Sterilised milk is milk that has been heated severely — typically around 120°C for several minutes, or held just above 100°C for twenty to thirty minutes — to destroy all pathogenic bacteria and the great majority of other microorganisms and spores. The treatment gives the milk a long shelf life at room temperature as long as the bottle or carton stays sealed, which made it popular before refrigeration was universal and in regions with unreliable cold chains. The severe heating changes the milk noticeably: it takes on a cooked, slightly caramelised flavour and a creamier colour from Maillard reactions between lactose and proteins, and some heat-sensitive vitamins are reduced. Once opened it must be refrigerated and treated like fresh milk.