Dear Mr. Wizard:
I have assembled the equipment to start kegging my beer and force-carbonating it in soda kegs. I will still want to bottle some of the beer for ease of transport. How much difference in carbonation levels can I expect, filling from a counter-pressure bottle filler or just filling quietly from the keg with an extension tube on a party tap?
Mr. Wizard replies:
The amount of carbonation lost during filling is heavily influenced by the carbonation level of the beer being filled. Highly carbonated beers lose more carbonation when bottled compared to beers with lower levels of carbonation. It is almost impossible to give hard numbers to directly answer your question, but based on personal experience, you will lose a considerable amount of carbonation if you simply fill bottles from a tapped keg.
I use a long-tube, counter-pressure filler to fill 22-ounce bottles. The fill tube extends to the bottom of the bottle and gently lets beer in as the counter pressure in the bottle is slowly vented. My filler allows me to fill one bottle in 40 seconds, which is fast enough for my needs. I know from carbonation measurements taken before and after filling that my beers lose about 0.25 volumes of carbon dioxide (they drop from 2.65 to 2.40 volumes).
Although I am happy with the carbonation level of my bottled beer, this is an appreciable loss of carbonation. I can guarantee that if I did not use a long-tube, counter-pressure filler, my losses would be much higher. In practical terms your beer will begin to seem flat when the carbonation level drops to around 2.2 volumes.
Another real issue to consider when bottling beer is oxygen pick-up. The rule of thumb in a commercial brewery is that oxygen pick-up becomes increasingly more important as the beer nears completion. Beer transfers following fermentation, filtration and packaging are three areas to be especially careful with, in respect to oxygen pick-up. The reason is simple; yeast is capable of mopping up oxygen when it is active and yeast activity rapidly decreases after fermentation. In the case of filtered beer, there is no yeast activity because there is no yeast!
There are some very fancy bottling systems, used by commercial brewers, that use a vacuum-evacuation technique to remove oxygen from the bottle and short filling tubes that allow the beer to cascade down the surface of the bottle. These fillers work very well in respect to carbonation retention, low oxygen pick-up and speed, but are out of reach for homebrewers. The best filler for the homebrewer is a long-tube, counter-pressure filler. This technique will fill your bottle without losing too much fizz, your main concern, and will also do an excellent job of minimizing oxygen pick-up. My advice is to use the hose from your keg to fill a glass or mug - but not to fill bottles.
Mr. Wizard, BYO's resident expert, is a leading authority in homebrewing whose identity, like the identity of all superheroes, must be kept confidential. To see more of Mr. Wizard, check out the latest issue of Brew Your Own available at better homebrew shops and newsstands.