gutter oil is a general concept, which refers to all kinds of inferior oils, generally including swill oil, frying waste oil, food and waste oil produced by related enterprises. There are two kinds of gutter oil: broad sense and narrow sense. The narrow sense of gutter oil is the regeneration of inferior oil in catering industry collected from sewers in various ways and refined by different methods. In a broad sense, gutter oil means recycled oil, including pan-oil and fried old oil used in restaurants.

On the one hand, we can make a preliminary judgment through sensory inspection, that is, the color and smell of gutter oil, while to accurately identify gutter oil, we must measure its physical and chemical properties, and we can further judge gutter oil by testing indicators such as water content, specific gravity, refractive index, saponification value, acid value, carbonyl value, peroxide value, iodine value, heavy metals, relative unsaturation of fatty acids, cholesterol, residue detection and oxidation products.