Making lager fizzy

It’s funny how sometimes what would on the face of it appear a fairly simple thing turns out to be much much more complicated…

If you are our regular reader, you will remember that way back in April we made a Czech-style lager, named Bohemian Wrenbury. This has been lagering (cold storage) for many weeks, but now summer is here (ho ho!) we want to drink it. But obviously being lager it needs to be fizzy.

As everyone knows, the bubbles in fizzy drinks are carbon dioxide. How do you make beer fizzy? Well there are two ways: first is to provoke a secondary fermentation. Fermentation gives off carbon dioxide, so if you keep the beer sealed it will dissolve in the beer. This is how we carbonate our ales. The second way is force carbonation, i.e. seal the beer and use bottled carbon dioxide.

As I said, for our ales we use a secondary fermentation. So why not just do this for the lager? Well, mainly because of the lack of yeast. After many weeks of lagering, I think most of the yeast will have dropped out of the beer. Plus the lager is in the fridge, so any secondary fermentation would take a long time. And finally, it’s a bit uncontrollable. You add sugar to power the secondary fermentation… if it didn’t ferment we’d end up with somewhat sweet lager.

So we decided to try force carbonation. First problem: how do you know what pressure you have in your keg? After much research, I bought a keg pressure gauge on eBay:

After a bit of drilling, tightening and testing it was fitted and seems to work OK.

Next problem – how much pressure do you need? The first thing to say is that the amount of carbonation in beer is measured in “volumes”. That is, carbonation to one “volume” means you have dissolved the same volume of carbon dioxide in your beer as you have beer. Two “volumes” means twice the volume of carbon dioxide has been dissolved as you have beer.

Generally lagers are served at between 2 and 2.5 volumes. So far so good. Now the amount of carbon dioxide you can dissolve depends on the pressure of the gas above the beer and also the temperature. A quick look on Google reveals some handy charts for the pressure (in PSI) required to attain a certain level of carbonation versus temperature. Yes, temperature plays a part: the colder the beer the more gas you can dissolve in it. This sounds counter-intuitive, but it’s right. For dissolving salt in water, you need it to be warmer to get more to dissolve. Not so for dissolving a gas – for that you can dissolve more gas at lower temperatures, because a higher temperature means more energy in the liquid, which can drive the gas out of solution and into the space above.

All fine, except that you need quite a lot of pressure, even at about 5C, to dissolve 2.5 volumes or carbon dioxide. More pressure, unfortunately, than the King Keg is supposed to tolerate (10-12 PSI max). Bugger. But given that I’d got this far I didn’t see that there was much else I could do but go for it, and see how much carbon dioxide I could get to dissolve. So far it turns out that the answer is “not enough”. When served, the lager comes out at quite a gush (because of the pressure above), but it doesn’t hold any bubbles. Just the very slightest hint of carbonation, like mostly flat mineral water. Arse.

On the plus side, the beer itself tastes and smells wonderful – just like Budvar. And it’s nice and clear.

So what was the “right” solution? To invest in a “cornelius keg”, which can hold much much more pressure. But I don’t think one would fit in my fridge. So this is going to take a lot more thought and experimentation for future lagers.