Why does pouring beer onto the sides of the glass prevent it from foaming, as opposed to pouring it straight down the middle?
In: 83
The carbon dioxide that is dissolved in the beer has a hard time coming out of solution by itself. It won’t collect into bubbles, it’ll just slowly make its way to the surface and off gas there. In order to form the bubbles, it needs something to stick to, called *nucleation sites*. When you pour the beer into the glass, you mix and swirl the beer with air, which creates little bubbles. Those bubbles make *great* nucleation sites for the CO2 to grab onto and come out of solution. When they do, it makes adds to the bubble and makes it bigger. A bigger bubble has more surface area for CO2 to stick to, which allows more CO2 into the bubble, which makes it bigger… and so on. The bubbles rise up and create that thick foam, also called the *head*.
The sugars and proteins in the beer increase the surface tension, so the bubbles are a bit tougher and don’t immediately pop.
This is also what happens when you shake a soda. The air bubbles form nucleation sites and the CO2 rapidly leaves the solution, forming thick bubbles that expand and make your soda “explode”.
By carefully pouring the beer (or soda) down the side, you’re preventing too many air bubbles from getting mixed in, so there are fewer of those nucleation sites and the CO2 has nowhere to go, so it stays dissolved in the beer.
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