Take a look. 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.

The second taste. 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.

Three flavors. 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 when necessary. In addition, based on the relationship between metal ion concentration and conductivity, the content of metal ions in oil can be judged by detecting the conductivity of oil. Many experiments show that the conductivity of waste cooking oil is 5 to 7 times that of first-class edible oil, so it can accurately identify waste cooking oil.