Bu Qingfeng, head of Huikang company, said in court that the oil his company sells is vegetable and edible oil, but he is not aware of how the company contacted Liu or decided to buy gutter oil from him.

In February, the Supreme People's Court, together with the Supreme People's Procuratorate and the Ministry of Public Security, issued a statement on the crackdown of the production and selling of gutter oil. People making or selling gutter oil as food oil can be sentenced to life imprisonment or even death if their products kill or severely injure consumers.

Prosecutors also exhibited the assessment on Liu's gutter oil by Beijing Food Supervision Center to further prove that it was poisonous. The assessment shows the sample contains polyaromatic hydrocarbons, excessive acid and cholesterol.

Yan Weixing, a researcher at the China National Center for Food Safety Risk Assessment, said that kitchen waste such as cooked oil may contain excessive acid value and cholesterol and even carcinogens.

Excessive acid value means that the oil has gone bad, he said, and if the oil contains cholesterol, it shows that the oil has animal fat, which is common in leftovers.

"The production and selling of gutter oil as food oil should be stopped at its source," Yan said.

"Some manufacturers blend gutter oil with edible oil and make it undetectable. They can even make gutter oil that almost feels the same as normal edible oil. As a result, to catch those responsible red-handed is very important."

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wangqingyun@chinadaily.com.cn