Haɗin kai wani tsari ne na rayuwa wanda ƙananan ƙwayoyin cuta kamar yisti ko ƙwayoyin cuta ke canza sugars da sauran mahadi masu sauƙi zuwa mahadi masu sauƙi kamar barasa, acid, da gas. Haɗin barasa yana nufin jujjuyawar sukari zuwa ethanol (giya) da carbon dioxide ta yisti ko wasu ƙananan ƙwayoyin cuta.
