Police in Shanghai have arrested five managers of a steamed bun baking factory that was allegedly involved in adding illegal chemicals to steamed buns to cheat customers, Xinhua reported.

In addition, the city's quality supervision bureau revoked the production license of the Shanghai Shenglu Food Co. subsidiary on Wednesday when unsafe materials and products were found at the plant.

Shanghai's public security, industrial and commerce, and food safety supervision departments are working together to investigate the allegations against the company, which emerged in an investigative report by China Central Television (CCTV) on Monday.

CCTV claimed the company had produced steamed buns, or mantou in Chinese, from stale buns that had been returned from local retailers. The company allegedly added illegal chemicals to the expired buns to make them appear fresh.

The bureau said it tested 19 batches of steamed buns from the company and found that four contained a yellow coloring that is forbidden in steamed food and confectionery. It said it also found sodium cyclamate, an artificial sweetener, in two batches in quantities that exceeded national standards.

According to the Shanghai public security bureau, related evidence has been collected, including chemical additives and the company's production and sales records.

The company's director, Ye Weilu, its sales manager, Xu Jianming, and three staff members have been detained.

The bureau said the company, registered in Songjiang district's Tianma Development Zone, mainly produced steamed buns and flour confectionery. The company had eight workers and an annual turnover of 1.8 million yuan (US$275,000), of which steamed buns accounted for up to two-thirds.

The five suspects acknowledged that the company had produced 84,000 packs (with four buns to a pack) since January. The buns were shipped to 10 supermarket chains in the city, including Dia, Hualian and Lianhua.