Why do coffee and milk run away and water just boils?

Why do coffee and milk run away but water just boils?

Myriams-Fotos Brewing Turkish coffee or boiling milk – it would seem, what could be easier? But it is worth being distracted even for a moment, and please – the capricious substance instantly spills over the hot stove, delighting with new aromas. We tell you how the boiling of milk and coffee differs from the boiling of water and what to do so that they do not run away.

Kitchen physics

It's all about the composition of liquids: milk contains molecules prone to polymerization – proteins, fats, lactose, and coffee – small particles of ground grains. When heated, water from the surface layers actively evaporates, and particles or molecules combine to form a thin but strong film. Meanwhile, bubbles begin to appear at the bottom of the pan, consisting of gas that has been dissolved in the liquid. They rise to the surface, after which the steam from them comes out.

Pure water boils without any problems: there are no barriers to steam, and, having freely risen up, the bubbles simply burst.

But in the case of coffee and milk, they collide with the film formed on the surface, accumulate and turn into foam. With every second there are more and more bubbles, new ones prop up them from below – and now all this mass is already confidently climbing over the edge.

How to save milk

There are several ways to keep milk in a saucepan. The most tedious is to constantly stir: the polymer film is destroyed, the bubbles freely rise up and burst, releasing steam. But this is not our way. Instead, an inverted saucer can be placed on the bottom of the saucer with milk: the bubbles under it connect and rise up only when they reach a solid size. Large bubbles easily break the film and do not form foam.

The industrial analogue of a saucer is a milk watchman, invented back in 1921. A simple device is a metal disk 6–8 cm in diameter. It is covered with grooves under which bubbles collect, and is equipped with a spout (or just a hole) for steam release.

The third method, the most scientific, is a water bath. A container of milk is placed in a pot of water, it heats up smoothly and no higher than up to 100 ° C. The dissolved gas escapes without the formation of bubbles – and the milk does not escape anywhere.

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