First of all, yes, under some circumstances, beer bottles can explode. So-called bottle bombs are one of the few actual dangers of home brewing. But bottle bombs are very uncommon in beer brewing, and many brewers have never seen one. In this regard, soda has given beer a bad name.
Bottle bombs in soda making
Traditional root beer and other traditional soft drinks can be made at home through a process similar to brewing. However, in soda making, the beer is bottled during what brewers would consider primary fermentation, so that the yeast's first effect is to create a great deal of carbonation without consuming too much sugar or creating too much alcohol. The result is a sweet, basically nonalcoholic beverage. However, unless the bottles are chilled at the appropriate point in the fermentation to stop the yeast from continuing, the bottles will become overcarbonated and eventually explode. For this reason, home soda makers now generally use plastic soda bottles rather than glass bottles.
Bottle bombs in beer brewing
In brewing beer, the carbon dioxide produced by primary fermentation is allowed to escape through an airlock, blowoff tube, or loose lid, so pressure buildup is a problem only if the airlock or blowoff tube gets clogged (for example, with hop residue); the most common result is for the stopper of a carboy or the lid of a plastic bucket fermenter to be blown off, which can make a mess but is usually not dangerous. This can be avoided completely by using a wide enough blowoff tube and leaving sufficient airspace at the top of your primary fermenter, or simply not sealing the lid of your primary.
By the time the beer is bottled, fermentation should be complete. Unlike with soda, the residual sweetness in beer comes primarily from unfermentable sugars, and so no further carbon dioxide will be created no matter how long the beer sits at room temperature. In order to naturally carbonate beer in the bottle, brewers add a small, measured amount of sugar (usually corn sugar), which is enough to carbonate the beer when fully consumed by the yeast, but not enough to cause overcarbonation.
There are a few things that can still cause bottle bombs in beer brewing, however. These include:
- Early bottling. If the beer is bottled before fermentation is complete, for example because of a stuck fermentation, the beer may continue to ferment slowly in the bottle, resulting in overcarbonation. This is why it is important to take a final gravity reading to ensure that fermentation is complete; if your final gravity is higher than expected, try to restart the fermentation (see the Stuck fermentation article for hints).
- Overpriming. Adding too much sugar to each bottle can result in too much pressure building up. This can be as simple as putting too many sugar tablets in the bottle or miscalculating the amount of sugar to add to the bottling bucket. It can also happen if sugar is added to the bottling bucket and not stirred in vigorously enough, so that the sugar or sugar solution is concentrated at the bottom of the bucket and some bottles get more sugar than others.
- Infection. Some bacteria can ferment sugars that are unfermentable by ordinary beer yeast; if a bottle is infected with one of these, it can become overcarbonated even if the beer yeast has done all it can.
Bottle bombs in mead making
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Bottle bombs are more likely in making sweet mead, since the final product is intended to contain extra fermentable sugars from honey. Additionally, fermentation of honey is less predictable than malt: a mead can sometimes stop working for long enough to trick the brewer into bottling, then go into an additional fermentation which may overpressurize the bottles.
Prohibition bottle bombs
Another reason beer bottles have some reputation for bursting goes back to Prohibition in the US. During that time, many people made their own beer and wine at home (it was sometimes legal to make it, but not to buy it--and at other times, the draw was that it was possible to make it even when you couldn't buy it). Unfortunately, brewing supply stores didn't exist, and homebrewers made do. A beer maker would typically use Blue Ribbon hopped malt extract (which was widely sold for baking), lots of sugar, and baking yeast. Baking yeast, unlike brewing yeast, is hybridized to produce maximum CO2.
To make matters worse, homebrewers didn't use the modern method of waiting for the fermentation to subside, then adding a measured amount of corn sugar to produce a fairly predictable amount of carbonation. Rather, they typically watched the brew until the hydrometer reading dropped to a certain level, then bottled. Since it's nearly impossible to tell just where the yeast will ferment out a brew, the result was often unpredictable: many batches were either flat or overcarbonated. And considering they used baking yeast, it's not surprising that many closets full of homebrewed beer self-destructed back in those days.
Handling bottle bombs
If you know or suspect that a batch has been overcarbonated, treat it carefully. Open each bottle carefully, wearing gloves and eye protection, and allow the excess gas to escape. You can then recap the bottles, or pour the beer back into the bottling bucket, allow it to become flat (and complete fermentation if necessary), reprime, and rebottle.