Why does milk boil and expand so rapidly at a low temperature though some quantity of water is present in it?

Water boils at 100 degree Celsius and 1 atm. pressure (i.e., pressure at sea level). Variation in boiling point and expansion of liquids are due to the presence of various other substances. When a low boiling liquid is present along with water, the mixture will boil at even lower temperatures. But only the low boiling liquid will be evaporated. In case of milk, during heating, fatty organic substances present in the milk separate out and float on the surface due to their weight. Those fatty matters cover the surface like a blanket, and prevent any loss of water or any other substance by evaporation. This increases the pressure beneath the layer and leads to expansion. read less

This question appeared in THE HINDU on April 12, 2001 and the answers were posted as well. Boiling of milk QUESTION: Why does milk boil and expand so rapidly at a low temperature though some quantity of water is present in it? C. Valliammal, Nagercoil, T.N. ANSWER 1: Water boils at 100 degree Celsius and 1 atm. pressure (i.e., pressure at sea level). Variation in boiling point and expansion of liquids are due to the presence of various other substances. When a low boiling liquid is present along with water, the mixture will boil at even lower temperatures. But only the low boiling liquid will be evaporated. In case of milk, during heating, fatty organic substances present in the milk separate out and float on the surface due to their light weight. Those fatty matters cover the surface like a blanket, and prevent any loss of water or any other substance by evaporation. This increases the pressure beneath the layer and leads to expansion. B. Narayanasamy, Tiruchi, T.N. ANSWER 2: Milk consists of mostly water and some fats, proteins, lactose and minerals. Milk fat is a mixture of glycerides of fatty acids with a density less than that of the milk serum. The solid fat is dispersed in the serum in the form of small globules. When the milk is heated up, these fat globules rise to the top and at a temperature around their melting point, about 50oC, form a layer of skin on the warm milk. The steam bubbles that form within the milk get trapped by this skin and accumulate under it. They grow and coalesce and build up a pressure that eventually raises the skin and makes some of the milk spill over, stirring breaks the skin, releases the pressure and prevents spilling over. Leo Christoper, Sirumugai, T.N. ANSWER 3: Milk is a white opaque fluid in which fat is present as an emulsion, protein and some mineral matter in colloidal suspension and lactose with some minerals and soluble protein in true solution. Milk expands when it foams. This expansion results in increase of volume. The volume increases due to foaming as a result of trapping of air in the liquid. This takes place when the milk is whipped or agitated. At the time of foaming air is trapped as bubbles, surrounded by thin layers of protein with fat interspersed to act as a stabilizer. The stability of foam depends on the surface tension of the liquid. Water does not readily foam unless an emulsifying agent is present, since it has higher surface tension, whereas milk readily foams since it is an emulsion by nature and also has low surface tension. The surface tension of milk is related to proteins, fat, phospholipids and few fatty acids present in it. When the milk is heated up to its boiling point, it gets agitated and its density also decreases, which result in foaming. This can be prevented by pouring some water on its foaming surface, which helps to increase the surface tension to some extent. In the beginning of heating, the low temperature at the top layer helps for the formation of a scum. This tenacious layer is formed by coalescence of fat globules, which are held apart originally in the unheated milk, by the surrounding protein molecules. When the milk is heated these protein molecules get separated from the fat globules, resulting in coalescence of freed fat globule, which in turn results in the formation of scum. Scum formation can be prevented by beating or stirring the milk while heating it or by covering the milk pan. Use of milk boiler will prevent the formation of scum. When allowed to cool, the drying out of the top region of the milk helps again for the formation of scum. The scum contains a small amount of coagulated proteins, minerals and fat globules. S. Palaniappan, Pudukkottai, T.N. read less