Why is it that when you heat milk to a point where it boils, the volume increases and it sometimes spills over?
David Muir, Edinburgh, UK
Advertisement
There are two factors that cause milk鈥檚 unfortunate tendency to boil over. The first is the abundance of surfactants, or foaming agents, within it, such as proteins, phospholipids, glycerides and free fatty acids. These lower the surface tension and stabilise the froth formed when milk boils.
The second is the so-called denaturing of milk proteins due to the heating. These altered proteins coagulate with fats in the milk to form a sticky skin called lactoderm. This film dries through surface evaporation and acts as a blanket, trapping the foam and protecting it from the open air, whose dryness would otherwise cause the bubbles to burst. The froth pushes up and over the pot rim, much to the irritation of the negligent cook.
鈥淐lean liquids heated in clean vessels tend to 鈥榮uperheat鈥�. When these liquids do eventually boil, they do so violently鈥�
Anna Butcher, Brookton, Western Australia
When milk boils, small bubbles of air form and create a foam that can spill over the side of the pot and make a mess.
This isn鈥檛 a problem in my kitchen as I have a 鈥渕ilk boiler鈥� that disrupts the formation of large bubbles from small ones, so the foaming doesn鈥檛 occur and the liquid doesn鈥檛 boil over. The milk boiler rattles on the bottom of the pot as the liquid reaches boiling point and so also alerts the cook that it has begun to boil.
I have a ceramic milk boiler, made by a company called Bristile, that was my husband鈥檚 grandmother鈥檚. This device works just as well in milk as it does in water when I cook pasta. It was made in Perth, Western Australia, back in the 1930s and has saved many a mess.
The Bristile company now makes roofing tiles as there isn鈥檛 much demand for milk boilers today. In the past, most farming families in Australia had a cow and there was always so much milk to do something with.
Ron Oren, London, UK
From a chemist鈥檚 perspective, milk is a mess of proteins and fats in water. Heating the milk disturbs the balance and forces the fats and proteins to form a layer on top. If you let the milk cool down again, that layer will solidify into the skin that most people dislike.
While the milk is boiling, bubbles that rise from the watery phase pick up a thin coat of this layer, which stabilises them so they pile up to form a foam. The foam will keep growing until it runs out of water to create bubbles or fats to coat them. Stirring the boiling milk helps as that breaks up the fatty layer and leaves room for the bubbles to escape.
Simon Goodman, Griesheim, Germany
Liquids boil when the molecules in them move as fast as the average speed of the molecules in the air above them. When heated, gases dissolved in a liquid will come out of solution as bubbles at so-called nucleation centres. These are particles in the liquid or microscopic irregularities on the walls of the heating vessel.
When clean liquids are heated in clean vessels, they tend to 鈥渟uperheat鈥� and go over their boiling point. This 鈥渂ubbling鈥� can be very dangerous in the laboratory because when these liquids do eventually boil, they do so violently. Inert particles are sometimes added when heating such liquids to avoid this hazard.
To answer this question 鈥� or ask a new one 鈥� email lastword@newscientist.com.
Questions should be scientific enquiries about everyday phenomena, and both questions and answers should be concise. We reserve the right to edit items for clarity and style. Please include a postal address, daytime telephone number and email address.
快猫短视频 retains total editorial control over the published content and reserves all rights to reuse question and answer material that has been submitted by readers in any medium or in any format.
You can also submit answers by post to: The Last Word, 快猫短视频, 25 Bedford Street, London WC2E 9ES.