How to prevent channeling

by Simone Forgia

Floating Puck Technique

The problem of channeling is a much more serious problem than many people think. When the water pressure hits the coffee, it disturbs the particles of the ground coffee and if the puck is not well prepared, cracks that ruin the extraction are created. The barista's skills are then put to the test, but it is not always easy to determine whether there have been any canalisations. In fact, very often these fractures are microscopic and therefore not visible to the naked eye. 

The causes of channelling can be many and the first of these can be the use of a low quality coffee grinder. In fact, if it is not of good manufacture, it grinds unevenly, the grains, being of various sizes, do not fit together well and the water passes more freely where the larger particles collide. A similar problem can also occur if clumps are present, which can also happen with better coffee grinders. To avoid this, it is possible to use a stirrer and the Weiss Distribution Technique (WDT) for a more even grind distribution.

A second reason may be the use of too much coffee for the size of the basket. When the grinds come into contact with water, they increase in volume and thus they need space to expand, otherwise they push against the showerscreen and channels can occur. Filters usually have an indication of the grams of coffee that can be fitted inside them, however, you cannot always rely solely on this data, as some coffees, for the same weight, are more voluminous than others.

The third and most common cause could be the use of an incorrect pressing technique. It is important to tamp perpendicular to the surface and in a single movement. Tamping several times, as is often seen, can in fact fracture the coffee puck internally, without it being visible from the outside. You must also be careful not to hit the portafilter when inserting it into the machine.

The causes listed above occur during the initial coffee preparation phase, but problems can also occur during extraction. If, for example, you raise the lever too quickly, you may create a vacuum effect that ruins the coffee bed. In addition, if you do not make a correct pre-infusion, the puck is not allowed to become completely saturated and a too high pressure when the coffee is dry may cause a shock with consequent cracks. A possible solution to reduce this impact may be to use a paper filter between the showerscreen and the coffee so that it protects and attenuates the water jet.

Another suggestion we can give you to stress the coffee less during the extraction, could be to use a reduced flow in the first seconds of pre-infusion. On lever machines unfortunately, it is not possible to precisely adjust the water flow with a precise command, however, with a little sensitivity and experience you can lift the piston until you feel that the water starts to enter the extraction chamber, but keeping the water inlet hole slightly obstructed so that the flow is reduced.

This trick allows for a better saturation of the ground coffee with less disturbance. It also leads to a lower migration of fines that would otherwise accumulate on the bottom of the filter and consequently obstruct it. By reducing the accumulation of smaller particles on the bottom, we will have a more even percolation, enabling us to grind the coffee finer and obtain a higher extraction percentage.

La Pavoni water flow comparison

Flow comparison between the completely open and the half-open hole

Let's assume now that a channel has formed, the water will flow with less resistance and we will be tempted to apply more force on the lever to maintain the same pressure. Acting in this way, however, will increase the water flow even more, which in turn will widen the channel even more and thus a vicious circle will be created.

Pressure drop due to channeling

Pressure drop due to channeling

In order to understand if there have been channeling problems during extraction, a drop in pressure may therefore be a first indicator. For more serious cases, it will be visible by looking at the coffee puck at the end of the extraction. Furthermore, if we have a bottomless portafilter, we can observe if there are any colour variations or splashes. The presence of light shades can in fact be a symptom of this type of problem. Finally, the most reliable tool to determine the presence of channeling problems always remains our palate. Indicators can be herbaceous notes and an unpleasant acidity, typical of under-extraction.

The "Floating Puck Technique", which is currently being studied by The Lever's team, is another technique that can be used to observe the presence of canalisations. This new technique is based on the fact that if there are channels inside the puck, then dipping it into the water should cause air bubbles to escape. In this way not only will we be able to determine whether channels are present, but we will also be able to locate where. In order to ensure the success of the experiment, however, you must make sure that the coffee is not shaken when you remove it from the filter. To avoid this, at the end of the extraction you can let the coffee puck dry leaving the portafilter attached to the espresso machine for 10-15 minutes and then you can remove it and rotate it carefully. In this way the coffee puck should come out intact and undisturbed. At this point you can immerse it in water and watch for air bubbles to escape. A poorly prepared coffee panel will disintegrate after a few minutes and if there are any major cracks in the panel, pieces of coffee may come off and sink. In the case of a well-prepared panel, however, it should remain compact and afloat for more than 12 hours.

Below you can see illustrative pictures of a coffee puck immersed in water. As you can notice, as soon as it is dipped in water it floats well, but air bubbles start to come out from the top, which can be an indicator of channels. After a few minutes, the upper part starts to dissolve, as it probably suffered from excessive stress caused by the water jet during extraction. After another few minutes, further horizontal cracks begin to appear, which could lead to a bad pressing technique. Over time these cracks expand further and further until they reach the point where a part of the panel detaches and sinks. So, as you can see, a panel that might have looked good at first actually proved to be poorly prepared.

Puck after 1 minute
Puck after 1 minute

Puck after 2 minutes
Puck after 2 minutes

Puck after 45 minutes
Puck after 45 minutes

Puck after 1 hour and 6 minutes
Puck after 1 hour and 6 minutes

Puck after 1 hour and 7 minutes
Puck after 1 hour and 7 minutes

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