Air pressure in cocktail shaker rises as shaken. Contents force out. It's cold. Pressure should go down. Why?

Question

When I make a martini in my cocktail shaker I add ice, gin and vermouth. The ice is cold (naturally) but the gin and vermouth are at room temperature. The shaker is kept in the freezer already filled with ice - ready to go when I get home. I add the gin and vermouth as above, put on the lid and shake. Air pressure rises in the shaker as I do this, occassionally pushing off the lid slightly so contents are forced out with a hiss as the air is expelled. This seems contra-intuitive. Once would expect the air inside the shaker to be chilled by the shaking and the pressure to fall, sucking the lid on tighter. Why is it so?

Answer 1

I am a graduate student in physics and martini purist, so I'm thrilled to see such a question...

This was puzzling at first, because the heat generated from shaking is absolutely negligible, (I can prove this mathematically if anyone doesn't believe it) and generally, I keep my shaker at room temperature and add ice (I think this protocol is more common). But by keeping it in the freezer, the temperature of the metal, ice and AIR in the shaker is well below 0 C. Thus everything in the shaker system is below 0 and you add something well above 0 (the alcohol). In this case, the very cold air gets slightly warmer and expands.

You might be skeptical that the air remains cold after you open the shaker top: how long does it take for warm air to circulate in?
I am also somewhat skeptical.

To test this explanation, take the cold shaker and ice out of the freezer and breath forcefully into the open shaker after adding the alcohol and QUICKLY cap it. This should ensure that the air is at room temperature and will now contract when you shake it.

Responses to other explanations__

The volume of dissolved gas in the alcohol is very small. Either way, this cannot be the explanation because a pressure increase is not observed if all components are warm.

The vaporization pressure of the alcohol is also very small, and cannot explain the phenomenon because of the same reason as above.

Answer 2

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Answer 3

The phenomenon can easily be explained thus:
When you shake a solution or liquids you are in fact reproducing what molecules do when heated. The friction caused between the ice and the two liquids creates hea,t therefore heating up the air and therefore creating air pressure.

Simply put, you convert mechanical energy to heat through friction. Heat makes gasses expand.

Answer 4

lorenztrans is on the right track.
The alcohol is the answer.
Shaker, air, ice are cold. Alcohol is room temp I assume. Produces vapour. Proof is that you can smell it. Expansion of ethanol liquid to vapour volume is huge ( not sure exactly but it is big).
Solution 1 - chill all ingredients
2 - At room temp mix and then pour over ice.
3 - worry about chilling out as you drink it not the hiss while making it

Answer 5

All wrong Grasshoppers. You're just annoying dissolved gasses in the mix causing them to bubble out. Same a a can of beer, but not as much fun at a redneck wedding.