|MadSci Network: Chemistry|
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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