Intending to prepare a custard or, more rarely these days, to sterilize raw milk bought directly from the farm, you put some milk to heat on the stove. A few minutes of inattention later, disaster struck. Half of the liquid left the pan and spilled onto the stove, with a “psssshhhhh” that did not fail to alert your ears, coupled with a characteristic burnt smell, announcing the long minutes that you will now have to spend scraping to clean it all up.
Yet, you knew it, milk is particularly sneaky. The expression “to watch like milk on the stove” does have an origin. But the facts are there: you have been had again, and it will not be the last time.
Bubbles and foam
But why this tendency to boil over, when a pan of water will never pose a problem (at most it will make the lid shake and generate a few splashes during a particularly intense boil)? Quite simply because of the particular composition of milk, which beyond its 90% water, contains 10% of various substances, some of which give it a foaming power. Because yes, for it to boil over, not only do you need bubbles (which is what boiling does, including in a pan of water), but above all they must be stable, in other words form what is called a foam.
For this, the presence of a “surfactant” is necessary, namely a molecule that likes to position itself at the interface between air and water, thus stabilizing the bubbles in the liquid. So who plays this role in milk? Mainly proteins, which due to their composition in amino acids – some of which are hydrophilic (which like water) and others hydrophobic (which “flee” water) – will come to position themselves around the steam bubbles…
- Slate.fr
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