Police in East China have detained a sales manager at Mengniu Dairy Group on suspicion of tampering with the production dates on batches of milk and yogurt.

Wang Sunfu, 28, and an accomplice identified only as Zhao are alleged to have changed the use-by information on 5,000 boxes of dairy products distributed to retailers in Yiwu, Zhejiang province.

According to public security authorities, the duo bought the milk and yogurt products, made in December, from a worker at Mengniu Dairy for 22 yuan ($3.50) a box.

After changing the production dates to May 19 or June 1, they resold the boxes for 45 yuan each to distributors in Yiwu and nearby Pujiang county, local police said on Wednesday.

After receiving complaints about the quality of the milk products, Pujiang's commerce authority on Aug 3 discovered 160 boxes of milk in a warehouse.

After an investigation, officials suspected more people and dairy products were involved and transferred the case to police.

Wang was detained on Aug 6, while Zhao turned himself in on Monday.

The investigation remains ongoing, Ma Jianli, of the Pujiang public security bureau, said on Wednesday.

A worker at Mengniu Group's base in Hangzhou, capital of Zhejiang province, declined to comment on Wednesday. However, the company confirmed on its Sina micro blog that Wang Sunfu was one of its sales managers.

Wang's alleged behavior severely violated government and company regulations, and the company is cooperating with police, it said.

Although the changing of the production date was an individual act, Mengniu should also take responsibility, said Wang Dingmian, former vice-chairman of the Guangdong Dairy Industry Association.

"Mengniu should supervise the whole process of production, transportation and distribution. In this case, it should bear at least 30 percent of the responsibility because the expired milk products will harm consumers and changing the production date is illegal," Wang said.

Zhou Yiyi in Hangzhou contributed to this story.

jinhaixing@chinadaily.com.cn