Puck Prep & Channeling: Extracting Evenly
Why does an espresso sometimes taste sour and bitter at the same time? Almost always, channeling is the cause: the water finds its own channels through the coffee puck. Play through how distribution, tamping, grind size and a puck screen change the evenness — and see where channels form.
The sliders: All three are relative scales from 0 to 100 with no unit — no grams, no kilograms, no microns. Distribution (WDT / grooming): 0 means clumpy and uneven, 100 means finely loosened and level. Tamping: this is where the most common mix-up lives — the number is not a force. “Tamping 70” does not mean 70 kilos or 70 newtons, but rather: already pressed fairly straight and evenly, though not yet perfectly level. 0 would be a crooked, uneven tamp, 100 a completely level one. Grind size: 0 is coarse, 100 is very fine. These are not microns either, just the direction from fine to coarse. Puck screen is a switch, on or off.
The results: Channeling risk and evenness are scores from 0 to 100, not measurements and not lab results. They show you the direction your puck is moving in, not how the espresso will taste. The two numbers are the same information seen from two sides: evenness is simply 100 minus the channeling risk. In the image you can also see whether the tool draws zero, one or two channels — this illustrates the score; it is not a prediction of where a channel would form in your basket.
The basis: The risk is made up of four contributions: poor distribution feeds in with a weight of 0.62, a crooked tamp with 0.50, a very fine grind with up to 0.25, and a puck screen on top takes off a flat 0.12. Grind size only counts at all from setting 55 upwards. The ranking of these levers is well supported — first distribution, then tamping, then grind size; this is consistent with the espresso modelling by Cameron and Hendon (2020). The numbers themselves are not. 0.62, 0.50, 0.25 and 0.12 are estimated weights, not measurements, and the threshold of 55 for grind size is estimated too. They put the levers into a plausible order; they do not quantify them. A model that shows the direction — and it shows that reliably.
This is a simulator. It works with a simplified model and shows you how things relate — not measurements and not a promise. What ends up in your cup depends on your machine, your water, your beans and the way you work.
Our recommendations are based on experience from advice, courses and the showroom. They are non-binding and come with no guarantee. We accept no liability for any damage resulting from their use. If in doubt, follow what your manufacturer says in the user manual.
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Straight to the results: Channeling risk and evenness are scores from 0 to 100 from our model — not measurements. They are based on weights we have estimated, and the channels in the image are drawn, not measured.
What happens during channeling
Water always takes the path of least resistance. If the puck is uneven — clumps of fines, loose spots, a crooked tamp — the water flows preferentially through these weak points. In the channel, the coffee is heavily over-extracted locally (bitter, astringent), while the rest of the puck stays under-extracted (sour, hollow). The result: both at once in a single cup.
Mathematical modelling of espresso shows why: very fine particles tend to clump together and create differences in density that encourage uneven flow. Even distribution and a level tamp are therefore the most effective levers for a clean extraction — more effective than most people think.
One point needs bringing together, because this page has so far answered it twice in different ways: the puck screen. Further down it says its effect is “rather small” — whereas the tool above puts it at 12 out of 100 points less channeling risk. Read together, this means: in the calculation, the screen replaces roughly what you would also achieve with 19 points better distribution or 24 points better tamping. That makes it the weakest of the four levers — and the 12 points are estimated, not measured; the 19 and the 24 are calculated from them. Measured effects on yield and flavour are small and inconsistent. So the screen does not replace good distribution; it complements it.
Source (scientific): M. I. Cameron, C. H. Hendon et al. (2020), “Systematically Improving Espresso: Insights from Mathematical Modeling and Experiment”, Matter 2(3), 631–648 (Cell Press). In addition: the review “Culinary fluid mechanics and other currents in food science” (2022). The weights in the simulator’s calculation (0.62 for distribution, 0.50 for tamping, 0.25 for grind size, −0.12 for the puck screen) are not derived from these sources — they are reasoned assumptions.
Clean puck prep — step by step
- Dose & level: A consistent dose, with the coffee distributed evenly in the basket — no hills, no holes.
- Loosen & distribute (WDT): Break up clumps with fine needles and distribute evenly. This is the biggest lever against channeling.
- Tamp level: Straight and with even pressure. A crooked tamp is a channel waiting to happen.
- Optional puck screen: A fine screen on top of the puck spreads the incoming water more evenly across the top and keeps the shower screen clean.
- Look & taste: With a bottomless portafilter, you see spurts and channels straight away. Sour and bitter at the same time? Then go back to step 1 — no right or wrong, just more even.
What helps against channeling
The most effective tools are a WDT tool / distributor for even distribution, a well-fitting tamper (or an automatic tamper such as Puqpress for consistent, level pressure) and a puck screen for even water distribution from above. Our Sensory 101 goes deeper into how over- and under-extraction then feel in the cup; you can play through the numbers in the Extraction Simulator.
Frequently asked questions
What is channeling?
Channeling means that the brewing water finds channels through the coffee puck instead of flowing through it evenly. In the channels it over-extracts (bitter), in the rest it under-extracts (sour) — and both end up together in the cup.
Does a WDT tool really help?
Yes. Loosening and distributing evenly breaks up clumps and reduces the differences in density that create preferred paths for the water. A level, even tamp remains just as important, though.
Do I need a puck screen?
It is not essential, but it helps: it spreads the water more evenly across the top and keeps the shower screen clean. The measurable effect on yield is small and inconsistent — against channeling, what counts most is the distribution in the basket. So that you read the simulator correctly: there, the screen lowers the channeling risk by 12 out of 100 points, roughly as much as 19 points better distribution. This figure is estimated, not measured.
Why does my espresso taste sour and bitter at the same time?
That is the typical sign of channeling: an over-extracted channel (bitter) and an under-extracted rest (sour) in the same cup. Better distribution and a level tamp usually solve it.
You will find all our interactive tools gathered together in our coffee tools.
