Why and how does beer foam up when sipping in a glass?

Question

Basically the title, why does the beer foam up significantly more when sipping it straight up, compared to when under an angle?

In: 1

Answer 1

Beer foams because it is infused with gas – usually either carbon dioxide or carbon dioxide and nitrogen. This gives beer its fizzy texture.

The gas is dissolved in the beer. So normally, it is just a component of the liquid. Specifically, the carbon dioxide reacts with the water in beer to form carbonic acid (H2CO3).

This compound isn’t strongly held together, and as a result carbon dioxide is only weakly dissolvable in water. If you release pressure or add a bit of energy, it can be enough to undissolved the carbon dioxide, which forms gas that, through the surface tension of the beer, gets trapped in tiny bubbles which form the foam.

This is the exact same thing that happens with soda, but because of the difference in composition between beer and soda, the foam on beer tends to be more structurally sound and taken longer to dissipate.

The reason why angle matters when pouring beer? Energy. It takes energy to separate the carbon dioxide in beer. When you are pouring a draft straight down, it travels a few inches to hit the bottom of the cup. This gives more than enough force to create the foam. But when you pour at an angle, you are pouring the beer a much shorter distance to the top of the cup, where it then runs down at an angle to the bottom. This significantly reduces the energy that goes into the beer, which reduces how much it foams up.