What is Espresso Extraction Channeling? What are the Consequences of Perforated Espresso Pucks?

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When extracting espresso, a common problem is unstable flow rate, which sometimes becomes faster and faster. The coffee feels thin when shaken, and the crema color is also light. This situation indicates that the coffee puck has "perforated."

What is "perforation"?

When you finish extraction and pull out the portafilter, you'll see that the puck surface is uneven and has some small holes, like this ⬇️

More severe cases look like this ⬇️

These holes are created when water breaks through during extraction. When the holes are broken through, the resistance of the entire puck to water becomes uneven. This allows more water to pass through these holes, making the flow rate appear faster. Because the water passing through the holes hasn't fully extracted the coffee grounds, the entire coffee becomes thin and the crema color is light. This phenomenon is generally called "channeling."

How to detect perforation?

Bottomless portafilters make it easiest to detect perforation. Let's first look at normal extraction versus extraction with perforation. This shows the flow rate and crema distribution during normal extraction. You can see that the flow rate is generally uniform without speeding up or slowing down, and the overall crema progresses orderly from dark to light.

This shows the extraction situation when perforation occurs. You can see that the coffee flow rate is on the fast side and getting faster, with two instances of spraying. The locations where spraying occurs are exactly where perforation happened. You can see that the flow rate at perforated locations is much faster, and the crema color is lighter than in other areas.

Why does perforation occur?

There are three main reasons: uneven distribution, insufficient coffee dose, and concentrated water flow.

Uneven puck density is a frequent cause of perforation. When the puck density varies, water easily breaks through areas with lower density, leading to perforation.

To solve this problem, distribution and tamping steps are performed before extracting coffee. Distribution ensures coffee grounds are roughly evenly distributed in the portafilter basket, preventing extreme differences in density between coffee layers. Tamping compresses the evenly distributed coffee grounds into a puck that won't scatter when hit by water.

Many friends know that extracting espresso requires distribution and tamping, but often they perform useless distribution - still uneven after distribution, and perforation still occurs when it should.

The reason lies in not properly receiving coffee grounds with the portafilter, or wanting to even out the coffee grounds after receiving them, but making the coffee layer more and more compact. A relatively easy way to achieve even distribution is to push the coffee grounds into a small mound (high in the middle, low on the sides), then place the distributor vertically over the portafilter basket and rotate several times. If there are no issues with distribution, then tamping only needs to ensure it's level.

In cases of insufficient coffee dose, the shower screen is farther from the puck. During extraction, the surface coffee grounds might be agitated by water impact, causing surface resistance to be inconsistent and making perforation more likely. Therefore, some friends increase the coffee dose or switch to a smaller portafilter basket to avoid this risk.

The function of the shower screen is to divide the powerful high-pressure water column into evenly sized water streams. However, if the shower screen is blocked by some back-sucked coffee residue, the water flow distribution becomes uneven. During extraction, one side might have excessive pressure and break through the coffee layer. This situation is also easy to detect - normal water flow should be evenly dispersed by the shower screen, forming 4-5 water streams. Abnormal situations show water converging into a single stream, at which point you need to remove and clean the shower screen.

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