Lipid is a general term for oil, fat and lipid. The fat in food is mainly fat. Generally speaking, liquid at room temperature is called oil, and solid at room temperature is called fat. Fat is composed of three elements, C, H and O. Fat is a triacylglyceride composed of glycerol and fatty acids, in which the molecule of glycerol is relatively simple, but the types and lengths of fatty acids are different. Fatty acids are divided into three categories: saturated fatty acids, monounsaturated fatty acids and polyunsaturated fatty acids. Fat is soluble in most organic solvents, but insoluble in water.