Dispense digest: What happens inside a cask beer as it conditions?

Is it just me, or does cask beer (also known as real ale) often have an air of mystery about it?

Perhaps because it’s not very common in the States or because I’ve only worked in bars that don’t sell real ale, I’ve always found its unique production method a bit hard to wrap my head around.

It’s often said that cask beer “finishes” in the pub’s cellar, but what does that actually mean?

What happens inside a cask beer as it conditions?

You’re about to find out.

Two pints of cask beer at Whitelock’s Ale House, Leeds (Credit: Five Points Brewing Co)

But first, a quick reminder of what we mean cask beer. 

According to the Campaign for Real Ale (CAMRA), cask beer, or real ale, is:

“a name for draught (or bottled) beer brewed from traditional ingredients, matured by secondary fermentation in the container from which is dispensed, and served without the use of extraneous carbon dioxide.”

-The Campaign for Real Ale (CAMRA)

We won’t delve much into the dispense side here, as our focus is on the maturation, but it’s important to note that, unlike kegged beer, all cask beer is dispensed without any additional top pressure on the cask.

A keg of beer is dispensed by applying pressure to it, effectively pushing the beer through the lines and to the tap. The only pressure acting on cask beer is simply atmospheric pressure (or the pressure of the gases in Earth’s atmosphere), which is acting on us all at all times.

(Sorry to get science-y so quickly, but this comes back up a bit later on.)

Now back to conditioning.

As a cask of beer conditions, it experiences three key changes:

  • the beer’s flavours will mature,
  • it will “drop bright” (meaning its clarity will improve, eventually becoming clear),
  • and it will develop the appropriate level of carbonation, also called condition.

Before we dive into the details of each, we’ve got to say a few words on what happens to cask beer before it leaves the brewery.

Generally, a cask will be filled with beer (importantly, still containing live yeast cells) and will have some priming sugar and finings added in. (I’ll explain the purpose of each of these ingredients shortly.) From here, the cask is sealed with a plastic bung called a shive and shipped out to the pub.

Once the cask arrives to the pub, it’s brought to the pub’s cellar (which should be set between 11-13° C, 50-55° F) and conditioning begins. (Update: It’s worth noting here that this is the recommended cellar temperature for cask beers specifically. Kegged beers should typically be stored at cooler temperatures, closer to 3° C, 38° F.)

Casks ready for dispense at a beer festival (Credit: Five Points Brewing Co.)

First up: flavour

Cask beer is considered a “living” product because it still contains live yeast when it leaves the brewery. This yeast is given an additional dose of sugar – called priming sugar – which fuels the yeast’s secondary fermentation.

Secondary fermentation takes place at a slower rate than primary fermentation.

It produces the beer’s condition or carbonation (which we’ll talk about shortly), but it also produces a broad range of aroma and flavour compounds. As secondary fermentation takes place in the presence of a higher alcohol content, the resulting mix of compounds is slightly different, affecting the finished beer’s flavour.

Additionally, if any hops were added into the cask (in a process called dry-hopping), the beer will take up flavours from the hops during this time.

Next: brightness

It’s important for the cask to be left undisturbed in the cellar while it conditions, as this allows it to “drop bright”.

During this time, the finings that were added in at the brewery will help to settle any yeast sediment, clarifying the beer. Finings carry a positive charge, while yeast is negatively charged, so the two clump together and sink to the bottom of the cask.

Historically, finings were typically made from the swim bladders of tropical fish, which is why many cask beers weren’t vegan, but these days veggie-friendly alternatives are available.

(Update: It’s also worth a mention that some modern breweries don’t use finings at all, leaving their beer with a natural haze and going against the traditional bright beer conventions. If this is the case, it should be stated on the pump clip so consumers know that a bit of haze is to be expected in the beer.)

Note that the beer can’t clarify while secondary fermentation is still active, as the carbon dioxide gas that’s being produced will continue to move any sediment about. The beer brightens towards the end of the conditioning process.

