After a bit of trouble with a thermostat on a 2nd hand fridge, I discovered that my latest batch of bottled, ready-to-drink brew had frozen.
What happens to beer when it freezes? Will the taste be affected? Does the level of CO2 change?
Great question (+1)
I have never frozen beer so could not comment on flavour changes.
With regard to CO2, this should be unaffected as long as the beer is in a sealed container - hopefully one that does not split as the beer (90+ % water of course) freezes and expands.
As the temperature of a liquid drops the solubility of CO2 increases. The drop in temp will not cause the CO2 to come out of solution. Careful thawing should reveal that the beer is still carbonated and that the CO2 has remained in solution throughout.
It might be that, if the beer was in a sealed container that had a large 'air' space, the CO2 might come out of solution as the temperature dropped, on account that the pressure of gas above the liquid drops (contraction). My command of the physics is not enough to know whether the solubility increase has greater effect than the partial pressure drop above the liquid. For further reading look up Henry's Law.
If your bottles don't explode, it won't make any difference. The CO2 will just dissolve back into the beer when it thaws. If you were letting the beer carb when they froze, it will take longer to finish. I end up freezing my beer frequently during the long Canadian winters.
That said, if you partially freeze your beer and syphon the liquid through the layer of ice, you'll be performing a form of distillation. This form of distillation is commonly used in the production of Eisbocks.
I often pop a bottled beer in the freezer for 10-15 mins to rapidly cool if I've forgotten to load the fridge. Problem is, sometimes I then forget. The next morning I find the cap has been forced off and there's a sticky mess. It seems the water content freezes and forces out all the sugar. Tasting the thawed liquid later, it's still bitter but with little sign of alcohol, so it looks like that gets forced out of the neck with the sugar.