Understanding What Is & How To Fix Espresso Coffee Channelling
Beyond how coffee grind size affects over/under extraction, channelling is another of the most common reasons behind an espresso shot not quite tasting right. Unfortunately it's also one of the hardest problems in espresso making to diagnose (and therefore to fix). Here we explain how to diagnose and troubleshoot channelling so that you can make better espresso coffees at home. Let’s get started.
What is coffee channelling?
Channelling occurs when your brewing water finds paths of least resistance in your ground coffee puck. Since your water is put under a lot of pressure when brewing an espresso coffee, once these low resistance pathways (channels) are formed, a lot of your brewing water will flow through it. This means that the ground coffee immediately around these channels will be over extracted and the ground coffee further away from your channels will be under extracted. This puts a combination of unpleasant sour notes from the under extracted coffee and harsh bitter notes from the over extracted coffee combined into your espresso. Severe channelling will probably make your espresso undrinkable.
How to diagnose coffee channelling
There are a few ways that you can diagnose espresso channelling and here we run through them from the easiest to the most difficult to spot:
1 Your espresso flow suddenly speeds up mid shot - this speeding up occurs when your brewing water finds these channels and starts rushing through them.
2 Your espresso flows slowly but is still sour - if your espresso shot pulls in over 30 seconds and is still sour, then this sourness is likely caused by channelling rather than simply by under extraction (we talk more about why this distinction is important later on).
3 You can taste sharpness and bitterness in your espresso - in all honesty it's not that easy to distinguish channelled espresso from over or under extracted espresso, so we’d recommend diagnosing your shot by looking at how it flows and tasting it rather than just by taste alone.
4 Note: if your espresso tastes sour and runs quickly through the entire shot, then under extraction is likely the issue rather than channelling.
How to troubleshoot coffee channelling
Here we run through the causes of channelling, from most to least common, and how to troubleshoot them:
1 Uneven Tamping - if your coffee bed is not level then water will flow faster through the thinner part of the puck than the thicker part. This has the same effect on your espresso’s flavour as channelling. To fix this, use a distribution tool (photo above, WDT, Weiss Distribution Technique) to break up the clumps in your ground coffee bed and make sure that you tamp with your portafilter on a level surface. Try to get your coffee bed as level as possible before you tamp.
2 Grinding too finely - doing this will make the resistance that your coffee puck offers against your brewing water too high. This often results in channelling occurring around the outside of the puck where your coffee tends to be more loosely packed. Since channelled espresso often tastes very similar to under extracted espresso, and under extracted espresso is generally caused by grinding too coarsely, your espresso is only likely to be channelling due to grinding too finely if it both tastes sour and flows slowly. To fix this, grind your coffee more coarsely. Remember if your espresso tastes sour and flows quickly (in under 20 seconds) then you instead need to grind finer. This is a problem that trips up a lot of people making espresso.
3 Inconsistent grind size - if your coffee puck is made up of coffee ground at different sizes then your brewing water will naturally flow through the coarser ground coffee as there is more space between each grind size. To fix this, purge an espresso worth of coffee through your grinder before grinding your next dose. This should ensure that any coffee ground at your grinder’s old setting won't be mixed in with your next dose.
4 Not enough headroom in your Portafilter - if you overfill your portafilter basket then you’ll end up mashing ground coffee into your brew head. This will immediately make your ground coffee puck uneven and increase your chances of channelling occurring. To fix this, lower your ground coffee dose, in which you want there to be at least a third of a centimetre headroom between the top of your ground coffee puck and the top of your portafilter basket. If you have a Sage or Breville espresso machine, they come with a dose trimming tool to make this as easy as possible.
5 Your espresso machine' settings are to brew at too high a pressure - if your machine is indeed set at too high a brewing pressure, your brewing water will be emitted at such a force that it breaks your coffee puck. This creates the very channels that we want to avoid. This is the least likely cause of channelling, and requires the most radical fix, so I’d only look into this if you’ve looked into every other potential cause and nothing has worked. To fix, you’ll have to open up your espresso machine and adjust its over pressure valve. This is pretty easy to do on some machines, but very challenging on others.
If you think you've channelling within your espresso shots, do try our pointers here and hopefully you'll be able to diagnose and find a solution towards making a better espresso coffee. Please do remember, the biggest mistake that people make when they encounter channelling is they think that because their shot tastes sour they need to grind finely. If your shot is pulling steadily between 25-30 seconds and your espresso is sour then try grinding coarser rather than finer. As for the other important part, if you are looking for coffee wholebeans perfect for espresso then our Blend / Darker coffee subscription is well worth a consideration - darker roasts extract more quickly than lighter roasts, making them ideal for espresso brewing.
GUSTATORY (adjective): curating excellence in taste.