From China Digital Space
地沟油 (dìgōu yóu): ditch oil
Cooking oil "refined" from leftover food, also known as “gutter oil.” The leftovers from homes 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 because of its negative health effects; it can be carcinogenic and contain hazardous chemicals.
Ditch oil first became a known problem in 2010, when authorities revealed that up to one tenth of cooking oil used in China might actually be ditch oil. Ongoing investigations have shown that ditch oil is widely used. Ditch oil is now a symbol 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)