polypin

A polypin is a rigid plastic container, typically holding 10.8 liters (equivalent to half a firkin or one pin in traditional brewery measures), designed to hold draft beer for delivery and consumption away from the brewery or pub. It consists of a food-grade plastic outer shell with an internal bag-in-box system that collapses as the beer is drawn through a tap mechanism, minimizing air contact and oxidation during dispensing. The standard polypin format became established in the off-trade (off-license and home delivery) sector during the late 20th century as a practical alternative to traditional wooden casks and kegs for lower-volume, shorter-term storage.

The container is constructed with a molded plastic body, typically high-density polyethylene or similar food-safe polymers, reinforced at stress points around the tap assembly. The internal bladder system uses a valve mechanism that allows CO2 to enter the space vacated by dispensed beer, maintaining carbonation pressure without requiring external gas supply or complex equipment. This makes the polypin significantly lighter and cheaper to distribute than metal kegs, reducing transport costs and handling labor. The tap assembly is usually a simple plastic or brass valve fitted through the container wall.

Common variants and applications

Polypins are manufactured in a standard 10.8 liter size, though 5 liter versions exist for home brewing. They are popular with cask ale enthusiasts and in the homebrewing community because they accept hand pumps or gravity dispensing without pressurized gas systems. The off-trade supply chain uses them extensively for delivery to pubs without cellar facilities and for retail sales of craft and specialty beers. The typical shelf life is considerably shorter than a keg, usually 2 to 4 weeks depending on storage temperature and beer style, because the internal bag can gradually allow oxygen migration if damaged.

Damage to the internal bag or valve is the most common failure mode, resulting in rapid beer deterioration or inability to dispense. The plastic shell can become brittle in cold storage or degraded by direct sunlight if not protected. The tap mechanism requires careful cleaning after use; residual yeast and protein deposits can block flow or cause contamination on the next fill. Returns logistics are important: some suppliers operate collection and refill systems, while others treat polypins as single-use containers, significantly affecting cost per pint for the end customer.

The term polypin combines the older brewing measure pin (half a firkin, roughly equivalent to 4.5 imperial gallons) with the modern material descriptor poly, short for polyethylene. The format occupies a middle ground between single-serve bottles and large kegs, serving venues and consumers who need convenient draft beer without the investment in professional cooling and gas systems. In regions with strong cask ale traditions, particularly the United Kingdom, the polypin remains a standard logistics unit for smaller pubs and clubs.