Bottle conditioning is the process of naturally carbonating beer by adding a priming solution (water + some type of sugar) to the flat beer immediately before bottling to initiate a "re-fermentation" in the bottle. The CO2 produced from the re-fermentation in the bottle is absorbed into the beer, creating carbonation. The amount of carbonation in the finished beer can be controlled by adjusting the amount of sugar in the priming solution. After priming and filling, the newly bottled beer should be stored at 70-75F for 2-3 weeks or until fully carbonated. After this time the bottles can be chilled or stored below 60 degrees to stabilize the beer.
Bottle conditioning also has an effect on the flavor and aroma of the beer due to the presence of active yeast in the bottle. The yeast in the bottle is able to reabsorb or process byproducts from fermentation eventually rounding out or mellowing the flavors. It can also extend the shelf life of beer by delaying the staling effects of oxidation.
Many different types of sugar can be used for bottle carbonation, including corn sugar (dextrose), table sugar (sucrose), or dry malt extract. In order to properly determine the amount of sugar to use, you need to take into account the temperature of the beer and the volume as well as the type of sugar and the level of carbonation desired. Northern Brewer’s Priming Sugar Calculator is a good resource for determining the amount of sugar to use.
When a batch of beer fails to carbonate in the bottle there are a few common causes. After fermentation, the yeast cells that remain in solution may be too stressed or too few to re-start fermentation in the bottle. The likelihood of this happening increases with the length of secondary fermentation and the alcoholic strength of the beer. To ensure proper re-fermentation, additional yeast can be added to the beer at bottling. You can use a fresh pack of the original yeast or use a neutral fermenting dry yeast such as the Danstar Nottingham ale yeast (y005).
Another common cause for lack of fermentation is storing the bottles in too cool an environment during conditioning. At cooler temperatures, the remaining yeast will not be able to re-start fermentation in the bottle. Make sure the bottles are stored above 65F until the beer is fully carbonated. Sometimes warming the bottles and rousing the yeast from the bottom of the bottle get the process started. If that doesn’t work you may have to resort to dosing each bottle with a small amount of dry yeast after opening and then recapping the bottles.
Forgetting to add priming sugar and not getting a good seal on the bottle with the cap can be other reasons for lack of carbonation in the bottle. The easiest solution to these problems is to dose each bottle with additional sugar. The best way to do this is to open each bottle and add the pre-measured conditioning tabs to each bottle. Only dose with more sugar if you are sure that you forgot to add priming sugar or that the caps had a bad seal, otherwise the extra sugar can result in too much carbonation. For more information on Bottle Conditioning, see our Advanced Guide to Bottle Conditioning.