Homebrewing: Things That Go Boom In The Night

carboy filled with golden liquid with lots of foam, layer of sediment and an airlock

src=”http://www.elspethpayne.com/wp-content/uploads/2012/02/carboy-working1-209×300.jpg” alt=”” width=”209″ height=”300″ />This manuscript was first published in Tournaments Illuminated, a quarterly magazine for medieval enactors.  It appeared in the Third Quarter 2010, Issue 175.  Copies may be ordered from: http://sca.org/links/shopping.html .

Please, if you quote me, give me credit.  Copyright applies.

 Things That Go Boom In The Night

While supporting local brewing events, Atlantia’s Royal Brewer hears a lot of stories and sagas from other brewers.  Here she tackles one of the more common problems brewers share:  what to do when your new five gallon batch of brew explodes in the closet.

Almost all brewers have this story.  You know the one – about the carboy that exploded in the hall closet, where you didn’t discover it for days or weeks. The one that got your spouse saying, “You can brew again when –.”  When you move to a bigger place.  When you have a spare bathroom.  When you aren’t going to make the house smell like that again.  When you get the closet and everything in it scrubbed and the closet painted.  When hell freezes over.

You’ve spent a lot of time lovingly preparing your mead or your beer, creating your “must” (mead) or “wort” (beer) by combining ingredients in the right order at the right temperatures, setting them up for the perfect fermentation.  You’ve carefully transferred your must or wort to a carboy, a thick-walled (usually) glass container with a narrow neck, holding the one or three or five gallons.  You’ve made sure there’s space between the brew and the narrow neck of your carboy; you need some oxygen in there to begin with (it’s bad later on, and I’ll talk about that down below). You’ve primed the yeast, and added it at the perfect temperature for it to take off and reproduce like mad (yeast eats sugars and gives off alcohol and carbon dioxide as byproducts).    You’ve put a stopper with an airlock in the carboy’s narrow neck, so that the carbon dioxide the yeast produces can get out, but ambient bacteria or wild yeast can’t get in.  There’s a lot of detail and care (and expense) that goes into assembling a batch of mead or beer. But now you have a system under pressure, as the yeast produces more and more carbon dioxide in a stoppered glass container that’s got to vent the gas fast enough to to keep the pressures inside and outside even…

In the worst cases, your carboy can actually shatter.  Bursting carboys are dangerous, as they can send shards of heavy glass flying.  At best, you get a puddle of your brew all over the floor (and probably walls).  It’s a sticky mess to clean, and you’re down one carboy and a batch of (expensive) brew.  One friend told me about having a hops stain on his ceiling until he spackled over it prior to moving out, many years later.  There are lots of those stories, too.

So what’s an eager brewer to do?  What can you do to minimize the Great Exploding Carboy phenomenon?

First, let’s look at what causes it.  Carboys are corked in such a way as to let carbon dioxide replace the oxygen and other gases in the carboy, in that “head space” between the liquid and the stopper with its airlock.  Oxygen is desirable when you’re first pitching yeast into the must, but after that can bring unpleasant off-flavors to your brew.  Growing, metabolizing, multiplying yeast produces Co2. The extra gas being produced in your carboy is allowed to vent through an airlock.  At first you’re venting mixed air, with extra carbon dioxide, but as the yeast continues its life cycle it fills the extra space with CO2, and if there’s a little extra it goes the same route as the oxygen.  The airlock is there to keep bad bacteria and wild yeast out of your brew (germs always fall down) as well as to release extra CO2 up and out.  Always remember, yeast is alive, and like most living things isn’t entirely predictable.  The amazing lab-produced brewing yeasts of today mostly do what you expect – but not always.

Sometimes that airlock gets clogged.  That’s easy to fix – it’s just debris in the way, which probably means spices or fruit.  To avoid this in the first place, put your spices or fruit into a (sanitized) mesh bag and suspend it in your must or wort, rather than letting it float freely in the must.  Make sure you can reach the neck of the bag, or tie a (sanitized) strong to it, for easy removal.  Once it’s clogged, remove it and clean it out – q-tips and special small brushes are very helpful – and re-sanitize both the airlock and the cork.  If possible, have a replacement cork and airlock sanitized and ready to swap out immediately – remove the first, replace it, then go clean the clogged one. You don’t want oxygen interacting with your must if you can help it.

