(Intro)

Beer. It's refreshing, delicious, bubbly, tingly and full of chemistry. We've talked about the science of making beer but there's also a science to the way we pour beer. You probably know the drill: always tilt the glass. But why? It's all about the foam.

Beer has lots of dissolved carbon dioxide - the same stuff that gives soda its bubbles. This CO₂ is created during the fermentation of sugar by yeast cells - the process that also produces the alcohol.

When you first crack open a can or bottle of beer, that whooshing sound you hear is the pressure inside the container dropping to match that of the outside world. In the process, a little bit of CO₂ is released but most of it stays trapped in the liquid. And when you tilt your glass while you pour your beer, you're stopping most of the remaining gas from escaping.

See, the carbon dioxide in the beer wants to come out, but it needs help - the molecules needs a place to come together in order to create bubbles. These spots are called nucleation sites. Have you ever watched as, like, a string of bubbles pop out of, like, one single place on a glass? That's a nucleation site - it could be a fiber or dust particle on the glass, an imperfection or bump, or even a tiny trapped air bubble.

And that's what you're trying to avoid by tilting your glass while you pour your beer: trapped air that can then turn into nucleation sites. If you pour your beer straight in, without tilting, you're forcing a bunch of air into the beer, providing lots of those little nucleation sites and helping the CO₂ escape. As the gas rises, it gets coated in proteins from the grain, which can provide enough surface tension to keep the gas trapped in bubbles on top of the beer.

That's what makes the foam on the top - also known as the head. Since you probably don't want a cup full of foam, when you pour your beer in you need to control the release of the CO₂ and the formation of the head. A gentle pour down the side of the glass traps much less air, which means fewer nucleation sites, which means more carbon dioxide stays in the liquid where it belongs. In other words: less foam, more tingly.

At the very end of your pour, you can stop tilting the glass - that way you end up adding a few centimeters of head to the top of your beer. Because having some foam can be a good thing! It helps you smell the beer while you're drinking it. You just probably don't want too much of it.

And it turns out this tilting method works for more than just beer. French scientists at the University of Reims found that tilting that glass is also the best way to pour champagne in order to preserve the bubbly bite of the drink. Cheers French scientists!

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