Metal kegs or bag kegs? Push with CO2 or with air?
We’ve probably already seen or heard more than once in blogs or in conversations with brewers about types of kegs, their characteristics, etc. That’s why I’m going to focus on something more specific: back pressure in beer kegs and, above all, when to use air or CO2.
The most widespread system or product is CO2, or carbon dioxide, which mainly serves two functions inside a beer keg: to push the beer or other liquids, and also to keep the keg atmosphere carbonated and prevent it from going flat.
That’s why we could never use air to pressurize a keg unless it is a keg with an internal bag. But are there kegs where you can use regular air and don’t need CO2?
Yes, this type of keg has a bag inside, which is where the liquid is stored and, unlike the typical metal keg, the gas that enters does not come into contact with the beer. This means that an air compressor could be enough to push the beer and therefore it would not be necessary to keep buying CO2 cylinders. The system is similar to a “bag-in-box”.
The main differences between these two types of kegs come down to costs. On the one hand, bag-in-kegs are usually disposable and single-use, so the cost may seem higher at first glance. But on the other hand, with metal kegs you must keep in mind that besides having to buy CO2, you’ll have to clean them one by one to reuse them, which also means time and money, in addition to the transport costs of collecting the keg.
Non-returnable bag-in-keg containers have been especially helpful for shipping product to different parts of the world, as they have enabled international beer sales without having to worry about returning the container: you simply ship it and forget about that part.
So what would happen if I counter-pressurized a keg without an internal bag using an air compressor? Nothing in particular will “happen” in the moment: the air will counter-pressurize it just the same and you’ll still get beer out. So what’s the issue? First, from a health and safety standpoint you can’t do this if you’re selling to the public, because you’re contaminating the beer. And then, as we mentioned earlier, that beer will lose carbonation quickly and oxidize, since we mustn’t forget that we’re counter-pressurizing with the ambient air from wherever we are, not injecting CO2 into the keg. Imagine that the environmental contamination in that place is quite high—everything in that air is going into the keg. The rest is obvious: off-flavors, flaws, odors…
That’s why we could never use plain air, that is, use an air compressor to counter-pressurize a keg, unless it is a keg with an internal bag that is not in contact with the beer.
