The following contents come from Yanhuang Health Website/In layman's terms, gutter oil can be divided into the following categories: 1. In a narrow sense, gutter oil refers to oily floating objects in sewers or leftovers from hotels and restaurants, which are simply processed and refined; The second is the oil produced by the processing and refining of inferior pork, pig offal and pig skin; Third, the oil for fried food should be reused after the number of uses exceeds the specified requirements, or reused after adding some new oil to the oil. The insiders also provided some methods to identify waste oil: 1. Look at the transparency, pure vegetable oil is transparent, and the transparency will be reduced due to the mixing of alkaline grease, wax and impurities in the production process; Look at the color, pure oil is colorless, oil will have color because the pigment in oil is dissolved in oil during production; Look at the sediment, its main component is impurities. Second, each oil has its own unique smell. You can put a drop or two of oil on your palm, rub your hands together and smell it carefully while it is hot. Odd oil indicates that there is a problem with the quality, and it is likely to be smelly waste oil; You can't buy it if it smells of mineral oil. Third, take a drop of oil with chopsticks and taste it carefully. Oil with sour taste is unqualified, oil with bitter taste has been rancid, and oil with peculiar smell may be "gutter oil". Listen, take a drop or two of oil from the bottom of the reservoir, spread it on flammable paper, ignite it and listen to its sound. Products with normal combustion and no noise are qualified products; If the combustion is abnormal, it will make a "squeaky" sound, indicating that the water content exceeds the standard and it is an unqualified product; When burning, there is a "crackling" explosion, indicating that the water content of the oil is seriously exceeding the standard, which may be an adulterated product and must not be purchased. Five questions, ask the merchant about the purchase channel, ask for the purchase invoice or check the sampling test report of the local food hygiene supervision department if necessary.