The initial step in yoghurt production is homogenization of the milk, which splits the fat into small globules. The milk is then preheated at 850C for 30 min or 90-950C for 5-10 min to destroy pathogenic and spoilage organisms. This
heating also affects the physicochemical structure of the protein and the stability of the yoghurt gel. The majority of the whey proteins are denatured by the heat treatment, with α-lactalbumin completely denatured. The milk is then cooled to around 42-430C and the yoghurt starter bacteria added. The mixture is held at this temperature until the desired acidity is reached. During this period the starter organisms are involved in the development and production of flavor. L. bulgaricus provided essential amino acids for the growth of S. thermophilus, while the latter organism provided formic acid and related compounds required to stimulate L. bulgaricus growth. The formation of lactic acid during the metabolism of the starter organism is crucial to yoghurt production. It destabilizes the casein micelle, causing coagulation of the milk protein and formation of a yoghurt gel.