Many espresso machines advertise pre-infusion. But what actually happens during this process and when is the function useful? We explain why we no longer use the term "pre-infusion," why 3 seconds is always too short, and when you should turn off the function. (This article builds on our Espresso Preparation Guide. You'll find the basics there.)
Why we no longer say "pre-infusion"
The term pre-infusion suggests a specific sequence: apply water to the coffee, pause, then continue brewing. We consider this pause - sometimes called "blooming" - to be pointless for espresso. More on that in a moment.
What we mean instead when we talk about this topic is a gentle pre-wetting of the coffee puck. That is, applying water to the coffee slowly with reduced pressure or reduced flow before full brewing pressure is applied. Without a pause. The difference may sound like nitpicking, but it's relevant to what happens in your portafilter.
What pre-wetting does
Coffee is hydrophobic. It initially repels water. Dry coffee grounds do not absorb water immediately. If you apply full pressure (8–9 bar) to a dry coffee puck, the water encounters resistance and seeks the path of least resistance. The result: channeling, uneven extraction, an espresso that can taste both sour and bitter.
If you pre-wet the puck slowly, something else happens. The coffee absorbs moisture, swells evenly, and when full pressure is then applied, the puck is prepared. The water distributes more evenly, and extraction becomes more homogeneous. For many coffees, this results in a rounder, sweeter, more full-bodied cup. Not for all - but for many.
A side effect: with pre-wetting, you can often grind finer without the puck clogging and still extract evenly. Finer grind size means more extraction, and that, especially with complex coffees, is often very interesting in terms of taste.
Filter coffee blooming and espresso pre-wetting: two different things
For filter coffee, there is a similar-sounding technique: blooming. You pour a small amount of water over the coffee grounds, wait 30 to 45 seconds - and watch the coffee swell and gas bubbles escape upwards. The CO₂ stored in the coffee cells during roasting freely rises from the open coffee bed. This is not a ritual but physics. If this CO₂ were still in the coffee bed, it would block the even flow of water and also introduce carbonic acid into the drink. The pause solves the problem. (We explain how this works in detail in our filter coffee preparation article.)
In the portafilter, this is physically impossible. When the portafilter is engaged, CO₂ has no way to go upwards - the puck is sealed by the shower screen. It can only drain downwards through the basket, and that happens extremely slowly without active water flow. Even when the pump is running and water is pushing through the puck, current findings show that it takes longer for the CO₂ to be completely out of the coffee puck than a shot even runs. A pause without water flow certainly doesn't achieve this.
The purpose for which blooming works for filter coffee therefore does not exist for espresso - not because a pause for espresso would be poorly executed, but because the physics of the system simply does not allow for a way. Pre-wetting for espresso therefore pursues a different goal: to evenly saturate the hydrophobic coffee puck with water before full pressure is applied. This is not a CO₂ problem, but a wetting problem. And for that, completely different rules apply.
Why the pause makes no sense
Many machines implement "pre-infusion" as follows: pump on, a few seconds of water, pump off, pause, then pump on again with full pressure. The problem is simple.
If the pump operates at full pressure in the first phase and then pauses, you have already shot water under pressure onto the puck. The puck is saturated in some places, but not in others. Then comes a pause during which nothing happens except that the coffee swells unevenly. And then full pressure comes again - on a puck that is already partially swelled but not evenly wetted. This is not gentle but counterproductive.
For pre-wetting to work, you need one of two things: either the pump can reduce its output for the first phase (less pressure or less flow). Or the machine has a second water path with a smaller diameter that is activated during the pre-wetting phase. Both result in the coffee slowly and evenly getting wet before the actual extraction at full pressure begins.
Three seconds of pre-infusion is always too little
This is where it gets mathematical. To completely wet a coffee puck, you need approximately twice the amount of water as the amount of coffee. For 18g of coffee, that's about 30–36ml of water.
How quickly does your machine deliver this water? Normally, about 4 ml per second flow through the puck. At 4 ml/s, you therefore need at least 8–10 seconds to get 30–36 ml of water through the puck. Only then is the coffee puck completely saturated.
Three seconds gets you 12 ml. The puck is perhaps one-third wet. Five seconds? 20 ml - still not enough. With reduced flow (about 3 ml/s), it takes even longer.
Vibration pump: Your machine is already doing it
If you have a vibration pump - and this applies to the vast majority of home machines under €2,000 - then your machine is already automatically performing a gentle pre-wetting. The vibration pump slowly builds pressure; the flow starts low and increases over several seconds. This is precisely pre-wetting.
What does this mean? If your machine with a vibration pump also offers a "3-second pre-infusion," exactly what the pump does anyway happens during these three seconds. In the worst case, the existing pressure is even relieved with the water in the portafilter. Your additional pause has no positive effect. The effect is nil. You only extend the brewing time without changing the extraction.
For some machines - the Ascaso Steel Duo PID is a well-known example - we therefore recommend simply turning off the pre-infusion function. It doesn't make the machine better, and the 5 seconds that can be set there at most are not enough for meaningful puck wetting anyway.
When pre-wetting really makes sense
It gets really interesting with machines with a rotary pump that can additionally reduce the flow. The Lelit Bianca is a good example: you can restrict the flow at the beginning via the paddle and slowly wet the puck - 9 to 10 seconds with reduced flow, then to full pressure. For many coffees, this results in a measurably more even extraction.
To summarize the requirements: You need a machine that can actually reduce the flow or pressure in the first phase (not just pause), and you need at least 8–10 seconds of pre-wetting time for the coffee puck to be fully saturated.
How pre-infusion affects total brew time
If your machine performs pre-wetting, this naturally extends the total contact time. How do you deal with this if you have a recipe from a roastery?
Starting point: The recipe says 27 seconds, made with a rotary pump.
You have a vibration pump: Add about +3 seconds, so around 30 seconds. This is because the vibration pump builds pressure more slowly and the first phase extracts less intensely. (We explain this in detail in the article Espresso Extraction Time: When does the brewing time really start?)
Your machine also has pre-infusion with a pause: Then the pause time is added on top. If the machine "breathes" for two to three seconds after pre-wetting before full pressure builds up, you also add this time. In the example: 27 + 3 (vibration pump) + 3 (pause) = approx. 33 seconds.
But - and this is important - don't get hung up on second-precise recipes. These times give you a framework, a guide. Maintain the brewing ratio (e.g., 18g in, 36g out), land approximately within your time window, and then decide based on taste. Your palate will get better with each espresso.
More on brewing time and why the first drop is not the right starting point: Espresso Extraction Time: When does the brewing time really start?
This article is also available as a video on our YouTube channel.
