Technique · 9 October 2026

Puck Prep: Why an Even Coffee Bed Decides Your Espresso

Sour and bitter at the same time? Then the water has found itself a channel. Here is how to prepare the ground coffee in the basket so the water flows through it evenly. — by Dieter Eitz

Water Takes the Easiest Path

When you pull an espresso, the machine forces hot water at around 9 bar of pump pressure through a bed of finely ground coffee. If that bed is uneven, the water finds the loose spots and shoots straight through them. This is called channelling. Along the channels too much is extracted, beside them too little. The result often tastes sour and bitter at the same time. A different grind size will only balance this out by chance, as long as the bed stays uneven.

You will find the basics of ratio, temperature and time in the article The Basics of the Perfect Espresso. This article is about the step before that: how the ground coffee gets into the basket.

1. The Right Dose in the Right Basket

Every basket is designed for a specific dose. Weigh it out rather than guessing, and stay close to the manufacturer's recommendation, within about a gram either way. If the basket is too empty, the risk of channels goes up. If it is too full, the shower screen presses into the coffee. After tamping, there should be two to three millimetres of headroom up to the shower screen.

To begin with, the double basket is the better choice. Single baskets are harder to fill evenly and are less forgiving.

2. Break Up the Clumps

Finely ground coffee tends to form clumps. A clump is a dense island in the bed that the water flows around. The remedy is stirring with needles, known after John Weiss as the Weiss Distribution Technique (WDT): push thin needles (about 0.2 to 0.5 mm) through the coffee down to the bottom of the basket in small circular movements until no more clumps are visible.

3. Distribute Evenly

After that, the surface should be level. A distributor smooths it out, and a light tap on the side of the portafilter at least settles the coffee roughly. Both come before tamping. The distributor mainly makes the surface smooth; it does not break up the clumps underneath. So the rule is: needles first, then distribute.

4. Tamp Level

When tamping, the exact weight matters less than pressing level and evenly until the coffee no longer gives way. A puck tamped at an angle is thinner on one side, and that is where the water runs first. The tamper should fit the basket so that no loose ring is left at the edge. Do not knock the portafilter after tamping, or the puck will come away from the edge.

5. Lock In and Brew Straight Away

The portafilter and group head should be preheated. Lock in the portafilter and start the shot immediately. That way every shot stays the same, and you can repeat the result.

How to Spot Channelling

• Bottomless portafilter: Without a spout you can see the underside of the basket. A good sign is an even flow that merges into a single stream in the centre during the first few seconds. Spurts, pale patches or an edge that runs first point to channels.
• Taste: sour and bitter at the same time, thin despite a fitting extraction time.
• The shot runs too fast even though grind size and dose are right.