Some supposedly edible cooking oil on store shelves in Beijing, Tianjin and Hebei province is being produced from so-called gutter oil and swill-cooked oil, Chinese media have reported.

Swill-cooked oil is waste animal oil and oil that has been used several times - usually by restaurants - for frying.

According to Beijing Times, some producers of the illegal cooking oil that is made from the gutter oil and swill-cooked oil are processing nearly 100 tons of their substandard and possibly dangerous oil each day. Their advanced refining equipment and production techniques make it difficult for buyers to tell which oil is produced properly and which is really processed garbage.

The newspaper said the oil produced out of swill-cooked oil flows to food processing businesses and into wholesale markets through illegal channels and even onto supermarket shelves.

An insider told the paper that the raw materials for the illegal oil include swill, oil that has been repeatedly fried, leftover pieces of pork from slaughterhouses and poultry fat. The oil is then blended and bleached.

Statistics from the capital's management agency in charge of the city's appearance said some 1,750 tons of food waste is produced in the city every day and kitchens produce 60 tons of waste oil daily. The agency said the city can only properly process 600 tons of food waste each day.

The person in charge of an illegal cooking oil plant in Tianjin said his factory processes more than 30 tons daily and he said several other local factories have similar capacities.

Industry insiders said the main equipment needed is large cans and boilers, which are connected with pipes. The oil becomes clear when refined and filtered and is then packaged as cooking oil.

According to the paper, reporters saw cylindrical cans that were 10 meters tall and 3 meters wide in illegal cooking oil plants in Tianjin and Xingtai city, Hebei, producing oil that was brighter than tea after impurities were removed with calcium bicarbonate and acidity neutralized with alkali.

Reporters from the newspaper sent samples of the "recycled" oil from several illegal plants in Tianjin and Hebei to the China National Food Safety Supervision and Inspection Center, and two bottles of the samples unexpectedly met the standards of the general indicators of edible vegetable oil and animal oil.

Wang Ruiyuan, vice-chairman of the oil branch of the Chinese Association of Cereals and Oils, said no effective way to detect illegal cooking oil has been found to date.