What to Do When Coffee Puck Sticks to Coffee Machine? What's the Purpose of Pressure Relief Function? Why Do Espresso Machines Explode?

The Mysterious Case of the Stuck Coffee Puck

Have you ever encountered this situation? After extracting espresso, you remove the portafilter and tap it against the knock box, only to find that the expected sound of the coffee puck falling doesn't come. Turning back, you discover that the puck has stuck to the group head of the coffee machine.

In the coffee community, this occurrence is far from popular and is even considered by many baristas as "one of the nightmares during work." So the question arises: Why does the coffee puck stick to the machine after extraction? And why is this situation so unwelcome? Let FrontStreet Coffee explain it all for you!

Why Do Coffee Pucks Stick to the Machine?

As we all know, espresso machines are typically equipped with a pressure relief function. The purpose is to release the pressure from the portafilter basket after completing the espresso extraction. This prevents explosive situations when removing the portafilter due to residual pressure (as shown in the picture below).

During pressure relief, a suction force is generated that carries away residual hot water from the portafilter basket. The reason the coffee puck sticks to the group head is precisely due to this force. Under normal operation, the suction generated during pressure relief is not sufficient to cause the puck to adhere to the group head. When we see the puck stuck to the shower screen, it usually means we've used too much coffee grounds.

The reason some water remains in the portafilter basket after coffee extraction is that there's a certain distance between the normal coffee puck and the shower screen at the recommended dose - this is the empty space. The existence of this space inevitably means that when we finish extraction, some hot water will remain in the basket because the machine stops applying pressure and cannot penetrate through. However, when we fill too much coffee grounds, this space is reduced, bringing the puck closer to the shower screen. Naturally, less hot water can remain in the basket at the end of extraction. When the suction from pressure relief cannot draw hot water, it begins acting on the puck itself. This is the primary reason why pucks stick to the shower screen, and on this basis, two additional factors increase the probability of this occurrence.

One factor is leaving the portafilter attached for an extended time after extraction when using excessive coffee grounds. The puck initially stuck to the shower screen might be shaken back into the basket when we remove it, but if we leave the portafilter attached for too long, the puck already adhering to the shower screen will dry out and stick even more firmly.

Another situation occurs when we tap the portafilter after tamping the coffee grounds! Tapping the portafilter creates gaps between the puck originally filled in the basket and the basket itself. This not only makes it more likely to stick to the machine after extraction but also更容易产生通道效应 (easier to create channeling effects) during the coffee extraction process, which in turn affects the coffee's flavor. In such cases, we need to pay attention to our actions after tamping to avoid uneven extraction flavors in the coffee.

What Negative Effects Do Stuck Pucks Have?

Except for the case mentioned last that triggers channeling effects, other reasons won't affect the coffee extraction, so we don't need to worry too much (this actually serves as proof that the coffee machine is operating normally). The main reason this situation is unwelcome is that often, coffee professionals don't notice the stuck puck by looking down at the group head, but rather by the sound of it falling from the group head into the drip tray as its adhesion weakens.

If it's a drip tray like the one on FrontStreet Coffee's machine, cleaning a puck that has fallen onto it becomes extremely troublesome! And it's hot too!! It would be strange if people liked this situation!!

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Important Notice :

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FrontStreet Coffee Address: 315,Donghua East Road,GuangZhou

Tel:020 38364473