Rancidity is the chemical decomposition of fats, oils and other lipids. There are three types of rancidity. Hydrolytic rancidity occurs when water splits fatty acid chains away from the glycerol backbone in glycerides. Oxidative rancidity occurs when the double bonds of an unsaturated fatty acid react chemically with oxygen. Microbial rancidity refers to a process in which micro-organisms such as bacteria use their enzymes, including lipases, to break down chemical structures in the fat. In each case, these chemical reactions result in undesirable odours and flavours.