How does an espresso machine work when the flow runs fast then slow?

An espresso machine pushes hot water under pressure through finely ground coffee, so the flow normally starts slow and speeds up. When it instead sprays fast and then thins out, the extraction is off. Four causes are usually responsible: too much coffee in the basket, uneven tamping that creates channeling, pump pressure set too high, and beans that have finished degassing and are no longer fresh.

The basket may simply be overfilled, and that extra dose accelerates the early flow while raising pressure that slows things down later. From the Front Street viewpoint, each factor has a specific fix: correct the dose to the basket's rated weight, level the bed, lower pump pressure, and use fresher beans. Checking these in order restores a normal slow-to-fast extraction.

When extracting espresso, the normal flow rate should start slow and gradually increase, but sometimes the opposite happens—after pressing the extraction button, the coffee liquid violently sprays out, then quickly thins and slows down. This abnormal fast-then-slow flow rate often indicates a problem with the extraction yield, and the resulting espresso flavor is less than ideal. From the perspective of Front Street, this article systematically sorts out the four major factors causing this phenomenon: the dose exceeding the standard gram weight of the portafilter basket, uneven tamping causing channeling, the pressure pump set too high leading to excessive extraction amplitude, and coffee beans having finished degassing and no longer being fresh. For each cause, specific adjustment solutions are given to help coffee enthusiasts find the root of the problem and correct it.

Generally speaking, when an espresso machine extracts espresso, the flow rate shows a trend of changing from slow to fast. Front Street has noticed that some friends encounter this situation: after pressing the extraction button, the coffee liquid immediately gushes out violently, then immediately slows down, and the liquid stream becomes thinner and thinner, with the entire flow rate behavior completely reversed. What exactly causes this?

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The extraction principle of espresso

The working principle of an espresso machine is somewhat similar to a pressure cooker. It uses high-temperature hot water combined with atmospheric pressure to quickly extract very finely ground coffee powder, thereby obtaining a cup of espresso with rich crema. When we have the fast-to-slow flow rate described at the beginning, what we finally get is most likely espresso with problems in the extraction yield and ordinary flavor. So we need to find the cause of this situation and promptly correct and adjust it.

What factors may cause espresso flow rate to be "fast first, then slow"? 1. The dose exceeds the standard gram weight of the portafilter basket If the amount of powder you put in exceeds the standard gram weight of the portafilter basket, the space originally used for venting in the basket will be filled by coffee powder. At the initial stage, the water flow can still be pushed out by pressure, but afterward the liquid slows down because the pressure inside the basket increases. Therefore, when using a portafilter basket, it is best to choose the middle value of its standard dose range. For example, if it is marked 18-21g in the picture, then take 19 to 20g, leaving enough room for venting.

‍ 2. Uneven tamping of coffee powder Uneven distribution of the grounds, or a shift in the center of force during tamping, will cause channeling, making hot water preferentially pass through areas with lower resistance. As a result, some coffee powder has not yet been soaked by water, while the coffee powder that has already been soaked by the water flow expands rapidly, and the resistance formed in turn blocks the channel. The best solution is that when we fill the portafilter basket, we can use tools such as a distribution tool to evenly distribute and level the coffee powder to every position, ensuring consistent powder distribution before tamping (the force should be balanced).

3. Stability of the pressure pump

If the pressure is set too high, extraction is prone to large fluctuations. For example, when the bar value is set to 15, extraction will immediately gush liquid under 15 bar of pressure at the beginning, and then suddenly drop to around 9 bar. For dark-roasted coffee beans, this situation easily extracts undesirable substances. We can handle this by adjusting the pressure. The first option is to lower the pressure setting to the pressure suitable for the beans being used, for example, 9-10 bar for darker-roasted beans, and one level higher for lighter ones; or, when conditions permit, use pre-infusion. For detailed information on pre-infusion, please refer to this article from a while ago ( How to make pre-infusion ). Reasonable pressure adjustment can bring out just the right aromatic substances.

4. Coffee beans are not fresh enough For coffee stored for more than two weeks after the roast date, the carbon dioxide and volatile aromatic substances contained in the beans have gradually been released, leaving only a small amount of flavor substances in the beans. With less gas obstruction, water easily passes through the puck at the beginning, and as the puck absorbs hot water and resistance increases, the flow rate slows down. Therefore, Front Street recommends drinking the beans while they are still fresh. If you have stored a lot of beans, you can use a sticker to seal the exhaust valve to reduce the breathing movement of degassing.

- END - FrontStreet Coffee

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The above content is compiled by CoffeeHunters, a coffee news website.