MadSci Network: Other

Re: Why does beer foam less when a glass is tilted?

Date: Sat May 8 10:33:53 1999
Posted By: Richard Kingsley, Grad student, Bachelor of Education (Science), OISE - University of Toronto
Area of science: Other
ID: 923925416.Ot

Message:

Hi Kevin,

There is plenty of physics involved in opening a bottle of beer.

Supersaturation occurs when the partial pressure of a gas above a liquid is lower than the partial pressure of the gas dissolved in the liquid.

1) Air pressure inside and outside equalises

The air in the bottle, before you open it, is pressurised. At this point, the partial pressures of the carbon dioxide in the air and in the liquid are the same; however, the partial pressure of the gaseous carbon dioxide is higher than outside the bottle. When you open the bottle, the air inside expands and the pressure decreases (That is happening when you hear the "whsshhh" and you will see vapour at the bottle entrance which is due to the cooling of the air as it expands). This happens because the pressure inside and outside of the bottle move towards equilibrium. Now the partial pressure of the carbon dioxide is greater in the beer than in the air above it. Therefore, the carbon dioxide in the beer becomes supersaturated.

2) Partial pressure of the gaseous and aqueous carbon dioxide equalises.

The partial pressures of the gaseous and aqueous carbon dioxide will tend to move towards a state of equilibrium. In order to obtain this, carbon dioxide will be forced out of solution, which then lowers the partial pressure of the aqueous carbon dioxide. The carbon dioxide will come out of solution at any nucleation site. This could be a microscopic ridge or fissure on the inside of the glass which is where you see bubbles after you have poured the beer.

When you pour the beer into the glass sideways you are doing two things. You are increasing the surface area of the liquid which allows the carbon dioxide to escape faster and form less foam. As beer moves down the side of a glass it will spread out and move at a slower rate which causes less disturbance to the stationary beer. The act of pouring straight into the glass will force air bubbles into the beer (you can see this effect when you pour water from one glass to another) which also act as nucleation sites and allow more carbon dioxide to come out of solution. I hope this answers your question.

Richard Kingsley

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