Dairy Group, said there are other reasons to believe the milk was safe.

"Students at another primary school in Hanzhong, Shaanxi province, drank 700 cartons of milk from the same batch," she told China Daily on Tuesday. "None of them experienced any discomfort."

Zhao said teachers at the school have said that the vomiting, stomach aches and other symptoms had at first appeared among only 16 students and then seemed to be spread to the rest.

"That might be caused by hysteria," she said.

Mental health experts said there could be a psychological cause at the root of the symptom's spread to a large group.

"If several children began to vomit and have stomach aches, it's possible that hundreds of others may be influenced by that phenomenon and undergo a collective reaction," said Ji Xuesong, a professor at Peking University's institute of mental health.

"If some students showed symptoms like nausea and vomiting, a teacher should avoid asking if others have the same feelings," Zhao Yuanhua said. "Asking that may make more children get infected."

Zhao Yuanhua said Mengniu and local affiliate dealers oversee the distribution of 2.45 million milk cartons to 5,500 schools in China every day.

Despite the company's assurances, experts in food safety said they were skeptical of the test results.

"Why do accidents always happen at branch plants?" said Sang Liwei, a food-safety lawyer in Beijing and a representative of the Global Food Safety Forum, a non-government organization. "The milk that caused the incident was manufactured in a local plant in Baoji, Shaanxi province. One month ago, some milk withdrawn from circulation was also produced by a subsidiary plant of the Bright Dairy and Food Co Ltd in Zhengzhou (capital of Central China's Henan province)."

"It's common that branches that have been merged can't keep up with the management of a parent company," Sang said. "But businesses have to make sure their headquarters and branches follow the same standards in production lines and in handling of raw materials."