Carbonating in Kegs or Growlers

This is a great question and one I always like answering. Beer can be conditioned, a.k.a. naturally carbonated, by capturing carbon dioxide produced by yeast in a conditioning tank, bottle, can, or keg. The most common home method of conditioning is in bottles, hence the general term bottle conditioning. In past discussions about this method, I have focused on the how. This answer is going to focus on the safety aspects of “bottle” conditioning, starting with keg conditioning.

Kegs are great for a variety of reasons, including convenience, minimal packaging labor, relatively low cost, and their safety features. Most homebrewers and beer consumers don’t think much about pressure safety when it comes to beer kegs because we very rarely hear about exploding kegs. Commercial beer kegs are not only designed to withstand pressure much greater than that used for beer dispense, they are also built with an integral rupture disk to prevent explosions. Beverage gas regulators also include a pressure relief device that vents gas pressure above ~55–65 psig. This gives commercial-style kegs three levels of protection.

Cornelius kegs used by most homebrewers have a pressure relief valve on the lid (the thingy with the pull tab) and are also protected by the beverage gas regulator; still a belt-and-suspenders level of safety! Outside of gross negligence, there is not much that’s going to cause a keg to blow up. Plastic kegs are a different product and all brewers need to know about their kegs because plastic kegs have exploded and resulted in at least one death. That’s a topic for another day.

Bottles are different than kegs because glass bottles do not contain relief valves and do indeed have pressure limits that are sometimes exceeded. But like kegs, we don’t hear too much about bottles exploding in the commercial marketplace. The rate of bottle failures is likely higher for homebrewers who don’t have labs or packaging quality assurance measures. Most commercial beer is carbonated before packaging or bottle-conditioned to a level that aligns with the pressure rating of the bottle because no one is keen to blow up glass beer bottles. However, there have been some high-profile bottle failures in the market associated with diastatic yeast, high levels of fermentables in fruit beers, and some near-misses related to hop-creep. In all of these examples, beer carbonation levels resulted in pressures exceeding the bottle pressure ratings.

What about growlers? Well, most of the growlers used to fetch beer from your favorite local places that do growler fills are not rated for any internal pressure greater than atmospheric.

Let’s pause here and review a few carbonation basics. Most beers in the world contain about 2.5 volumes of carbon dioxide, or about 5 g/L. The equilibrium headspace pressure of beer at 38 °F (3.3 °C) containing 2.5 volumes of carbon dioxide is about 11.25 psig; it doesn’t matter if the beer is put into a bottle, can, keg, or serving tank, the pressure needs to be 11.25 psig to satisfy the level of carbonation at this temperature. Because packaged beer is in a sealed container, pressure increases with temperature. The carbon dioxide contained in the beer in this example increases the headspace pressure to 29 psig when the beer is warmed to 68 °F (20 °C), and then to 56 psig when the beer temperature rises to 104 °F (40 °C). (Note that this does not appear like a linear relationship because we normal folk use gauge pressure instead of absolute pressure.)

Technical details aside, a hot bottle of beer is packing about 95 psig of pressure if the carbonation is pushed to 3 volumes (6 g/L) and the temperature rises to 122 °F (55 °C) in the back seat of your car on a hot and sunny summer’s day.

Back to your question. There are no safety concerns at all to condition in a keg, even if you somehow managed to over-carbonate your brew and heat it up 122 °F (50 °C). And if your hot, gassy beer is in a Corny keg you may have some beer foam in your car when the relief valve blows, but nothing more than a beer mess. The same beer in a growler? No bueno! Depending on the type of bottle and its history (new versus used), that hot bottle in your car may up and explode. And if that does happen, just hope you are not in the car.

Here are a few factoids that I hope will help folks with glass safety:

1. Screw-top growlers are intended for short-term transfer of beer from tap, back to a refrigerator, and into a glass. Tales about cold growlers of ale blowing up in the fridge are not something floating around the web, so it’s safe to say that this is not a problem.
2. Growlers are known to burst in cars when beer tourists are hauling home precious pints, take a long pit stop for lunch, and return to their car only to discover a pool of beer and shattered glass in the back seat.
3. Heavy-duty growlers are really just large versions of re-usable flip-top bottles. These containers are made from much thicker glass than growlers and are designed to contain carbonated liquids over a wide-range of pressures, including the high temperatures used for pasteurization. No problems bottle-conditioning in these types of containers.
4. For all practical purposes, liquids are non-compressible. However, liquid density does change with temperature because liquids do expand with temperature. This is why bottles should never be filled completely full. Although the effect of temperature on volume is negligible, for example 1.000 gram of water occupies 355.000 mL at 4 °C (39 °F) and 355.331 mL at 70 °C (158 °F), a bottle of beer without headspace can result in bottle failure when no headspace is available for expansion. Easy enough, just don’t fill a flip-top full and expel all gas when closing . . . accidentally been there, done that, and can attest to bottle failure within an hour as the beer warmed.
5. Re-used glass bottles fatigue with time, especially if they are heated to sanitize. Commercial bottling lines designed for use with returnable bottles use in-line scanners to detect fissures in glass that are signs of fatigue and reject these bottles before the filler. Homebrewers don’t have glass scanners, so it’s a good practice to visually inspect bottles. When I began homebrewing, returnable glass bottles were common and we would collect good-looking bottles for use at home. One-way glass bottles are the norm these days and are not designed to be used multiple times. Bottle failures are much more problematic when these single-use bottles are used for homebrewing.

The last thing I will say about glass safety relates to eye protection. Wear safety glasses whenever using a counter-pressure bottle/growler filler! Wear safety glasses or face-shield and gloves if a bottle-conditioned bottle/growler blows up and you decide to open all bottles from that batch. Assuming that all bottles/growlers were filled from a bottling bucket dosed with priming sugar, it’s a safe assumption that all bottles contain about the same pressure. If one bottle/growler failed, others may follow and you do not want to be near a bottle when it fails without proper protection. Brew safely and happiness will follow!

Response by Ashton Lewis.