Lipids are oils, fats, and lipids. Fats in food are mainly oil and fat, generally called liquid oil at room temperature, and called solid fat at room temperature. Fat is composed of three elemental types: C, H and O. Fat is composed of glycerol and fatty acid triacylglycerol ester, of which the glycerol molecule is relatively simple, and the type and length of fatty acid are not the same. There are three main types of fatty acids: saturated fatty acids, monounsaturated fatty acids and polyunsaturated fatty acids. Fat is soluble in a number of organic solvents, but not in water.