Why does pouring beer onto the sides of the glass prevent it from foaming, as opposed to pouring it straight down the middle?
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The foam in beer is a lot like the bubbles in soda. It stays dissolved when the beer is under pressure in the can/bottle/keg, but comes out as foam when the beer is poured.
And just like with soda, it comes out faster if the beer is agitated. (For example, if you shake the can before opening, it’ll spray you in the face.)
When you pour beer straight down the middle of the glass it has more time to fall and hits the bottom of the glass harder. When you pour down the side of the glass it doesn’t hit the glass as hard.
Agitation is well known to cause beer to foam. You just don’t give a beer a good hard shake and pop the lid right off, it will spew all over the place.
Pouring gently down the side of the glass reduces the amount of turbulence in the beer and reduces foaming.
I, for one detest this practice. Draft beer served in a bar is supposed to have a head on it. I pour a bottled beer gently right down the middle to get that head where it belongs.
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.
When you pee and hit the water it splashes and makes foam. When you pee on the side it slides the toilet and foam less.
Agitating the beer causes the carbon dioxide to activate. Like shaking a cola. You do want some foam on a beer though. If you pour it so it doesn’t foam at all you’ll almost assuredly get a stomach ache.
same reason pissing on the side of toilet bowl results in less splashback then going straight down the middle.
To add to foaming prevention: rinsing out the glass beforehand will also prevent a lot of foam from being created. Otherwise the sides likely have some water scale/other imperfections that acts as a nucleation site (like coke and mentos)
Why is going down a slide fun and jumping off a building hurts?
Pouring down the side causes less if a disturbance in the beer, causing less carbonation to be allowed to come out of solution and form foam.
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