By the way, if your airlock is clogged from debris, you’ve almost certainly over-filled the carboy.  Try to fill only to the shoulder, where the carboy curves in toward the neck.  Once you’ve oxygenated it initially (to help the yeast start its aerobic fermentation) and got the carboy corked, you don’t want more oxygen coming into contact with the wort or must.

If you think your recipe is going to produce more must than your carboy can take, either scale it down or keep a spare 1-gallon carboy, cork and airlock ready (that means sanitized).  Now, if your overflow only partially fills a carboy it brings up another challenge – it takes much longer for the yeast to generate enough CO2 to fill the empty space, if it ever does.  Remember oxygen can damage the flavor of your brew.  Ergo, you don’t want to fill a carboy only half-full; no matter how healthy your yeast is, it may never fill all that space with its own CO2.  Do your best to plan so you fill your vessels appropriately.  Topping off with water just dilutes the brew, which is not an optimal solution.  Generally, if you’re careful following a recipe, you’ve already noted what size batch it’s calibrated for and have planned for it.  Almost all modern recipes are calibrated for a five-gallon batch.  Historic recipes can be be for hundreds of gallons!

Now, if your carboy is properly filled and you have a very vigorous ferment going, it’s still possible that the must or wort will back up into the airlock.  Suddenly you notice that, instead of clear water in the airlock, you have murky liquid.  That’s another way to clog the airlock and reduce its effectiveness.  If you see backflow putting wort or must in your airlock, sanitize a fresh airlock and swap them out carefully, being careful to wipe off any scum around the neck of the carboy that might block the bottom of the airlock or act as a conduit for bacteria.  Make a note (you do keep notes, right?) that this particular recipe is a vigorous fermenter, so you can be ready next time.  I have a beloved (to me, anyway) braggot recipe that is so vigorous, it will blow the locking lid off a brew bucket every time!  It’s likely to make a small geyser through the airlock if I don’t keep an eye on it.  I plan for it, now that I know – for with this recipe, it can go on for the first two or three order nexium weeks.  Thank goodness positive pressure has so far kept infection out!

When you find your brew in the midst of an overly vigorous ferment, you need a way to release the pressure from your carboy now.  Your best bet is probably a blowoff tube.  You’ll need an inexpensive length of plastic tubing from your hardware store (sanitized, of course) that will fit over the venting tube in your airlock, and a bucket.  Get hose that is the right diameter to fit snugly over that central tube.  Take the top lid off your airlock, and fasten that inexpensive length of clear plastic hose (sanitized) to the central tube inside your airlock, where CO2 is vented up and out.  Put the other end in a bucket or other large container, so the rising foam and debris has somewhere to go other than your clean kitchen floor.  (Yes, I even sanitize the blowoff bucket most of the time.)  Need I say to keep an eye on the tubing to make sure it doesn’t clog, especially if you have berries or something in your brew?  As soon as the pressure abates, dismantle the blowoff tube and put the lid back on the airlock.  Positive pressure is all that’s keeping stray bacteria etc. out, so as soon as that pressure is released you must put the complete and unclogged airlock back in place.  And now we return to our normally scheduled fermentation…

Extra-vigorous fermentation may happen because you double-dosed your yeast – esteemed modern meadmaker Ken Schramm, author of The Compleat Meadmaker, often creates recipes often use double doses of yeast,.  It’s an effective way to get a quick, clean ferment, which reduces the chance of producing unpleasant off-flavors from slow or difficult yeast metabolism.  Your recipe may simply be one with a particularly good balance of nutrients and simple sugars, like my own vigorous braggot recipe.  It may happen because you used more yeast nutrient than strictly necessary.  It is less likely to happen with a sweet mead, but in early phases of primary fermentation it’s certainly possible.  Honey abounds in sugars but is comparatively nutrient-poor.  It happens fairly commonly with melomels or fruit beers, since fruits have not only sugars but also nutrients that yeasts thrive on. (Almost all the exploding-carboy stories I hear are from melomels.)  If you put fruit in your secondary fermentation vessel, you tend to get another burst of activity, when you’re not used to having to watch for it.

Your best bet is to keep the carboy where you can eyeball it every day.  I keep mine in a corner of the kitchen, often for the first six months or so.  I wrap the carboy to keep the must or wort protected from light (old t shirts work well, or scrap fabric); I like to be able to see the cork and airlock easily.

