Starter cultures are an essential component of all fermented dairy foods including cheese, yoghurt, sour cream and lactic butter. The primary function of these bacteria is the conversion of lactose and other sugars in milk to lactic acid. This acidification contributes to a preservative effect with the result that many pathogenic and spoilage bacteria are inhibited. The associated drop in pH also results in the loss of water from the curd as whey. In addition, starters are responsible for the production of a variety of secondary metabolites, including a number of compounds which are necessary for flavour development.