How can milk foam hold up a spoon when it’s only made of water and air?
HE CAPPUCCINO LOOKED all right to me, but then I’m not much of a coffee drinker. It had a pretty pattern in the foam on top, and it had apparently been made using fairly swanky coffee beans. But that wasn’t good enough for the friend I was with. She picked up the accompanying metal teaspoon and placed it flat on top of the foam, with the handle of the spoon resting on the side of the cup. We watched in silence as the spoon sank slowly beneath the bubbles. “Rubbish,” she sniffed. The waitress returned with my hot chocolate, which was more than half milk foam, and we repeated the test. The spoon sat there quite happily, and apparently this foam passed the test. Isn’t that odd? Spoons fall through air and they fall through milk. So how is it that when you mix those two things together, they make something that behaves like a solid and can hold up a spoon?
I have a small milk-frothing device at home, and it’s always hot chocolate o’clock, so the next day I roped my in neighbour to assist and we did some experiments. We tried cold semi-skimmed milk first. Milk contains both protein and fats, and as the air was whisked in, the cup filled up with foam really quickly. The secret to a foam is a molecule with both a water-loving and a water-hating end. These coat the surface of each bubble, making a sort of cage around it. In the cold milk, the fats were playing that role, but there weren’t very many of them. As we watched, the bubbles joined together to make bigger bubbles, and these eventually burst. The foam vanished almost as quickly as it arrived.
Then we tried heating the milk. Cold protein molecules are wound up into little balls, with their hydrophobic (waterhating) ends tucked safely away inside. But as the milk was warmed, the proteins unwound to reveal those ends. Suddenly, there were far more molecules that could act as a coating. The foam grew just as quickly, but this time it stayed put because there were lots of stable little cages for the bubbles. But it doesn’t tell us why the mixture could hold up the raisins that we scattered over the surface.
In really smooth milk foam, the bubbles are too small to see – each one measures about one-tenth of a millimetre in diameter and the coating stops them from joining together. These foams are quite wet, and the bubbles are spherical squishy packages that pack together just like ping-pong balls in a bucket. Pushing on them just squashes the bubbles a bit, so they’ll push back and can hold an object up. But if you push a bit harder, the bubbles squish enough to start sliding past each other. The more liquid there is between the bubbles, the less hard you have to push to get them to shuffle around. This was the issue with the failing cappuccino. There was too little air and the bubbles could easily move out of the way of the spoon.
Like richer hot chocolate? Full-fat milk isn’t so good for foam, because the fats and proteins stick to each other and not to the bubbles. But add a bit of cream, and fat takes over from protein to make even more decadent foams. So, to finish the experiments, we made some more milk foam using our most successful technique, and turned it into hot chocolate. I’m still not a coffee drinker, so I’m not going to turn into a coffee snob. But I love the thought that you can build a solid structure out of a liquid and a gas. And even more, I love the idea that you can add this structure to chocolate and drink it.
“I’m still not a coffee drinker, but
I love the thought that you can build a solid structure out of
a liquid and a gas”