Question sent by JORDI PORTAVELLA (La Garriga, Barcelona). FERNANDO SAPIÑA answers:
Dear Jordi, Milk is a fluid segregated by the female mammary glands of the mammals. It is the only food consumed by all mammals alike during the first stages of their lives. This is the reason why it is normal that it has all the needed nutrients for life sustainment either dissolved or dispersed in water. In the case of cow milk, this has an average of 4.6% of sugars, 3.2% of proteins, and 3.9% of fats.
The most important part, in relation to your question, has to do with fats and proteins. Fat in milk appears as drops that are surrounded by a membrane and scattered in water. Regarding proteins, there are two types: casein micella and lipoprotein aggregates. All of this indicates that in milk we find tensoactive substances.
Tensoactives are compounds that stabilise fat dispersions in water and also stabilise the formation of air bubbles in water. This is a key point. When boiling milk bubbles of water steam are formed, which are stabilised due to the presence of these tensoactives. These bubbles take long to burst and, therefore, as the milk boils more and more bubbles are formed, increasing the volume of foam formed until it overflows. In the case of water, water steam bubbles burst immediately after becoming, and foam is never produced.
You can perform an experiment at home so as to simulate what happens with milk using a simpler model system. You take water, put it into a receptacle and add a drop of washing-up liquid which, like soaps and detergents, contains tensoactive substances. When the dissolution boils foam is produced and it overflows the receptacle.
Fernando Sapiña. Institute of Science of Materials. Scientific Park, University of Valencia. Author of the «Science at the Table» section in Mètode.