Thursday, 11 October 2012

Liquid Nitrogen Cocktails

I borrowed the picture from the Guardian, via Google
When I first read headlines about liquid nitrogen in cocktails, I thought the media had gone mad. Nitrogen is liquid at almost 200 degrees below zero. Above that temperature it becomes gas.
Liquid Nitrogen has many uses, but one example is in the NHS where it is used to freeze warts, that gives an idea how cold it is. There is no way you can drink it. Pour it into your mouth and your lips and tongue would freeze solid. If you did get it into your stomach, it would explode because of the rapidly expanding gas.
That is not what happened to the poor girl who lost her stomach; she didn’t explode on the spot or freeze her mouth solid.
As far as I can work out, these cocktails have a small amount of liquid nitrogen added, which rapidly evaporates, making an interesting mist and adding to the drama of the drink. So why is it dangerous?
That's where the physics comes in.
If you add alcohol to water, it lowers the freezing point. The more alcohol you add then the lower the temperature you need in order to freeze the mixture.
Most cocktails have between 15% and 40% alcohol, which means that it would not turn into ice cubes unless you got it below the temperature that you find in most domestic freezers.
The table shows the freezing point of alcohol water mixtures.

Freezing point

Next question, how do they make these cocktails? I am surprised that the media don't seem to provide details, but from what I've seen, the barman makes up the cocktail and tips a little liquid nitrogen into the mixture.
That will cool the cocktail; exactly how cold it gets will depend on the temperature that it started at and how much liquid nitrogen is tipped in.
When you mix something that is at minus 190 degrees with something that has just come out of the fridge behind the bar, the final temperature depends on the volume of the two quantities that are mixed. The average cocktail is around 100-125 ml. Cocktail mixing is not an exact science but on average, something like a teaspoonful of liquid nitrogen would drop the temperature of the cocktail to about minus 15 degrees.
If you stuck your fingers in it, you would rapidly get frostbite. Bang down a couple of those cocktails and there is a good chance that your stomach gets frostbite. In other words, chunks of the stomach lining would freeze solid and die. The next thing you know you are in hospital having a gastrectomy.
Why the stomach, rather than the mouth or the gullet? I think the answer is that the fluid goes through those parts quite quickly, not allowing enough time for significant heat transfer to happen. In the stomach, on the other hand, the fluid stops moving and sits there doing damage. It won’t take long.
There will obviously be demands that such drinks be banned. That is probably not necessary, they can be made safe just by training the barman to use only minimal amounts of nitrogen and checking the temperature. If the drink is more than a few degrees below freezing, then don’t drink it. Not even if your friends dare you.
If you must drink one of these try asking the barman if he has 'O' level physics, but anyway take your time, pose a lot, sip it slowly and if you want to be really safe, drink a pint of warm beer first.