SHANGHAI - Mayor Han Zheng on Tuesday ordered a thorough investigation into allegations that tainted and expired steamed buns had been sold in the city.

The announcement followed an investigative report by State broadcaster China Central Television (CCTV) on Monday that claimed the Shenglu Food Company in Shanghai's Baoshan district had sold expired buns that had been returned to the producer from local retailers.

TV footage shot by an undercover reporter also appeared to show workers adding illegal chemicals in various quantities to the stale buns.

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The processed steamed buns were then sold in the Hualian and Lianhua chain supermarkets in the city, according to CCTV report.

On Tuesday, Shanghai Mayor Han Zheng demanded the establishment of an investigation team to immediately probe the scandal.

He said any offenders should be strictly punished according to the law.

The mayor stressed that the whole investigation process will be made known to the public.

The Shanghai Administration for Industry and Commerce kicked off a citywide inspection of the safety of processed food on Tuesday, following its massive recall of tainted and expired steamed buns on Monday night after the CCTV story aired.

Local supermarkets have been required to carry out checks and take off the shelves any products produced by Shenglu Food Company, the administration said.

By early Tuesday, more than 2,000 steamed buns produced by the company had been impounded by the administration. The sample was sent for further analysis.

Officials from the administration said it will take further action when results of the examination are known.

CCTV also reported that workers at Shenglu Food Company had re-labeled buns that had been made two days earlier, suggesting on the labeling that they were fresh.

Illegal chemicals were also allegedly added casually, such as sodium cyclamate -- an artificial sweetener that is about 30 to 40 times sweeter than sugar -- and potassium sorbate -- a food preservative -- the report said.

None of the additives were listed on the label, which is required by law, it added.

The report claimed that yellow coloring was used to produce the so-called corn flour buns that, it said, actually contained very little corn.

During the manufacturing process, most workers did not wear sanitary overalls or gloves and worked in unsanitary conditions, according to the report.

Each day, about 30,000 buns were produced by the company and sent to 400 outlets in Shanghai, including the Hualian and Lianhua chain supermarkets, according to CCTV.

China Daily visited several supermarkets on Tuesday, including Hualian, Lianhua and Dia supermarkets, major destinations for the steamed products from the Shanghai Shenglu Food Company, and found that the tainted steamed buns had been taken off the shelves.

"The buns from Shenglu have been removed, together with other food products from that company," said a staff member at Lianhua Supermarket's North Chongqing Road Branch.

"The buns made of corn flour and black rice are especially popular among older people because they are cheaper and thought to be healthier."

There was a similar situation at other supermarkets.

"We took them off the shelves last night," said a female staff member at one. "This brand is very popular among the old people. It's terrible to see such things happen."

While the tainted and expired steamed buns are now off the shelves, consumers were still cautions.

"We ate this brand's buns very often," said a male customer in his 50s. "It's really immoral to ignore public health like this but I think the buns from other manufacturers are OK."

A shopper named Chen said changes are needed.

"I hope it's just an individual case, rather than the tip of the iceberg," said Chen. "Food safety is a very important issue and the government should not only enhance its punishment measures but educate businessmen to make money in moral ways."

Lu Yue contributed to this story.