Dear student, Curds are a dairy product produced by bacterial fermentation of milk. Bacteria that is used for this purpose is Lactobacillus delbrueckii. This is a kind of bacteria which can convert a sugar into an acid by means of fermentation. Milk contains a sugar called lactose, when milk is heated to a temperature of 30-40 °C and a small amount of old curd added to it, the lactobacillus in that curd sample starts to grow and convert the lactose into lactic acid, which imparts the sour taste to curd. Regards