NINGBO - A court in east China's Zhejiang province on Wednesday began to hear a case involving 20 people suspected of turning a profit by manufacturing and selling "gutter oil."

The suspects have been charged with manufacturing and selling fake and substandard products, as well as manufacturing and selling toxic and hazardous food, said a written statement issued by the Intermediate People's Court in Ningbo.

China's Supreme People's Court assigned the national public case to the court.

"Gutter oil" refers to recycled oil made from kitchen waste dredged from gutters behind restaurants in China.

Prosecutors said that from December 2007, seven of the defendants refined substandard oil from waste cooking oil and sold it to wholesalers as edible oil, seeing total sales reach up to 99.2 million yuan ($15.6 million), said the statement.

The wholesalers, including companies and individuals, mixed the gutter oil with edible oil and sold the mixture to retailers, reaping profits in the process.

The court did not hand down a verdict at the hearing.

The Ministry of Public Security said in April that it has stepped up a crackdown on underground workshops that produce gutter oil. The oil has been deemed unsafe for consumption, as it may contain carcinogenic substances.