Why does my pint of lager go flat

A. You’re probably referring to the fact that lager seems a lot fizzier when it's first pulled then when you’re near the bottom of your glass and the answer relates to the solubility of gasses, specifically carbon dioxide, CO2. The solubility of CO2 increases as the temperature of the liquid it dissolves in decreases.

Your pint of cold lager has CO2 added to it either at the brewery or at the pump as it's being pulled, which makes it fizzy. Because it's cold, it can hold more dissolved CO2. However as the lager heats up in the pub, aided by your warm hand as you drink it, it can't hold so much CO2 in solution and so becomes flat.

In warm countries like Spain lager is served in much smaller glasses than in the UK so that you can drink all of it while it is still cold and therefore fizzy.

Q. What happens to the CO2
A. It escapes, via bubbles, into the atmosphere and your lager goes flat.

Q. Why does lager seem to go flat faster than bitter
A. There are two reasons for this. The first is that lager is usually served at lower temperatures than bitter and hence contains more CO2. Because of the lower temperature its rise in temperature will be faster than bitter and so will lose CO2 faster than bitter as can be seen by the constant stream of bubbles rising in a cold glass of lager.

The second is that lagers have CO2 added usually at the pump while with a traditionally brewed bitter the CO2 is produced naturally by the action of the yeast and sugar in the brewing process. Because of this lager starts out with more CO2 than bitter and with its fizz more prominent it appears to go flatter faster than bitter.

Q. What's the solution
A. Well other than sitting outside in the winter, stick to half pints and don’t hold onto your glass for long periods.

Do you want to know how something changes Click here to ask.

by Lisa Cardy