Espresso channelling
Water blasts through one path in the coffee puck instead of flowing evenly, often visible as an uneven, spurting or fast-then-slow stream.
Applies to: Espresso machine, Espresso
What channelling is
CoffeeHQ explanation
Water under pressure takes the easiest route through the coffee. If one part of the puck is less dense than the rest, such as a crack, a gap at the edge of the basket or a patch that was clumped, most of the water goes that way. This is a channel.
The coffee along the channel is stripped of everything it has and turns bitter. The rest of the puck is hardly extracted and stays sour. The shot in the cup is a blend of both, which is why channelling tastes sour and bitter in the same mouthful and why it varies so much from one shot to the next.
Channelling is usually treated as a preparation fault: it is addressed before the shot starts, by how evenly the grounds are distributed and how level they are tamped. Grind is not wholly separate. One study of espresso inferred, from extraction falling at its finest settings, that a very fine grind can itself make flow uneven by partly clogging the bed; that was an inference from one machine, grinder and coffee, not something the authors observed directly.
Causes, most likely first
There are 3 likely causes, listed with the most probable first. Work down the list and change one thing at a time, so that when the problem goes away you know which change fixed it.
Cause 1 of 3 · Reported, not independently verified
Uneven distribution of grounds in the basket before tamping
- What to change
- Distribute grounds evenly across the basket before tamping, using a distribution tool or careful technique.
Cause 2 of 3 · Reported, not independently verified
Tamp not level
- What to change
- Tamp with even, level pressure so the puck surface is flat.
Cause 3 of 3 · Reported, not independently verified
Gaps between the puck and the basket wall
- What to change
- Ensure grounds fill the basket edge to edge before tamping.
If none of these fixes it
Check the cleaning and descaling guide. Equipment that needs maintenance can cause several of these symptoms at once, and no change of recipe will correct it.
Work through it for your setup
The causes above are general. These go to the part of a diagnostic that takes this symptom one check and one change at a time.
- Make my coffee better: Espresso machineEvery reading for this brewer: the check, and the one thing to change first
- Espresso puck preparation: the sequence
- Dialling in espresso: the workflow
Easily confused with
- A grind that is too coarse — A coarse grind gives a fast shot that is evenly sour every time. Channelling is inconsistent and adds bitterness. If two shots prepared identically taste different, think channelling.
- Spraying from a bottomless portafilter — Spraying is channelling made visible. A spouted portafilter hides the same fault; it does not prevent it.
What not to change at the same time
Do not adjust the grind while you are working on channelling. Until the puck is extracting evenly, the shot time tells you nothing reliable about the grind setting.
How to tell it is fixed
Two shots prepared the same way should taste the same. With a bottomless portafilter, the coffee should bead evenly across the whole basket and draw together into a single central stream.
Understand the science: Coffee extraction — Uneven extraction, where one part of the coffee gives up far more than another, is explained there.
Related issues
What to change
Specific adjustments for this symptom, with what each one trades off.
Work it out
Interactive tools that take you through this one change at a time.
Learn the technique
Step-by-step guides to the skills this problem comes from.
- Distributing espresso grounds (WDT)Leave the coffee bed uniformly dense throughout, so water meets the same resistance everywhere and does not carve a channel.
- Tamping espressoCompress the coffee bed into a level, uniformly dense puck, without introducing a slope or a crack.
- Diagnosing a shot with a bottomless portafilterRead what the underside of the basket is doing during extraction, and match what you see to the preparation fault that caused it.