China's top quality and quarantine watchdog on Tuesday ordered an investigation into a food safety scandal in Shanghai, where steamed bun makers reportedly added illegal chemicals to steamed buns to cheat customers.

Li Yuanping, a spokesman for the General Administration of Quality Supervision, Inspection and Quarantine (GAQSIQ), told Xinhua that GAQSIQ has ordered its subordinate quality supervision department in Shanghai to thoroughly investigate the case and severely punish those responsible.

"We must gravely crack down on those illegal activities and will never relent," Li said.

According to media reports, workers at a Shanghai Shenglu Food Co plant in Baoshan district relabeled buns made two days earlier with new production dates. Employees added buns that were more than a week over their expiration date and had been returned by retailers back into mixers to make a batch of "new" buns.

Local public security in Shanghai detained managers of the food company for further investigation while collecting evidence, such as chemical additives and production and sales records.

Quality supervision officials have demanded that the company suspend production immediately and recall the suspected tainted steamed buns, according to GAQSIQ.

In the reported scandal, chemicals were added in random amounts, though they did not appear on the list of ingredients as required by law. The added ingredients included sodium cyclamate, an artificial sweetener nearly 30 times sweeter than sugar, as well as potassium sorbate, a food preservative. Yellow coloring was used in so-called corn flour buns that actually contained little corn.

After media reports of the contamination, stores were ordered to take steamed buns from a major Shanghai supplier off their shelves.