Milk is a natural food that all baby mammals live on. It comes from the breasts of the baby’s mother. It contains all the essential constituents a baby needs. Milk is a mixture of water, fats, sugar, starch, protein, vitamins and minerals. People use milk obtained from many animals, such as the cows, buffaloes, goats, camels and even reindeer. Milk is used to make many other products like curds, butter, cream, cheese and ice cream. When a small amount of curd is added to milk, it turns into curds within about 4 hours in summer and 12 hours in winter. Do you know, what turns the milk into curds? Milk contains a protein called ‘casein’. This protein gives milk its characteristic white colour. It is of high nutritional value because it contains all the essential amino acids required by our body. Curd is a result of a chemical reaction between lactic acid bacteria and casein. When curds are added to milk, the lactic acid bacteria present in it cause coagulation of casein and thus, turn it into curds. Man has been using curds for a long time. In the ancient times, curds are believed to have been sold as a medicine. Curds are found helpful in the digestion of food. Bacteria found in the curds help clean up the digestive system. According to some experts the regular consumption of curds keeps the stomach free from ailments. Curd is used in different ways by different people. Some eat it with salt while some others with sugar. In western countries, flavoured curd is called ‘yoghurt’. In West Bengal, sugar is added to milk before converting it into curd, which is known as ‘mishti doi’. In Maharashtra, a special kind of sweet, flavoured curd, called ‘shrikhand’ is very popular.