The Science of Fermentation

• In the fermentation process, the yeast present in the mixture consumes sugars from the mash, converting them to energy. The yeast release two byproducts: carbon dioxide and alcohol. Once the alcohol level in the mixture rises to a certain level, it inhibits fermentation. Until this happens, however, the yeast continues to release both alcohol and carbon dioxide. This carbon dioxide causes the bubbles in beer, but too much of it can have dangerous consequences for your new batch.

Preventing Bad Bubbles

• One of the challenges of brewing is to get the desired level of carbon dioxide to give the beer its fizz. Too much carbon dioxide will result in an excessively fizzy beer or, worse, can burst the container. The container in which fermentation takes place is fitted with a fermentation lock, which releases the carbon dioxide generated by the process. Once bubbles have stopped appearing in the lock, fermentation has stopped (although the process can sometimes stop temporarily for other reasons, such as a change in temperature). Once fermentation has stopped the brewer adds a small amount of priming sugar and then bottles the beer. The yeast consumes the priming sugar, creating the carbon dioxide bubbles that give the beer its fizz.

The Bottle Bomb

• If fermentation hasn't completely stopped before bottling, the combination of continued fermentation and the added carbonation created by the priming sugar leads to a buildup of carbon dioxide within the bottle. This can result in too much pressure within the bottle. In extreme cases, this can lead to the so-called "bottle bomb," in which the carbon dioxide pressure forces the cap off the bottle, spraying beer everywhere. The end result is a sticky mess and a wasted batch of beer.

Avoiding the Beer Bomb

• The only way to avoid excess carbonation is to make sure that fermentation has completely finished before bottling. This doesn't necessarily happen when bubbles stop appearing in the fermentation lock. For safety's sake, Beersmith recommends allowing two weeks or longer for fermentation to complete, even if bubbles have stopped appearing, since the lack of bubbles may be the result of a temporary pause in fermentation. Waiting until fermentation is really complete is the only way to prevent over-carbonated beer and the problems it can lead to.