A vendor arranges tofu on her stall as she waits for customers at a market in Huaibei, Anhui province.

SHANGHAI (Reuters) - A criminal gang in eastern China has sold almost 100 metric tons (110.23 tons) of toxic tofu onto the local market, the latest in a string of scares that have thrown light on shady practices in the country's food industry.

The gang added industrial bleaching agent rongalite to make dried tofu sticks brighter and chewier, the Shanghai Daily reported on Monday, citing official media in Shandong province. Rongalite is banned in food production as it can lead to cancer.

Gut-wrenching food scares erupt regularly in China and highlight the challenges firms face to control supply chains.

A supplier to KFC parent Yum Brands Inc and McDonald's Corp came under fire in July after a report showed workers using expired meat. U.S. retailer Wal-Mart Stores Inc recalled some donkey meat products in January after tests showed they contained traces of fox. 

Media reports made no mention if the tofu factory had supplied any large local or global chains operating in China.

The local Qilu Evening News newspaper said the gang, led by three cousins, had bags of rongalite powder stacked against the walls of the factory.

The chemical was then added into the mix to make the dried tofu sticks, also called "fuzhu", a popular snack in China.

"The factory floor was filthy and there was a choking odor in the premises. Workers were busy making fuzhu with utensils that were covered in dirt," police official Zhang Qinchao told the local paper.

Police detained four men involved in the operation and seized nearly 10 metric tons of toxic tofu, but said a further 100 tonnes had already been sold by the gang within Shandong and nearby Henan and Jiangxi provinces.

No one has yet been reported ill from eating the affected products, the report said.