Channelling is water finding a route through the puck that is easier than the rest of the puck. Once it finds that route it commits to it, so part of your coffee gets flooded and over-extracted while the rest barely gets wet. The shot arrives bitter and sour at the same time, which is a combination that confuses people because it feels like it should be impossible.

Everything called "puck prep" is an attempt to make the bed uniform enough that no such route exists.

Clumps are the main enemy

Ground coffee arrives from the grinder in clumps, held together by static and by the geometry of the chute. A clump is denser than the coffee around it. Water goes around it, not through it, and the boundary between clump and loose grounds is exactly where a channel starts.

Stirring the dry grounds in the basket with something thin breaks those clumps. This is the Weiss Distribution Technique, universally shortened to WDT, and it is among the largest single improvements available in puck prep. A set of fine needles works better than a single toothpick, because you want to pass through the whole bed without dragging it into new clumps. In our own testing below, only a blind shaker beat it, and not by much.

Spritzing the beans with a few drops of water before grinding, sometimes called RDT, cuts the static that causes clumping in the first place. It also reduces the mess considerably.

Level beats hard

Tamping pressure is the most argued-about and least important variable in this article. The often-quoted 30 pounds has no particular basis. Once the bed is compacted enough that water cannot push it around, extra force achieves nothing: coffee is not meaningfully more compressible past that point.

What does matter is that the tamp is level. A puck that is even slightly thicker on one side gives water a shorter, easier path down the thin side, and that is a channel. A self-levelling tamper is worth more than a strong wrist.

Consistent and flat, at whatever pressure you can repeat every time. Nobody has ever tasted the difference between 15 kg and 20 kg. Everybody can taste a sloped puck.

Dose and headspace

Baskets are designed around a dose. Overfill one and the puck presses against the shower screen, which can block flow and disturb the surface. Underfill it and the puck expands unevenly into the empty space when it gets wet.

The coin test is the old check: put a coin on the tamped puck, lock in, remove the portafilter, and see if it left an imprint. An imprint means you are close to the screen. Aim for a couple of millimetres of clearance.

What we measured

Rather than keep arguing about this from first principles, we put seven tools through our own testing and recorded what each one did to extraction yield.

Each tool got an episode of its own, with the shots and the numbers on camera: puck screens, WDT tools, cheap against expensive, the distributor, clicking tampers, the blind shaker, flat base against ripple base, and the blind shaker and WDT used together.

Two results are worth sitting with. The blind shaker came out on top, ahead of WDT, which is consistent with its mechanism: it declumps and levels in a single motion without a tool ever entering the bed, so there is nothing to drag grounds into new clumps on the way out.

The coffee distributor was the only tool that made things worse, and the condition attached to that result is the whole point. That figure is for a distributor used instead of WDT. Used after a proper WDT, it neither helped nor hurt: the shot came out the same as WDT alone.

The mechanism explains both halves. A spinning distributor reaches only the top few millimetres, and what it mainly does is press grounds downward. On a bed that has already been declumped there is nothing left for it to fix, so it changes nothing. On a bed that has not, pressing down leaves you with a puck of uneven density: compacted in some regions, still loose in others.

Water then races through the loose parts and crawls through the dense ones. Some of the coffee is over-extracted while the rest is still under-extracted, in the same puck, in the same shot. That is exactly the sour-and-bitter-at-once cup described at the top of this article, only now you have built it deliberately.

A distributor is not a declumping tool. Used after one it is harmless. Used in place of one it makes the problem it appears to be solving.

Which upgrades are worth it

The three levelling tampers landing within 0.35 points of each other is its own finding. Flat, ripple and clicking bases are marketed as meaningfully different and they did not behave that way. What they share is that they tamp level, and that is evidently the part doing the work.

Reading the shot

A bottomless portafilter turns channelling from a guess into something you can watch happen.