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Introduction
Food safety has gained worldwide attention. In China, one major food safety concern is the widespread use of gutter oil, also known as 'swill-cooked oil'. According to the second Tracking Survey Report on Food Safety in China, approximately 85% of the participants were concerned about gutter oil ([1]). A study ([2]) indicated that about two to three million tons of edible oil containing cancer-triggering substances are produced and re-used in restaurants every year in China, and approximately one tenth of the meals consumed by the Chinese could be cooked with gutter oil ([3]).
Gutter oil is a type of cooking oil recycled from the gutter, household drains and grease traps. It may also contain other wastes including reused cooking oil from restaurant fryers and kitchens, and animal fat (e.g. leftover animal parts, chicken fat, pig skins, internal organs, and expired meat, which are mainly from slaughterhouses) ([4]). Gutter oil is turned into edible oil by a series of simple processes that involve mixing, filtering, boiling, and refining. The edible oil made from gutter oil is then sold on the markets by unscrupulous merchants. Not only is it distributed to low-priced canteens and hole-in-the-wall restaurants, but also to workplace cafeterias and school canteens. Gutter oil has become a lucrative business, earning as much as 200 million Chinese Yuan (about US$331m) per year ([5]). However, long-term consumption of gutter oil may cause many foodborne illnesses because the oil may contain cholesterol, trans-fatty acids, heavy metals, and bacteria ([6]).
Incidents and regulations
The gutter oil crisis in China was first made known to the public in 2000 when a vendor was caught selling gutter oil extracted from restaurant garbage disposals ([7]). It did not create any heated debate because the authorities claimed that it was an 'isolated incident.' The public and the media started paying attention to the gutter oil issue due to an incident in 2003. People's Daily (a newspaper in Beijing) reported that two manufacturers of gutter oil were discovered by the Beijing Industry and Commerce Bureau and that over 2.3 t of gutter oil and 97 kg of illegal food were seized ([8]). Nevertheless, people still believed that gutter oil was only produced by small, localized workshops. Driven by the high profits, the...