Police have detained 32 people in a nationwide crackdown on "gutter oil", the Ministry of Public Security said in a statement on Sept 13.

"Gutter oil" is illegally recycled cooking oil, often scooped up from sewage drains and gutters behind cooking establishments and then sold to restaurants. Sometimes the oil does not end up in the gutter. Restaurants will just sell their used cooking oil after collecting it in buckets and the term "gutter oil" is used to describe all illegally recycled oil.

More than 100 tons of "gutter oil" were seized after the exposure of a criminal network spanning 14 provinces, including Zhejiang, Shandong and Henan, the ministry said.

The crackdown is part of a campaign to improve food safety following several scandals, including infant formula contaminated with melamine, an industrial chemical, and pork tainted with clenbuterol, a banned chemical that makes pork leaner but is harmful to humans.

"This is the first time that we busted a major network and destroyed an underground industry that collected, sold, refined, and distributed illegal cooking oil," Liu Shaowu, director of the ministry's security management bureau, said.

The operation dated back to March, when police began to investigate after receiving reports from residents in Ninghai county, Zhejiang province, that people were purchasing used oil from local restaurants.

In late March, Zhejiang police detained six people for buying and refining used cooking oil, and learned that they usually sold it to traders from Jiangsu and Shandong.

After further investigation, Gelin Bio-Energy Co Ltd in Jinan, Shandong, came under police suspicion for processing and manufacturing oil to be used for cooking that was extracted from discarded cooking oil.

On July 4, police from Zhejiang and Shandong launched a joint operation and detained nine people, including Liu Liguo, general manager of the company, and seized 70 tons of illegal oil at the scene.

The police then detained another group of five people, led by wholesaler Yuan Yi, an alleged trader in "gutter oil" in the food markets of Zhengzhou, capital of Henan province. They seized 100 barrels of the illegal oil that had already been labeled with fake brand names and another 30 tons in bulk tanks.

Police uncovered six underground workshops and two illegal production lines in the province, according to the ministry. Another 12 people were detained for selling and distributing "gutter oil".

Liu Liguo, the prime suspect, admitted to reporters in Dinghai county detention house in Zhejiang that the oil is dangerous.

The wholesale price for legitimate cooking oil is around 10,000 yuan ($1,560) a ton. But "gutter oil" sells for around 8,000 yuan to 9,000 yuan.

"We purchased the used oil from the collectors for 5,500 yuan to 6,000 yuan a ton, and after refining, we sold it for 8,100 yuan to 8,300 yuan to the wholesalers," Liu said.

Liu Shaowu from the Ministry of Public Security, said police are stepping up vigilance.

"Before the end of year, we will order police to fully investigate clues leading to any illegal cooking oil cases, and crack down on any workshops, factories or dens to ensure food safety and guarantee public health," Liu said.

Dai Peng, a professor at the Chinese People's Public Security University, said that it was difficult to capture people involved.

"Those involved in 'gutter oil' were hard to detect. The suspects hid in different regions, making it difficult for police to seize them in one operation.

"On top of this, various administrations, such as health, industrial and commercial departments, were not supervising properly, leaving loopholes for 'gutter oil' manufacturers."

Unscrupulous restaurant owners tried to make a quick profit from their used oil as authorities couldn't help them dispose of it efficiently, Dai said.

Wang Weibiao, deputy director at the public security bureau in Ningbo city, Zhejiang province, said that it is hard to tell the two kinds of oil apart.

"It is difficult for police and residents to distinguish real edible oil from illegal cooking oil by color and appearance.

"There are no viable technical standards to identify 'gutter oil'," he said.

People convicted of producing or selling toxic or harmful food face life sentences, said Hong Daode, law professor at China University of Political Science and Law.