Many espresso machines advertise pre-infusion or pre-brewing. 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're better off turning the function off. (This article builds on our espresso brewing guide. You'll find the basics there.)
Why we no longer say "pre-infusion"
The term pre-infusion suggests a specific sequence: add water to the coffee, pause, then continue brewing. We consider this pause — sometimes called "blooming" — pointless for espresso. More on that in a moment.
What we actually mean when we talk about this topic: a gentle pre-wetting of the coffee puck. That is, applying water with reduced pressure or reduced flow slowly to the coffee before full brewing pressure arrives. Without a pause. The difference might sound like splitting hairs, but it's relevant for what happens in your portafilter.
What pre-wetting does
Coffee is hydrophobic. It initially repels water. Dry coffee grounds don't absorb water immediately. If you apply full pressure (8–9 bar) to a dry coffee puck, the water meets resistance and finds the path of least resistance. The result: channeling, uneven extraction, an espresso that can taste both sour and bitter at the same time.
If you pre-wet the puck slowly, something different happens. The coffee absorbs moisture, swells evenly, and when full pressure arrives, the puck is prepared. Water distributes more evenly, extraction becomes more homogeneous. With many coffees, this produces a rounder, sweeter, more body-forward cup. Not with all — but with many.
A side effect: With pre-wetting, you can often grind finer without the puck becoming too dense and still extract evenly. Finer grind size means more extraction, and that in turn is often very interesting flavor-wise, especially with complex coffees.
Filter coffee blooming and espresso pre-wetting: two different things
With filter coffee, there's a similarly-sounding technique: blooming. You pour a small amount of water over the coffee grounds, wait 30 to 45 seconds — and watch as the coffee swells and gas bubbles escape upward. The CO₂ stored in the coffee cells during roasting rises freely from the open coffee bed. This isn't a ritual but physics. If this CO₂ were still in the coffee bed, it would block even water flow and additionally introduce carbonic acid into the drink. The pause solves the problem. (We explain how this works in detail in our filter coffee brewing article.)
In the portafilter, this is physically impossible. Once the portafilter is locked in, CO₂ has no way upward — the puck is sealed by the shower screen. It can only escape downward through the basket, and without active water flow, this goes remarkably slowly. Even when the pump is running and pushing water through the puck, current research shows it takes longer for CO₂ to completely leave the coffee puck than a shot even lasts. A pause without water flow certainly won't achieve this.
The purpose that blooming serves with filter coffee doesn't exist with espresso — not because a pause would be done poorly with espresso, but because the system's physics simply doesn't allow a way for it. So pre-wetting with espresso pursues a different goal: saturating the hydrophobic coffee puck evenly with water before full pressure sets in. This isn't a CO₂ problem, but a wetting problem. And different rules apply for that.
Why the pause doesn't make sense
Many machines implement "pre-infusion" like this: pump on, a few seconds of water, pump off, pause, then pump back on with full pressure. The problem with this is simple.
If the pump works at full pressure in the first phase and then pauses, you've already shot pressurized water onto the puck. The puck is saturated in some spots, not in others. Then comes a pause where nothing happens except the coffee swells unevenly. And then full pressure comes again — onto an already partially swollen but unevenly wet puck. That's not gentle, that's 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's 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
Now we get into the math. To fully saturate a coffee puck, you need roughly twice the amount of water as the amount of coffee. For 18 g of coffee, that's around 30–36 ml of water.
How quickly does your machine deliver this water? Normally about 4 ml per second flows through the puck. At 4 ml/s, you need at least 8–10 seconds to push 30–36 ml of water through the puck. Only then is the coffee puck fully saturated.
Three seconds gets you to 12 ml. The puck is maybe one-third wet. Five seconds? 20 ml — still not enough. At reduced flow (about 3 ml/s) it takes even longer.
Vibration pump: Your machine already does this
If you have a vibration pump — and this applies to the vast majority of home machines under €2,000 — then your machine already automatically does a gentle pre-wetting. The vibration pump builds pressure slowly, flow starts low and increases over several seconds. That's exactly what pre-wetting is.
What does that mean? If your vibration pump machine additionally offers a "3-second pre-infusion," exactly what happens in those three seconds is what the pump is already doing. In the worst case, the pressure already built up is even reduced with the water in the portafilter. Your additional pause has no positive effect. The effect is zero. You're just extending the brew time without changing extraction.
With some machines — the Ascaso Steel Duo PID is a well-known example — we therefore recommend simply turning off the pre-brewing function. It doesn't make the machine better, and the 5 seconds that are maximum adjustable there aren't enough for meaningful puck wetting anyway.
When pre-wetting really makes sense
It gets really interesting with machines that have a rotary pump and can additionally reduce the flow. The Lelit Bianca is a good example: using the paddle, you can throttle the flow at the start and slowly wet the puck — 9 to 10 seconds at reduced flow, then full pressure. This produces measurably more even extraction with many coffees.
The prerequisites summarized: 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 so the coffee puck is fully saturated.
How pre-brewing affects total brew time
If your machine does pre-wetting, that naturally extends the total contact time. How do you handle 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. That's 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 Brew Time Really Start?)
Your machine additionally has pre-brewing with a pause: Then add the pause time on top. If the machine takes two to three seconds to "breathe" after pre-wetting before full pressure builds, you also add this time. In the example: 27 + 3 (vibration pump) + 3 (pause) = around 33 seconds.
But — and this is important — don't drive yourself crazy with second-precise recipes. These times give you a framework, a guideline. Follow your brew ratio (e.g., 18 g in, 36 g out), land roughly in your time window, and then decide by taste. Your palate will get better with every espresso.
More on brew time and why the first drop isn't the right starting point: Espresso Extraction Time: When Does the Brew Time Really Start?
This article is also available as a video on our YouTube channel.
