This article was originally published in What's Brewing in December 2015 by CAMRA's Technical Advisory Group. It was adapted from an article by Trafford & Hulme's John O'Donnell which appeard in Opening Times magazine in November 2015. The original can be read here. Although some thinking and CAMRA's outlook has changed since it was originally published, it has been reproduced here to complement the original.

When is keg not really keg? Real ale from membrane kegs explained.

Back in April (2015), delegates at CAMRA’s Members Weekend in Nottingham voted to back the idea that real ale could come from a Key‐Keg. Many commentators saw this as a major step towards CAMRA modernising and recognising the legitimacy of new forms of real ale, while others saw it as the thin edge of a keg‐shaped wedge.

In fact, the motion was passed a full four years after CAMRA’s Technical Advisory Group (TAG) first expressed the view that Key‐Kegs can contain “CAMRA approved” real ale, a decision it reached after taste trials held at the Great British Beer Festival. What was obvious at Nottingham was that even an audience made up of CAMRA activists was unclear about the Campaign’s position or what membrane kegs are all about. This piece aims to demystify these clever containers.

When CAMRA was established in 1971 it fought against the sweeping trend for cask‐conditioned beers to be replaced by recarbonated “keg” beers. Four decades on the word keg still has massive negative connotations for many CAMRA members, leading to their dismissal as “fizz”. However, what those pioneering members were really fighting against wasn’t the physical containers the beer was served from but the product inside them, often made with low quality ingredients, usually filtered and often even pasteurised, killing much of the flavour in the process.

The second renaissance of British brewing over the last ten years or so has seen a significant resurgence in kegged beers, the vast majority of which have little, if anything, in common with the keg beers of the 1970s & 80s. Unlike their predecessors, some of these new brewers understand that flavour is reduced by filtering and particularly by pasteurising their beers so they don’t do it. In some cases the beers these brewers put in their kegs is exactly the same as they put in their casks – complete with live yeast that will provoke secondary fermentation in the keg. The only thing that stops these beers being real ale is that traditional kegs require the application of compressed gas to propel the beer to the bar.

This is where membrane kegs come in. Invented in 2006 by a Dutch company as a one‐way container (i.e. filled once, used, then disposed of), the key to the system is the “bag in a ball” principle. The beer is sealed in a strong, flexible synthetic bag held inside a rigid plastic outer layer – originally a sphere but these days more commonly a tall cylinder. To serve the beer, the space between the bag and the rigid outer layer is filled with gas under pressure, forcing the bag to collapse and pushing the beer out to the bar. Effectively, the membrane keg is a flexi‐walled cask that shrinks as the beer leaves it.

The gas does not come into contact with the beer so no extraneous CO2 is introduced as it a pressurised CO2 system and so it makes no difference which gas is used to do this. pubs will use CO2 because it is on hand anyway but if CAMRA beer festivals  prefer to use compressed air dispense, so be it, although the use of a handpump and vented container will result in levels of carbonation more typical of conventionally-served cask beer. The choice of gas makes no difference to the level of CO2 in the end product.

Importantly for CAMRA, the way membrane kegs work means that if what went into the bag was real ale – unpasteurised, unfiltered beer containing live yeast – what comes out can still be real ale, matured by secondary fermentation in the container from which it is dispensed but it may be much more highly-conditioned (gassy) than normal. 

Membrane kegs, being disposable, are too expensive to replace returnable traditional casks, but they have advantages for breweries in certain circumstances. For example, real ales can be delivered to irregular or far away venues, including for export, without the worry of retrieving expensive casks.

The downside of this disposability is some will consider them to be not very environmentally friendly.

They also have some technical advantages. Not only does CO2 not come into contact with the beer, neither does oxygen – the agent that causes real ale to go off within a few days. As a traditional cask empties, the space above the remaining beer is filled with air, which causes oxidation. In a membrane keg the bag collapses, thus keeping oxygen out, meaning the beer stays fresher for longer. This allows real ale to be served in places that do not normally have enough throughput to sell a cask in three or four days. It also allows pubs to increase their range by stocking slower selling, stronger, speciality styles alongside their regular cask offerings.

