To answer this question, we must first understand what "gutter oil" is: gutter oil in a broad sense is all discarded cooking oil, and in a narrow sense is gutter oil fished out of sewers and sewers. However, because it is impossible to have standard products, there is no clear definition of gutter oil so far. It is difficult to distinguish from the appearance and senses, and the diagnosis can only be made by five detection methods of the Ministry of Health.

Secondly, the complexity of gutter oil ingredients has endless possibilities like permutation and combination, which also makes the research into a maze. "The gutter oil is usually mixed with a variety of substances, and may even be a variety of mixed oils. For example, the mixture of corn oil and soybean oil, vegetable oil and animal oil, the mixture of many substances is accidental. It is difficult to find substances that must exist and only exist in waste oil as testing standards. " At this time, the national standard of edible oil GB27 16 has long been invalid. Because after refining, the acid value, peroxide value, heavy metal content and other common indicators of waste oil can reach the standard.

In addition, even though the special research on gutter oil began more than ten years ago, there is still a lack of gutter oil experts in China, and the understanding is not deep enough. For example, whether the detection of waste oil can be quantitative or qualitative is still controversial by many experts. "The so-called experts must have both theoretical knowledge and long-term practical experience, but there are very few experts or research institutions that have reached this level in China, and it is very difficult for many units and institutions to sample. It is naturally difficult to conduct effective research. "