In mechanised production, skimmed milk is mixed with dried skimmed milk and heated at 82–93ºC for 30–60 min to destroy contaminating micro-organisms and to destabilise 178 Food processing technology K-casein. It is inoculated with a mixed culture of initially S. thermophilus and L. bulgaricus. Initially S. thermophilus grows rapidly to produce diacetyl and lactic, acetic and formic acids. L. bulgaricus possesses weak protease activity which releases peptides from the milk proteins. These stimulate the growth of S. thermophilus. The increased acidity then slows the growth of S. thermophilus and promotes L. bulgaricus, which is stimulated by formate produced in the initial stage. L. bulgaricus produces most of the lactic acid and also acetaldehyde which, together with diacetyl, gives the characteristic flavour and aroma in yoghurt. Details of the production are described by Davis (1975) and Tamime and Robinson (1999).