Curd is formed by a chemical reaction which involves casein, the protein that occurs in milk. The coagulation of this protein results in the formation of curd. This occurs if bacteria called Lactobacillus acidophilus is present in milk. This bacterium produces lactic acid by the fermentation of the milk sugar lactose. The lactic acid so formed by the bacterial action contains positive hydrogen ions which are attracted to the negative particles of the protein casein. As the latter are neutralized, these protein molecules no longer repel one another but coagulate. The optimum temperature at which Lactobacillus acts is around 40°C. So milk is usually warmed to this temperature before setting it to curdle.