Remember that your yeast has several stages of metabolism.  The first stage is that pleasantly active aerobic one where you see bubbles rising through the airlock.  Some folks time them and record the timing in their brewing notes.  Your yeast is reproducing like mad, and giving off the by-products that give your brew that green taste as well as carbon dioxide and alcohol.  Once the yeast has used up most of the oxygen in your wort or must, it changes to anaerobic fermentation.  It’s a much less efficient form, but this is where most of your fermentation actually happens.  Your yeast uses up its former byproducts that made it taste green, and the flavor of your brew matures.  You can’t see it happening, generally.  It doesn’t produce much if any visible bubbling.  It can go on for a pretty long time, and it’s still producing extra CO2 that needs to be vented. If you bottle a brew before it’s mostly done fermenting, you may get bottles ejecting their corks in a gyser of brew, or even shattering.  Either way it’s a mess and a waste of brew!

There are several brewing handbooks that have good information and detailed descriptions (beer brewers in particular can be very scientific) of how fermentation works, what equipment is available, and what common cautions are for beginning (or not-so-beginning) brewers.  I find that almost all books about brewing describe the how-to.  Alas, the good technique guides are twentieth century or more recent, as it’s taken the development of modern chemistry and biology to let us really understand the science of what makes a brew work.  There are a plethora of period recipes that give us insights to how beverages were made, and a lot of the period technique was very exacting – they may not have known exactly what yeast was, but they sure knew how to feed and propagate it.  Most present-day brewers use modern equipment and ingredients, though, and want to control their product more carefully than was available in period.  I’ve included the texts I mention here in my bibliography.

Ken Schramm’s The Compleat Meadmaker offers a very good discussion of fermentation and meadmaking process, along with a dose of history; Ken is frequently invited to speak on the history of mead.  Brewing Classic Styles, by Jamil Zainasheff and John Palmer, also describe in detail where beer ingredients come from and how they’re processed for different styles of beer.  Charlie Papazian’s The Complete Joy of Homebrewing has been a brewer’s classic for years.  All of these gentlemen are national-award-winning brewers.  Just to give an example of the degree of science common to modern brewers, Papazian’s book is listed in Amazon under the categories Beer, Reference, and Biotechnology.  Don’t let that alarm you, you don’t have to be a chemist yourself to make good brew!

I would be remiss if I didn’t mention that your local brewing club, or chapter of American Homebrewers’ Association (AHA, at http://www.beertown.org/), is a very good source of advice and suggestions.  Brewers love to talk about brewing with other brewers!  I find many members are aware of the SCA, though they don’t share our fascination with making period beverages.  The AHA has a listing of its local chapters (“registered clubs”) here:


and a national list of homebrew suppliers (members of AHA) here: http://www.beertown.org/apps/shops/index.aspx

The Brewers’ Association supplier list, which includes professional breweries as well as homebrewers, is here: http://www.brewersassociation.org/attachments/0000/0233/Allied_Trade.pdf

Podcasts such as The Jamil Show (Jamil Zainasheff again) and Brew Strong offer helpful information, too.  Brew Strong is a beginner-intermediate show; Jamil is flat-out talking tech.

Fermentation vigor is a good thing.  It means your yeast is growing and multiplying in a particularly healthy https://oclaserdental.com/early-signs-of-gum-disease/, vibrant way.  It just needs a little judicious management, and to manage it you need to understand its basics.  This will allow you to be the envy of friends and neighbors as you provide them with an array of delightful beverages, avoid messy cleanups and possible injuries, save the cost of replacement equipment, and perhaps even encourage a degree of domestic tranquility and support for your hobby.



Bravery, H.E. (1970).  Home Brewing Without Failures.  ARC Books, Inc., New York, NY.

Calagione, Sam (2006).  Extreme Brewing.  Quarry Books, Beverly, MA.

Gayre, Robert and Charlie Papazian (1986).  Brewing Mead: Wassail in Mazers of Mead. Boulder, CO: Brewers Publications.

Papazian, Charles (2003).  The Complete Joy of Homebrewing.  Harper Paperbacks, third edition.

Schramm, Ken (2003).  The Compleat Meadmaker.  Brewers Publications, Boulder, CO.

Zainasheff, Jamil and John J. Palmer (2007).  Brewing Classic Styles.  Brewers Publications, Boulder, CO.







Related Post