Waste oil can be divided into three categories:
One is gutter oil in a narrow sense, that is, oily floating objects in sewers or leftovers from hotels and restaurants (commonly known as swill) 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 used for fried food should be reused after being used more than the specified number of times, or after adding some new oil to the oil.
When law enforcement officers check, they use the test paper for rapid detection of edible oil rancidity to compare whether the peroxide value and acid value of the detected oil exceed the standard, and once it exceeds the standard, it can be determined that it is a problem oil.