Espresso Channeling and Extraction Gushing: What to Do

Normally, when extracting espresso with a coffee machine, the flow rate transitions from slow to fast. FrontStreet Coffee has noticed that some friends encounter this situation: after pressing the extraction button, the coffee liquid suddenly gushes out violently, then immediately slows down, with the liquid column gradually becoming very thin - a completely inverted flow rate pattern. Why does this happen?

The Extraction Principle of Espresso

An espresso machine is essentially similar to a pressure cooker. It uses high-temperature hot water and applies atmospheric pressure to extremely finely ground coffee powder for rapid extraction, resulting in a concentrated coffee with rich crema. When we experience the fast-to-slow flow rate described at the beginning, we will most likely get an espresso with problematic extraction rate and mediocre flavor. Therefore, we need to identify the causes of this situation and make timely corrections and adjustments.

What Factors Might Cause the "Fast Then Slow" Espresso Flow Rate?

1. Exceeding the Standard Weight of the Portafilter

When you use more than the standard weight of coffee powder, the exhaust space reserved in the portafilter is filled with powder. The initial water flow can be forced out by pressure, but the subsequent liquid slows down due to increased pressure inside the portafilter. Therefore, when using a portafilter, it's best to use the median value of its standard powder amount. For example, for 18-21g as shown in the picture, we should take 19-20g to leave ample space for exhaust.

2. Uneven Tamping of Coffee Powder

Uneven distribution of coffee powder or offset center of gravity during tamping will cause channeling, allowing hot water to flow first from areas with less resistance. This will result in some coffee powder not being moistened, while the coffee powder soaked first by the water flow will expand rapidly, creating resistance that blocks the channel.

The best solution is to use auxiliary tools such as a distributor to evenly distribute the coffee powder when filling the portafilter, spreading it evenly to every place. Ensure the powder amount is evenly distributed before tamping (apply force evenly during tamping).

3. Stability of the Pressure Pump

When your pressure setting is too high, roller coaster-like extraction fluctuations easily occur. For example, if the bar value is set to 15, it will be subjected to 15bar pressure at the beginning of extraction, causing liquid to gush out violently, then instantly drop to around 9bar. For dark roasted coffee beans, this can easily extract undesirable substances.

We can solve this problem by adjusting the pressure. The first option is to reduce the pressure setting to the appropriate pressure for the beans being used - for darker roasts, 9-10bar can be used, while lighter roasts can be set one level higher. Alternatively, if conditions permit, we can use pre-infusion. For details on pre-infusion, you can refer to our recent article on how to make pre-infusion. Appropriate pressure adjustment can bring out just the right aromatic substances.

4. Coffee Beans Not Fresh Enough

For coffee stored more than two weeks after the roasting date, the carbon dioxide and volatile aromatic substances contained within the beans have gradually been released. Only a small amount of flavor substances remain inside the beans. Without the obstruction of gas, water can easily pass through the coffee puck at the beginning. As the puck absorbs hot water and increases resistance, the flow rate slows down.

Therefore, FrontStreet Coffee recommends consuming the beans as fresh as possible. If you have a large stock of beans, you can use stickers to seal the exhaust valve to reduce the breathing movement of gas release.

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FrontStreet Coffee
No. 10, Bao'an Qianjie, Yandun Road, Dongshankou, Yuexiu District, Guangzhou, Guangdong Province

Important Notice :

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Tel:020 38364473