Q: How much difference in carbonation levels can I expect, filling from a counter-pressure bottle filler or just filling from the keg with an extension tube on a party tap?

A: 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, 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 (650-mL) 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). More sophisticated fillers typically lose only about 0.05 volumes of carbon dioxide during filling. 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. 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.