If you’ve ever wanted to try your hand at cask-conditioned homebrew but want to keep things simple and inexpensive, a polypin may be just the thing for you. A polypin is a plastic, food safe, flexible container—kind of like the bladder that holds boxed wine, but sturdier. Popular among homebrewers in the United Kingdom, polypins are available in the United States under the brand name Cubitainer. Poly refers to the polyethylene from which the container is fabricated, and a pin is simply a unit of measure (half a firkin).
The beauty of these little containers is that they collapse as they are emptied. Unlike rigid firkins and kegs, which require that gas displace lost volume as the beer is dispensed, a polypin crumples in on itself, changing shape to accommodate the amount of beer it holds. No air in the polypin means no oxidation, which means that your beer stays fresh. And there's no need for top-up carbon dioxide, which means minimal expense and complexity. Some commercial British brewers even sell polypins of their flagship beers to-go, kind of like growlers for real ale.
And polypins couldn’t be easier to use. You just prime your fermented homebrew with corn sugar, siphon the beer into one or more sanitized polypins, seal each polypin with a screwcap, and wait a couple of weeks. When it’s time to serve, you carefully remove the screwcap and attach a quick serve spigot. As you gravity-dispense beer from the polypin, à la wine from a box, the plastic vessel collapses and keeps oxygen from getting to your beer. Kept at a cool 50–60°F (10–15°C), your cask-conditioned ale will stay fresh for as long as you can keep your thirsty paws off of it.