Channelling is water at nine bars taking the path of least resistance through the coffee bed. If one part of the puck is less dense than the rest, water goes there — and the coffee it passes through is over-extracted while the coffee it went around is barely touched.
The result arrives in the same cup: sour and bitter simultaneously, which extraction alone cannot produce.
The causes, in order
Clumps. Ground coffee leaves the grinder in knots, and a knot is denser than what surrounds it. This is the biggest single cause.
An uneven or tilted tamp. A thin side of the puck is a channel waiting to open.
Knocking the portafilter after tamping. Tapping to settle the puck cracks it, and a crack is a channel.
An under-filled basket. Headroom above the puck lets water pool and escape round the edges.
Seeing it
A bottomless portafilter turns this from theory to picture. A good shot comes down as one even column that converges into a single stream. A channelling shot sprays sideways from one or two points, often before the rest has started.
Fixing it
Stir the bed with a WDT tool through its full depth before tamping. That breaks the clumps, and it costs about twenty euros. Then tamp level and do not knock the basket.
Grinding differently does not help, and grinding finer usually makes it worse.
Questions people actually ask
Can channelling happen on a bean-to-cup machine?
Yes, and you cannot see or fix it — the brew unit doses and tamps internally. It is one reason automatic machines have a quality ceiling regardless of the grinder inside them.
Last reviewed 8 August 2026