Milk is converted into curd or yogurt by the process of fermentation. Milk consists of globular proteins. During fermentation, the milk sugar (lactose) produces lactic acid. Lactic acid acts on the globular proteins present in the milk and denatures them. This denaturation destroys the tertiary and quaternary structures of proteins and the globular proteins are converted into fibrous proteins thus giving a thick texture to the milk due to coagulation of the proteins.
