Police in China have arrested 32 people in an operation to prevent the sale of illegally reprocessed cooking oil.
More than 100 tonnes of oil were seized in raids across 14 provinces.
Some of it had been collected from drains behind restaurants to be sold on.
Six underground factories were found to be producing the oil, which some scientists say can cause cancer.
One firm, which was supposed to be turning kitchen oil into fuel, was selling it as fresh cooking fat.
Gutter oil, as it is known, is well-named, says the BBC’s Michael Bristow in Beijing, with some of it collected by dredging the drains behind restaurants. The name is now used for any cooking oil that is illegally recycled.
The raids took place following a four-month police inquiry.
“This case, through a difficult process of investigation… not only struck down a criminal chain of gutter oil producers, but also uncovered hidden details of the offenders’ greedy and unconscionable production of poisonous and harmful cooking oil,” a ministry of public security statement said.
The sting operation comes more than a year after Chinese state media reported that up to one-tenth of cooking oil was made from recycled waste oil.
The trade has been a problem in China for years – the business is said to be very profitable because of the low costs of the waste oil and refining process.
There have been a number of nationwide campaigns to stamp out the illegal trade.
Scandals over contaminated food have caused considerable public alarm in China in recent years.
In the most serious case in 2008, milk products mixed with the industrial chemical melamine caused the deaths of at least six infants and nearly 300,000 fell ill.