When you go to a coffee shop, do you wonder how they make those pretty patterns on the coffee froth? And did you ever wonder why you get a thick froth in the shop coffee, but just a thin one at home?
Why does milk froth?
When boiling milk, it forms a thin frothy layer on top, which expands like a balloon. Why does this happen?
Milk is a complex liquid, with fats, proteins and sugars in it. The main protein of milk is casein. It is a long, string like molecule, which is folded up tightly, just like a ball of wool. Casein is very good for health because it’s a complete protein. That means it has all the 20 amino acids the body needs for growth.
Amino acids come in two types. Some of them dissolve in water easily, while some don’t like water (The chemical word is ‘hydrophobic’, which is Greek for ‘water-hater’!). Normally, the water-hating amino acids are inside the tightly wound up string. When boiling milk, the casein unwinds, and all amino acids face the water. At this time, it acts just like soap, with parts that dissolve in water, and parts that repel it. And that’s why milk froths – because of its soap like casein!
Making froth in a latte
When you order a latte or a cappuccino in a coffee shop, look carefully at the barista (that’s the guy that makes the coffee). He’ll take a little coffee brew in a cup, add a spot of milk and then place it in front of the coffee machine. The machine shoots a jet of steam into the coffee. The steam stirs up the milk and makes it froth. The more you stir it, the frothier the coffee becomes. This is because the bubbles of steam are really very tiny.
The same thing happens when you blow soap bubbles. But because you blow a lot of air from your breath, the bubbles are big. When you beat eggs in a bowl, the air from above gets beaten into the egg, forming small bubbles.
So how does the barista make those pretty patterns? That’s where the chemistry comes in. Milk, foam and coffee broth are colloids. A colloid is a material in which two materials that normally don’t mix are forcibly mixed. Foam is a colloid of air in water. Milk is a colloid of milk fats in water, held together by the casein. The coffee broth is a colloid of coffee oil in water.
What the barista does is to mix these colloids around very carefully, by drawing the foam over the coffee broth and milk. If you leave the colloid alone, it’ll stay for a while. If you stir your latte, the colloids will break up.