How can milk foam hold up a spoon when it’s only made of wa­ter and air?

Focus-Science and Technology - - Discoveries - DR HE­LEN CZ­ER­SKI is a physi­cist, oceanog­ra­pher and BBC science pre­sen­ter whose most re­cent se­ries was Su­per Senses

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”

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