Talking Tech – The battle between good and bad carbon dioxide

An extended version of the article from Beer Buzz April – June 2019 issue.

In our last article we looked at the some of the containers that beer comes in. This time we are taking a look at something which divides many beer lovers – the role of carbon dioxide (CO2) in beer.

Let’s start by getting one thing out in the open – all beer should have carbon-dioxide dissolved in it. Whether it is a pint of Holt’s cask conditioned mild or Fosters made in Heineken’s factory in Moss Side, there is CO2 in your beer. There are many things that make the two drinks different – that one would be considered ‘flat’ and the other ‘fizzy’ is just one of them.

Carbonation, also referred to in brewing as ‘condition’, is measured in ‘volumes of CO2’. A volume is the space that the carbon-dioxide would take up at a standard atmospheric pressure at a temperature of 0° C. In other words, if a gallon of beer contained 2 volumes of CO2, the CO2 by itself would occupy the same space as two gallons.

A well-kept cask ale at cellar temperature should contain approximately 1.1 volumes of carbon-dioxide. A typical mass-market lager would be expected to contain 2.4 to 2.6 volumes of CO2 where a carbonated soft drink will typically have between 3 and 3.5 volumes. The reaction of the palate to the dissolved carbon-dioxide in your beer is part of the flavour profile – be it the gentle tingle on the tongue of good cask ale or the more pronounced ‘bite’ of a keg beer.

‘Nitro-keg’ beers (including the ubiquitous Guinness) are another category again – these tend to have low volumes of dissolved carbon-dioxide – in the 1.2 – 1.5 volumes region but also have nitrogen injected into them. The nitrogen has smaller bubbles which gives the characteristic ‘creamy’ appearance and mouthfeel. 

There are several ways carbon-dioxide gets dissolved in beer, but the most common is that it is generated naturally during the fermentation process. As yeast sets to work converting sugars in the wort to alcohol, the main biproduct is CO2.  

Many brewers will ferment in sealed tanks so that the naturally generated gas carbonates the beer while others ‘force carbonate’ either by applying CO2 at high pressure and low temperature or by forcing CO2 through the beer using a device known as a carbonation stone (for those of a certain age – think of a SodaStream in action).

A ‘Closed Conical Vessel (CCV)’ at Manchester Union Brewing Company

During cask conditioning, the carbon-dioxide generated during secondary fermentation in the cask is trapped in the sealed cask and absorbed into the beer. Contrary to what some believe, fermentation in a cask does not stop under pressure. There is a relationship between pressure and fermentation rate but at the pressures found in beer production, the effect is negligible and does not slow down secondary fermentation. 

Although commonly referred to as a process which takes place in the pub cellar, beer packaged with live yeast and sufficient fermentable sugars and held at suitable temperature will undergo secondary fermentation whether its in the brewery, in a warehouse or in the pub cellar. 

The levels of carbonation in a sealed cask can be significantly in excess of the expected final 1.1 volumes but the brewer must take care not to allow too much fermentation as the closures on a cask – the shive where a cask will be ‘vented’ and the keystone where the tap will be placed – will only hold back a relatively low pressure.

Cask beer which has been allowed to become too warm will frequently ‘blow’ one of its closures, increased temperature resulting in an increased rate of secondary fermentation and increased generation of carbon-dioxide.

The amount of carbon-dioxide which remains dissolved in a beer is determined by two factors – temperature and pressure. Physics determines the amount of CO2 which will remain soluble in a liquid at a given temperature – the lower the temperature, the more CO2 will remain dissolved.  

In a cask ale the CO2 produced during secondary fermentation stays dissolved in the beer until cask is ‘vented’ in the pub cellar – at which point any excess CO2 will then slowly escape to the atmosphere until it reaches the level which is soluble at cellar temperature. At 13⁰C this is 1.1 volumes and a vented cask beer needs time for the carbonation level to settle. Typically the time taken to vent off excess CO2 also allows other unwanted flavours in the beer to dissipate and desired flavours to develop. 