The Key‐Keg company market their product to cask ale brewers as Key‐Cask but the only difference in practice is the name – the container holding the beer is the same. There are also currently three other disposable keg systems on the market made by different companies.

Key‐Kegs / Key‐Casks are very awkward to vent, meaning that the beer within them can become naturally over‐carbonated compared to the 1.1 volumes of carbon dioxide in every volume of beer that is the norm for cask-conditioned ales vented at normal cellar temperatures (12-14 °C). Over-carbonation can occur with pressurised gas even though it does not come into contact with the beer, due to the combined effects within the container of the higher pressure and carbon dioxide generated in secondary fermentation.

One of the newer brands of disposable keg, Eco‐Keg, is much easier to vent and is also designed to work with traditional hand‐pulled beer engines – i.e. “through a handpump”.

These containers, however, cannot be cooled by other than cellar cooling, which can potentially cause problems at some beer festivals, and there is no guarantee that the dynamics of the collapsing bag will not disturb the sediment at the bottom of the container.

While real ale can be served from membrane kegs, not all membrane kegs contain real ale as the format can also be used to serve filtered and / or artificially carbonated beers. However, some brewers believe strongly in natural conditioning. Mark Welsby of Manchester’s Runaway Brewery is now packaging all his draught beer in membrane kegs, holding them in the brewery for several weeks to allow lengthier conditioning, generating a higher level of natural CO2 in the container. Beers conditioned in this way will tend to retain the carbonation generated at the brewery – meaning they are better protected against losing condition through poor handling in the pub.

Which takes us full circle back to Motion 13 at CAMRA’s Members Weekend. Those who were there will remember the passionate speech by brewer Phil Saltonstall, another brewer who ensures all his membrane kegs contain naturally conditioned ales. While Phil’s Brass Castle Brewery at Malton (North Yorkshire) also produce award‐winning traditional cask beers, he argued successfully that CAMRA should support brewers by allowing them to produce real ale in a membrane keg, provided it is differentiated by CAMRA‐approved labelling.

The labelling scheme is yet to be launched but some CAMRA festivals are already moving ahead with this and beginning to feature membranekegged real ale alongside cask. London’s Pigs Ear and York Beer Festival were among the first, and from 2016 will be joined by the Manchester and Leeds festivals, the former featuring a dedicated “Real Ale from Key‐Keg” bar.

CAMRA is currently drafting guidelines for festival organisers to serve these beers safely, easily and attractively. There are potential health and safety issues, and serving via a handpump, although preferred, is not easy on every system. We will publish these as soon as they are completed.

Finally, we must remember why the awful keg beers of the 1960s came into being. It wasn’t just that the big brewers found cheaper and therefore more profitable to produce, it was also that the inability of some publicans at that time to keep cask beer properly led to it being inconsistent and unpopular.

It can be predicted that more and more breweries will experiment with this format and it remains to be seen how popular it will or will not become. At this stage, provided beers can be labelled appropriately CAMRA members and festival customers can try them and feed back their own opinion. Some branches may also wish to hold their own controlled blind comparison tastings and feedback on the views expressed.

The pros and cons of the membrane keg vs the broached cask


  • Less oxidation so the beer takes longer to spoil
  • Good for smaller turnover venues and for exports
  • Enables lower demand special beers to be stocked
  • Avoids the cost of expensive lost casks
  • Allows for longer conditioning at the brewery
  • Beers retain a high condition


  • Some models are difficult to vent
  • Difficult to cool except in cellar conditions
  • Some models not compatible with handpumps
  • More costly and less eco-friendly if non-recyclable
  • The collapsing bag can cause haziness
  • Beers are very likely to be over-conditioned

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