Ask Mr. Wizard

Cask Ale Beer In A Bag


Justin Herrmann — Homer, Alaska asks,

Here in Alaska, before the pandemic, two breweries served real ale from a beer engine. Now it’s just one, and it’s a few hundred miles away, so I’m attempting it myself, conditioning in a bag (bag-in-a-box style). So far, I’m not achieving what I hoped. I’m priming with wort, and put some in bottles, some in bags. The beer from the bottles is fine, but the hand-pumped beer from the bags lacks proper carbonation. The bags expand quite a bit, so maybe the additional headspace is the problem? Some have suggested I should vent the bags, but I worry the venting will also keep the beer from reaching optimal carbonation; we don’t vent bottle-conditioned beer, right? Please help.


The bag-in-box method has never really been common among homebrewers, but is a technique used by many pubs around the world. The reason your beer is not carbonating is that a rigid vessel is required to house the bag. This allows the beer to be pressurized above atmospheric pressure and to become carbonated. The challenge with a bag-in-box type package for carbonated beverages is the seal required between the bag, the container, and the outlet to the environment.

The standard for this design uses stainless beer serving tanks to house specially designed liners equipped with an outlet spout that seals to the vessel with a special ring mechanism. These vessels are very common in Europe and are becoming more common in North America. Although it is possible to carbonate or condition beer in the bag, provided the tank pressure is high enough to prevent gas bubbles from forming, the standard method used with these tanks is to carbonate beer prior to filling. Since the liner separates beer in the bag from the atmosphere in the tank, high pressures can be used for beer dispense without affecting beer carbonation level. This same benefit also allows compressed air to be used for dispense without damaging the beer (shelf life varies by the type of liner). The liner also makes vessel cleaning much easier, and reduces water, cleaning chemical, and carbon dioxide consumption. Although the liners are recyclable, they are made from plastic and that is one of the few downsides to the general design.

Unfortunately for homebrewers, there are few small-scale choices of this type of container on the market. Mueller, Duo Tank, and Bier Drive are the primary suppliers of these vessels. Perhaps this idea could be explored as a new offering to the homebrewing market!

Your question is really more about producing cask-conditioned beer at home, versus how to serve beer from a bag. My suggestion is to use a soda keg to carbonate your beer if you are looking for the simplest approach. You can either rack your beer into your keg with some residual fermentables or rack into your keg after fermentation is complete and add priming wort or sugar. Whatever method you choose, a spunding valve (adjustable pressure relief valve) is a handy way to control the pressure in the keg. Although you can reliably dose sugar and carbonate without using a spunding valve, the method is not as easy. Conditioning method aside, you want relatively low carbonation if the plan is to serve something akin to cask ale.

I am a newer generation U.S. brewer, cutting my teeth in the late 1980s, and never really bought into the whole anti-bottled gas thing espoused by CAMRA (Campaign for Real Ales). If the idea is to serve beer with about 1.8 volumes (or whatever you want) of carbon dioxide using a beer engine, the easiest way to keep your beer fresh and the carbonation level consistent is to keep your keg pressurized. Beer at 50 °F/10 °C with an equilibrium carbon dioxide pressure of 9 psig contains 1.82 volumes of carbon dioxide. As long as you maintain this combination of temperature and pressure, your beer will be consistently carbonated at a pretty low level.

Although beer engines are simple to use, do interesting things to beer mouthfeel and foam, and look super cool, they are not designed for use with pressurized liquid because the design of the pump allows pressurized liquid to freely flow through the pump and out the nozzle. Two ways to prevent this from occurring is to either store the beer at a level that provides sufficient liquid head pressure to offset the keg pressure (in this example about 20 feet below the beer engine) or to simply install a check valve in the beer line. Check valves are normally designed to prevent back-flow, however, when used with a beer engine and a pressurized keg, check valves prevent flow out of the keg when the beer is not being actively pumped. Easy peasy!

If you do want to go old-school CAMRA, you can condition your keg, vent the keg to atmosphere, hook it up to your beer engine, and start pulling pints. The Achilles heel of cask ale is beer oxidation and the growth of aerobic beer spoilers, especially acetic acid bacteria. If you do decide to go
without external gas, you will want to consume your beer
in 3–5 days.

Response by Ashton Lewis.