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    Explosive Fermentation

    How I learned to start worrying that my airlock is not enough and love the blow off tube!


    There is an old adage, “You are not a true brewer until you have mopped the ceiling” we all chuckle when we hear it but some of us know the true power behind our fermenting wort.

    How can you avoid this mess? Well first we have to start with the right airlock for the job.

    Right Tool for the Job

    Home brewers would have some pretty gnarly products without the help from our airlock friends.

    There are 2 primary airlocks when you are fermenting beer, wine, cider, or mead. The 3 piece airlock and the S-type (double bubble).

    3 Piece Airlocks: These airlocks are the most common, you probably remember them from your first home brew kit. These airlocks are easy to clean and great for short term aging. The 3 pieces of this airlock include the airlock body, piston and vented cap.
    S-Type (Double Bubble): These airlocks are fairly common at any home brew shop (like ours!). These are great for long term storage because the liquid in the airlock will evaporate at a slower rate.

    Why does all of this matter? If you are doing a beer that is 1.055 or less these are great! However problems can start to creep up when you have a beer that is a 1.060+ gravity. That said, let’s talk about all the factors that need to be considered to avoid an explosive fermentation.

    Starting Gravity: I mentioned above that if your beer is above 1.055 then you will start to run into problems. Well this has some truth but I always use a blow off tube with “bigger” beers. When you have a fermenter with 1.090 gravity barley wine it has some potential energy. As fermentation gets under way the krausen will grow in respect to the fermentation activity.

    Head Space: Head space in the fermenter is a huge factor that has to be considered. When you have a small amount of headspace then you limit the space the krausen has to grow. When you limit this head space it has to go some place (usually out the airlock). Anything fermentable tends to be sugary which can lead to plugging up your airlock. This is the perfect recipe for building pressure to rocket off the bung and airlock.

    Yeast: Different yeasts are another thing to consider. Do you have a basic English yeast or a rocking Belgian yeast working for you? Your mileage will vary from strain to strain but it is something to consider before using your nice towels to clean up.

    Temperature: Fermentation temperature is one the hardest things most home brewers struggle with, but temperature is one of the most important parts of fermentation. When you are fermenting a beer at an ambient room temperature of 65F the product is likely heating up 4°-10°F from the fermentation alone! When you are a single yeast cell that is a huge temperature difference, these higher temps can make the yeast work faster, harder and produce more esters.

    All these factors can blow the lid off your fermenter! If you control these factors you are in for a better and cleaner fermentation. However some beers can be exceptionally excitable, that is where the blow off tube comes in.

    Blow Off Tube: The blow off tube is an extremely complex piece of equipment that can help make sure that you aren’t redecorating the room with malt and hops. A blow off tube is made of length of tube and a jar/growler filled with sanitizer, the tube is fixed to the top of the fermenter. Yep, that’s about it. Those using carboys, a 1 inch diameter tube will work great and allow for maximum blow off volume. If you have a hard time finding that tube or want to use something a little more flexible, you can use 5/15ths tubing and either fit it into the airlock gasket or hole in the stopper.

    Wrap up

    There is absolutely no wrong way to make a blow off tube as long as it keeps a nice tight seal and stops the lid or stopper from launching into low earth orbit. It is always a good idea to check on your fermenter even with a blow off tube.

    Bottle Bombs

    Not just messy, but dangerous!

    Bottling is usually the first form of packaging a new home brewer will learn, some brewers will bottle forever while others will switch over to kegging. Anyone that has bottled knows there is a sweet spot when adding priming sugar (or other sugars) to reactivate the yeast for bottle conditioning/carbonation. Too little sugar, low carbonation and flat beers. Too much sugar, gushers or even the dreaded bottle bomb. Bottle bombs occur when the carbonation is so high that the glass bottle shatters, sending glass and beer at high velocities in every direction.

    How did it get to this point?

    Bottle bombs can occur for several different reasons. These are some of the most common causes of excessive Co2 build up in a bottled products:

    Bottled Too Early: Bottling too early can leave a lot of sugar in solution, when you bottle you are adding more sugar on top of that. The yeast does not really care which sugar is which, it’s in it for a free meal. The yeast will continue to chow down on sugars, releasing Co2 until the bottle explodes. The best practice is to wait a full 2-3 weeks and check your gravity using a hydrometer to confirm that fermentation has completed, do not rely solely on your airlock.

    Too Much Priming Sugar or Alternative Sugar: Fermentation is complete and you have confirmed this with your hydrometer. The beer you’re bottling is a honey porter, but you just wish you had more honey flavor so you add a little more on top

    Improper Storage: Heat can break down compounds in beer and cause additional unintended fermentation. It is best to carbonate a beer at the same temperature that it was fermented at.

    Infection: Bacteria and some wild yeasts are really good at eating sugars that are normally unfermentable. Final gravity of a beer will show you how much sugar you have remaining after fermentation. These sugars are fair game to any other bacteria or yeast that can digest it. When that happens you get extra unexpected Co2


    How do you safely handle a ticking time bomb? If this batch of beer is worth saving, then slap on some safety goggles and a face shield, a thick heavy jacket, and some thick or leather gloves. Safety gear is a requirement, bottle bombs are extremely dangerous.

    Under Pressure

    1. Equip all safety equipment before going any further.
    2. Check a few of the bottles outside to get a scope of how much over carbonation you are dealing with.
    3. If they pop and foam up and out in a rather controlled manner then you might be able to save the batch.
    4. Give the beer a taste if you can, if the batch tastes sour you could have an infection that is eating the remaining sugars.
    5. If the beer tastes sour (and it was unintended) or if tastes outright bad, then cut your losses and dump the batch. Review your sanitation and cleaning technique and try again.


    1. If you followed the steps above and the flavor is good but the bottles are a little too jazzed up.
    2. Secure the bottles in a heavy box or cover with a thick towel/canvas bag.
    3. Place the bottles in a cold fridge freezer. NOTE: If you are putting the beers in the freezer keep a close watch on them because you are not trying to freeze the beer. You are trying to cool the product down.
    4. Once you have bottles cold, put on your safety gear and one by one open the bottle, release the pressure and put a new cap on. The cold temperature should temper the explosive nature of the beer.
    5. If the bottle starts foaming too much or too hard just cut your losses and toss the batch.
    6. Keep all the bottles in the fridge going forward, this will slow any additional fermentation.
      Enjoy the beer quickly.

    Wrap Up

    Bottle bombs should be dealt with before they become a bigger problems. Safety should be a number one concern and when in doubt throw it out. You can always buy more bottles or make more beer, you only have 2 eyes.