From China Digital Space
dìgōu yóu 地沟油
Cooking oil "refined" from leftover food; also known in English as "gutter oil." Leftovers from home cooking and restaurants are usually fed to pigs, but some unscrupulous people will gather the slop and "recycle" it by selling it as low-cost cooking oil. Refining ditch oil is illegal and unhealthy; it can be carcinogenic and contain hazardous chemicals.
Investigations starting from 2008 have shown that ditch oil is still widely used. In 2011, authorities revealed that up to one tenth of cooking oil used in China might actually be ditch oil. In June 2015, China Times reported that regulations of the collection and refining of cooking oil in Beijing have backfired by encouraging the formation of oil collection monopolies which skirt health and safety requirements.
Ditch oil is symbolic of China's ongoing food safety issues. A 2012 study by the Pew Research Center found that 41% of Chinese considered food safety a very big problem for the country, up from just 12% in 2008.
Yifushitang (@一夫食堂): Since using ditch oil also incurs costs, why not add a little less? It's not easy eating out. (May 25, 2015)