In a kegged beer higher carbonation levels are maintained by applying pressure to the liquid to keep the CO2 in solution (the skills of applying the right pressures in the cellar are for another day).

In many mass market lagers, carbonation is used to stimulate the tongue and mask the generally low flavour profile. However, for many modern brewers producing ‘craft’ beers for keg dispense, the intended carbonation is very much part of the design of a beer.

‘Craft’ brewers will design their keg beers to have anything from 1.2 to 3.0 volumes of CO2. A carbonation of 2.4 – 2.6 volumes would be typical but lower levels are often used for stouts and porters while highly hopped IPAs may use higher volumes to push out hop aromas from and prevent them tasting cloying.  Too high a carbonation for a given style and ‘carbonic bite’ can become overpowering and masks flavour.

It’s important to remember that all carbon-dioxide is the same gas – there is not ‘good’ and ‘bad’ CO2.  Whether it is generated during fermentation in a tank at the brewery, comes from a cylinder or is generated by secondary fermentation, it is all made up of one molecule of carbon and two of oxygen and once it is in your beer, you have no way of telling how it got there.

Whether you enjoy your beer gently or highly carbonated is a matter of taste.

Cask ale lovers enjoy the creaminess that comes with low carbonation while other drinkers find cask ales ‘flat’ and ‘dull’ and seek the lift from carbonation.  Different beer styles suit different levels of carbonation. There is no right and wrong.

Just enjoy the beer.

 

Explaining Beer Containers – Casks, kegs & KeyKegs

This is an expanded version of the piece which appeared in Beer Buzz January-March 2019

On the way from the brewery to your glass, beer is delivered from a variety of containers, including bottles, cans, kegs, casks and KeyKegs. You all know about bottles and cans, you probably know about casks but what about keg and KeyKeg?

Before we go on, we had better briefly mention the difference between container conditioning and brewery conditioning – we will return to this subject in more detail in future issues.

If beer is container conditioned it means that when put into the container, it still has some yeast and sugars and can continue to ferment and mature in the container. This gives a natural carbon dioxide (CO2) ‘condition’ to the beer – gas is dissolved in the beer naturally and produces the head and ‘bubbles’ when released into your glass. If the beer is pasteurised and/or filtered, then the yeast is killed and/or removed and the beer doesn’t ‘condition’ in the container . It will typically have CO2 (or nitrogen) injected under pressure to give the head and bubbles.

Some beers are conditioned in tanks at the brewery before putting into casks so the beer in the container is ‘bright’ – it contains very little yeast but is not pasteurised. Brewery conditioned beer can be “primed” with the addition of sugar and/or a small amount of yeast at packaging to gain condition in the container without having any significant impact on the flavour.

Generally, most cask beer contains yeast, while mass market keg lagers and “smooth” beers are filtered/pasteurised and artificially carbonated. The modern generation of keg & KeyKeg beers blur the boundaries as the beer they contain can be container or brewery conditioned.

The Cask

The cask is the container from which most ‘real ale’ is served. Most casks are made of stainless steel, although many smaller breweries use lower cost plastic casks. Some breweries still use some traditional wooden casks for special beers – these impart additional flavours to the beer. The Society For The Preservation of Beers from the Wood continues to encourage brewers to keep the traditions of wooden casks alive.

Usually containing some yeast when filled, cask real ale continues to ‘condition’ in the brewery and/or pub cellar. Cask beer is “vented” before service, allowing the carbonation to settle to a natural low level prior to being served. Lower carbonation is one aspect which sets cask ale apart from other formats. Cask ale is most commonly served from hand-pumps but is sometimes directly from the cask (as at beer festivals). In a small number of pubs it is pumped using electric or gas driven pumps to taps either on the bar or on the back wall.

Cask beer can be ‘fined’ (where a material, usually isinglass, is added to help the yeast drop to the bottom), or ‘unfined’ when the yeast drops naturally, sometimes leaving a ‘haze’ to the beer which is quite natural and not a fault.

