Yogurt, yoghurt, or yoghourt (/ˈjoʊɡərt/ or /ˈjɒɡət/; from Turkish: yoğurt; other spellings listed below) is a food produced by bacterial fermentation of milk.The bacteria used to make yogurt are known as "yogurt cultures". Fermentation of lactose by these bacteria produces lactic acid, which acts on milk protein to give yogurt its texture and characteristic tang. Worldwide, cow's milk, the protein of which is mainly casein, is most commonly used to make yogurt.