MadSci Network: Chemistry

Re: Why does milk boil over so quickly?

Date: Wed Sep 24 00:55:00 2003
Posted By: Werner Sieber, Research Scientist, Coating Effects,
Area of science: Chemistry
ID: 1064313097.Ch

Hi Jo,
With your phrase "proteins are very good at coating bubbles 
and stabilizing foams" you indeed have at least half of the answer. In the 
case of milk, the molecule involved is casein (Word derived from the latin 
term for "cheese"). Milk is an watery emulsion in which very small fat 
droplets are coated by casein in a way that protects them from flowing 
together (coalescing). On heating, this subtle architecture is destroyed: 
At a certain temperature, even below boiling point of water, protein 
molecules generally are irreversibly changed in their spatial arrangement 
("denatured"). Casein, together with other components, thereby forms a 
tough film which surrounds the water vapour bubbles of boiling milk, 
preventing them from breaking (The milk does NOT reach boiling point 
faster than water). This has the effect that the milk is transformed into 
a relatively stable foam, which occupies a much larger volume than the 
original liquid. Expansion continues a little while after removing it from 
the hotplate because of the heat stored in the pan. 
Best Regards
Werner Sieber

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