Traditionally, the cask is placed on its side and beer poured or drawn from a tap inserted at the bottom. A more modern method of service allows casks to be stored upright and served using ‘spears’ that draw beer from the bottom or ‘widges’ that float just below the surface of the beer. The float systems mean the beer is always drawn from the top and therefore avoids the risk of drawing yeast sediment into the feed – assuming of course that the cask has been allowed to settle in the cellar!

As the beer is drawn out, air is drawn in either through a porous spile placed in the shive (the opening at the top of the cask) or, if the cask is vertical, through a vent that forms part of the extraction device.

Air is cask beer’s enemy and will result in the beer oxidising and spoiling if not sold within around 3 days. There is a device called a ‘cask breather’ that draws in CO2 at low pressure to replace the beer drawn out – this can help to prolong the life of a cask beer before it spoils. Some drinkers don’t approve of cask breathers and for many years CAMRA barred beers served on cask-breathers from being listed in the Good Beer Guide. This has now beer withdrawn with CAMRA neither encouraging or discouraging the use of cask breathers.

The Keg

This is the container that most of the major mass-market beers come in – the heavily branded ales and lagers which form the majority of beer drunk in pubs.

These mass-market beers have no yeast in the keg and the beer is pressured from a gas bottle (usually either pure CO2 or a mix of 60% CO2 & 40% nitrogen) in order to get it from the cellar to the tap. Keg beers usually pass through a flash cooler to give the ‘ice cold’ beer that the marketing folks love.

‘Smoothflow’ keg beers (including Guinness) use nitrogen instead of CO2 and are served using a mix of 70% nitrogen and 30% CO2.

Many newer ‘craft’ beers are also served from kegs. Most of these are not ‘conditioned’ in the keg but they will often have lower carbonation than the mass-market lagers and are intended to be served warmer.

The keg connector applies pressure from the top, forcing the beer out from the bottom of the keg via the spear. The pressure applied and the gas used can affect the product in the glass and it is up to the landlord to set the pressure appropriately for the beer. Unfortunately, many pub cellars are set up for serving mass market lagers without the adjustment needed for serving lower carbonation ‘craft kegs’ which can result in ‘craft’ keg beers not being served as the brewer intended.

Most kegs are made from stainless steel, but there are now several variants of plastic keg in widespread use (brands including Dolium & EcoKeg) – these are intended to be disposable, removing the need for brewers to collect empty kegs.

You should not confuse a keg with a KeyKeg, which is a relatively recent invention…

The KeyKeg

KeyKegs are an invention of a Dutch company called Lightweight Containers. They consist of an outer plastic container with an inner non-porous flexible bag that contains the beer. Pressurised gas or air is fed into the gap between the outer and inner containers and the beer is forced out of the feed at the top of the Key-Keg to the tap – the bag collapsing as beer leaves.

The gas never touches the beer so the carbonation level is set by the brewer and cannot be changed by settings in the pub cellar (although it I possible to “vent” a KeyKeg – or any keg for that matter – to reduce the carbonation).

If the beer is conditioned in the KeyKeg, as the gas used for serving it never touches the beer, such beers meet CAMRA’s definition of ‘real ale’. Some brewers are intentionally producing KeyKeg conditioned ales which contain active yeast although the majority of KeyKeg beers are brewery conditioned.

The beer exits via the top of the container with any yeast/sediment collecting at the bottom of the bag – KeyKeg conditioned beers need to be settled in the cellar just like a cask (although KeyKeg conditioned beers are rarely fined so will likely have a ‘haze’).

The majority of the plastic of a KeyKeg is PET, the same material as plastic bottles, however as they need to be dismantled, the UKs recycling industry still makes recycling KeyKegs difficult.

To counter this, the manufacturers have initiated a “OneCircle” project to build a network of collection hubs as part of a logistics operation to route used plastic kegs for recycling, initially in the Netherlands.

Hopefully, this helps you understand how your beer gets from the brewer to your glass. Now what the brewer puts into the container, that is an entirely different discussion!

This article has been adapted from an original piece written by Jack Summers-Glass for InnQuirer, the magazine of CAMRA’s Furness branch.