Milk tastes sour when it is left in the open for some time. This happens due to the formation of
(1) Lactic acid
(2) Citric acid
(3) Acetic acid
(4) Carbonic acid
Answer: (1) Milk contains a sugar called lactose, a disaccharide (compound sugar) made by the glycosidic bonding between glucose and galactose (monosaccharides). When Milk is heated to a temperature of 30-40 degrees centigrade and a small amount of old curd added to it, the lactobacillus in that curd sample gets activated and multiplies. These convert the lactose into lactic acid, which imparts the sour taste to curd.
