The owner of a local hotpot restaurant and her nephew pleaded guilty Monday to using recycled cooking oil to make seasoning oil.

The restaurant owner, Zheng Pin, 39, and her nephew, Zheng Wei, 26, were both charged in Huangpu District People's Court with producing and selling food containing a hazardous substance. The court did not issue a sentence at Monday's hearing.

Police caught the Zhengs in July 2012 after receiving an anonymous tip that they were using so-called gutter oil in the food at the elder Zheng's restaurant, the prosecutor said in court.

In court, Zheng Pin took responsibility for the pair's illegal actions. "I came up with the idea in June 2012 to collect the waste oil from a restaurant kitchen and turn it into hotpot seasoning oil," she said.

Zheng Pin said that she learned the practice from hotpot experts in Chongqing. It was supposed to enhance the hotpot's flavor, while saving her time and money.

She told the idea to her nephew, who was working as a chef at another hotpot restaurant on Jinling Road East at the time. She asked him to turn the waste oil at his restaurant into hotpot seasoning oil at night after his coworkers went home.

Zheng Wei secured five kilograms to 10 kilograms of waste oil for his aunt every three to four days.

He filtered the food waste from the oil, mixed in fresh oil and seasoning, then boiled it until any water evaporated. "I learned all of this from hotpot experts in Chongqing," he said in court.

During questioning, Zheng Wei acknowledged that he knew that his aunt would end up feeding the recycled oil to her customers.

In his defense, he pointed out that he only did it as a favor to his aunt. "I did not benefit financially from any of this. I did it because Zheng Pin is my aunt and I wanted to help her improve her business," he said.

The defendants' lawyer asked the court to give his clients lenient sentences because they cooperated with police and prosecutors in the case.

Zheng Pin's lawyer argued that she never intended to harm anyone, only that she had a "weak legal consciousness."

The prosecutor in the case argued that the defendants knew that gutter oil was unclean and unsafe.

"If they themselves would not eat it, why should they be able to serve it to their customers?" the prosecutor said.