A food safety official in Shanghai on Thursday apologized for the contaminated steamed buns scandal, pledging a thorough investigation and harsh punishment.

"We're sorry for the people of Shanghai," said Wang Longxing, director of the Office of the Shanghai Joint Inter-department Food Safety Meeting. "The relevant departments are responsible for harsh crackdowns and there won't be any lenient punishment."

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

The plant's legal representative and four other managers have confessed to illegal production and sales of 334,864 colored steamed buns worth a total of 200,000 yuan (30,600 U.S. dollars) since January of this year.

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.

Of 19 batches of steamed buns or materials in stock, four batches were found to contain a yellow coloring and two batches contained sodium cyclamate, an artificial sweetener nearly 30 times sweeter than sugar, the team said.

Shanghai's Industry and Commerce Administration ordered an inspection late on Monday, hours after China Central Television (CCTV) reported that the plant had been adding coloring to make wheat buns look like corn flour buns and black rice buns.

CCTV also reported that its workers re-labeled buns made two days earlier with new production dates and added buns that were more than a week over their expiration date, and had been returned by retailers, back into mixers to make "new" buns.

The joint team has ordered the tainted steamed buns be taken off shelves at Shanghai stores.