Shanghai has been making progress in swill oil testing, as a local government research institute has identified a chemical substance which can help experts tell accurately and quickly whether cooking oil was dredged from gutters.

The breakthrough works by spotting and measuring a benchmark chemical - triglyceride oxidation polymer. Local scientists discovered that in swill oil, its concentration on average is nine times higher than that in the normal cooking oil. Even more of the chemical is present if the swill oil went through more refining procedures, according to the Shanghai Institute of Food Science.

At the moment, China has no national testing standard for swill oil, or a means to identify it. The Ministry of Health has been collecting testing methods nationwide, and the application of the city's breakthrough is likely to be adopted by the national authority to help with drafting the standard.

Sun Wei, an official with the institute, told Shanghai Daily yesterday that the testing method has not been applied in routine checks, because the research is ongoing and they hoped to find another marker or two to identify swill oil.

"Results would be more accurate if we had more indices," he said.

He also revealed that the institute has already developed a two-minute testing method to identify swill oil. The accuracy of the quick method is over 70 percent, according to their lab statistics.

The institute, which is also the only place in China that keeps samples of swill oil, has collected more than 1,000 samples of various types. Most of the samples were provided by insiders, ranging from refinery businesses to five-star hotels.

Swill oil, which has no specific definition, usually refers to oil refined from eatery drains and the oil being repeatedly used. The price of swill oil is about 8,300 yuan (US$1,230) a ton, while the standard soybean oil is sold for more than 10,000 yuan a ton.

The profit margin would be much larger if swill oil was added to sesame oil as the price for sesame oil is about 28,000 yuan per ton.

"It won't be easily caught if people added some swill oil into the sesame oil which is darker in color and with stronger smell compared to cooking oil," according to the institute.

In Shanghai, two companies are allowed to deal with waste oil and transfer it into biodiesel. But the companies have a hard time collecting enough waste oil due to its lower purchasing price of 4,500 yuan a ton, as the manufacturers of swill oil offer several hundred yuan more to collect the eatery drains.

Last month, Chinese police detained 32 people for allegedly making and selling tons of cooking oil dredged from gutters.

The criminal network, which covered the production, distribution and sales, operated in 14 provinces, including Zhejiang, Shandong and Henan. Six underground factories were sealed and over 100 tons of swill oil were confiscated.