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Unless foods are treated very soon after harvest or slaughter, most will deteriorate rapidly due to the action of bacteria and other micro‐organisms. This microbial deterioration usually manifests itself by the typical signs of putridity, fermentation or mouldiness, but most hazardous are those bacteria that can grow to dangerous concentrations without obvious signs that they have done so. The main aim of food preservation processes is to create an environment unsuitable for microbial growth or, with processes such as canning, to destroy the micro‐organisms in the food and to pack the food so as to prevent subsequent ingress by other organisms. Micro‐organisms will not grow at very low temperatures, in low water concentrations, in high acid, salt or sugar concentrations or in the presence of certain chemicals. Thus, freezing, dehydration, pickling and salting are effective in preserving foods. Heat also destroys micro‐organisms and is used in canning and bottling and in pasteurisation, which is a partial preservation process.