Lastly: condition or carbonation

The bung tapped into the cask as it leaves the brewery (called the shive) is intended to help contain the build of up pressure from the carbon dioxide gas produced during secondary fermentation.

Under the high pressure in the cask, some of that carbon dioxide will dissolve into to beer, while some remains present as in the headspace of the cask as a gas. Before the beer can be dispensed, that excess gas needs to be vented off.

This happens through a process called spiling. Inside the center of the shive is a small recess containing a plastic plug called a tut. The tut is punched through, creating a hole in the shive that’s quickly filled with a small, porous reed peg called a spile. Any undissolved carbon dioxide gas exits the cask through the spile.

Once the gas in the headspace has left the cask, the gas that was dissolved into the beer starts bubbling out of solution.

As discussed in last week’s dispense digest on nitro beers, molecules are always looking to move from areas of high concentration to low concentration to achieve equilibrium.

Before venting, the beer in the cask is super-saturated with carbon dioxide gas. Once it’s been vented though, the cask is no longer pressurized and that gas isn’t being forced to stay in the beer.

So it starts to leave, reducing the beer’s carbonation until it reaches a new equilibrium with the gas in the headspace.

What happens next is pretty fascinating.

At cellar temperature (11-13° C, 50-55° F) and at atmospheric pressure, a cask of beer will hold just over one cask’s worth of carbon dioxide gas.

Why? As we discussed last week, beer carbonation – measured in volumes of CO2 – is an absolute measure that’s relative to the volume of the container holding the beer.

But, the level of carbonation in a beer responds to changes in storage and serving conditions – namely, temperature and pressure.

In a cellar a constant temperature (11-13° C, 50-55° F) and pressure (1 atmosphere), a venting cask will always reach equilibrium at the same point – 1.1 volumes of CO2. So, in theory, all cask beer should eventually end up with the same level of condition or carbonation.

This chart shows that at cellar temperature (13° C) and atmospheric pressure, a cask of beer will hold 1.1 volumes of carbon dioxide gas. At a constant pressure, if the temperature increases to 20° C, the beer will lose carbonation and go flat. Alternatively, if the cellar is too cold (10° C or less), the beer will gain carbonation and become over-saturated. (Credit: Cellarmanship)

Of course, not all cellars are kept at a consistent temperature, atmospheric pressure varies with a city’s elevation, and not all beer is given the same amount of time to vent to be able to hit that equilibrium point, but it’s interesting to think through the science behind how the standard level of condition or carbonation in a cask beer is achieved.

During this process, it’s important to check the soft spile regularly to see that gas is still venting. (Be sure to replace it with a fresh one if it gets wet or covered in sediment).

When gas is no longer leaving the cask and the soft spile is clean and dry when checked, that’s a good indication the ideal condition has been achieved. (As seen above, that’s typically around 1.1 volumes of CO2, but may range anywhere from 0.8 to 1.5 volumes.)

At this point, a hard spile is inserted into the shive, in place of the porous soft spile. As no gas can escape through the hard spile, this step preserves the carbonation level in the beer until it’s ready for service.

One final check:

The time required for conditioning varies greatly from style to style and cask to cask. As a generalization, most beers need at least two to three days from venting to the start of service.

Then it’s time for one final check.

A beer should always be inspected in the cellar before it’s served to make sure conditioning has fully finished. If it’s served too soon, it will be hazy (if it hasn’t dropped bright) and it will have aromas of green apple from compounds that indicate that secondary fermentation hasn’t fully finished (hence why unfinished beer is often referred to as “green beer”).

A beer that’s ready for service should be clear, gently carbonated, and smelling and tasting appropriate for the style.

And that’s how a cask conditions.

Five Points Brewing Co’s brand new casks (Credit: Five Points Brewing Co.)

Note: This post has been updated after receiving some helpful feedback from bar manager extraordinaire Mauritz Borg. Thanks, Mauritz!


Sources: Cellarmanship (5th ed) by Patrick O’Neil

Image sources: Five Points Brewing Co Instagram

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