Micro-organisms such as Lactobacillus, Streptococcus lactis, and others commonly called lactic acid bacteria (LAB) grow in milk and convert it to curd at 40^oC. During growth, the LAB produce acids that coagulate and partially digest the milk proteins. A small amount of curd added to the fresh milk as inoculum or starter contain millions of LAB, which at suitable temperatures multiply, thus converting milk to curd.