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'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 think this pause—sometimes called "blooming"—is pointless for espresso. More on that in a moment.
What we actually mean when we discuss this topic: a gentle pre-wetting of the coffee puck. That means adding 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 to what happens in your portafilter.
What pre-wetting does
Coffee is hydrophobic. It repels water initially. Dry coffee grounds don't absorb water right away. If you hit a dry coffee puck with full pressure (8–9 bar), the water meets resistance and takes the path of least resistance. The result: channeling, uneven extraction, an espresso that can taste 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 getting too tight 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 named technique: blooming. You pour a small amount of water onto 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. That's not ritual but physics. If this CO₂ were still in the coffee bed, it would block even water flow and on top of that bring 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 path upward—the puck is sealed by the shower screen. It can only escape downward through the basket, and without active water flow that goes extremely slowly. Even when the pump is running and pushing water through the puck, it currently takes longer for CO₂ to completely leave the coffee puck than a shot actually runs. A pause without water flow certainly won't achieve it.
So the purpose for which blooming works with filter coffee doesn't exist with espresso at all—not because a pause with espresso would be done badly, but because the system's physics simply doesn't allow it. That's why pre-wetting with espresso pursues a different goal: to saturate the hydrophobic coffee puck evenly with water before full pressure hits. That's not a CO₂ problem, it's a wetting problem. And that follows completely different rules.
Why the pause makes no 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 is simple.
If the pump works at full pressure in the first phase and then pauses, you've already shot pressurized water at 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 returns—on an already partially swollen but unevenly wetted 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 gets activated during the pre-wetting phase. Both ensure the coffee gets wet slowly and evenly before the actual extraction with full pressure begins.
Three seconds of pre-infusion is always too little
Now it gets mathematical. To fully wet a coffee puck, you need roughly twice the water amount to coffee amount. With 18 g of coffee, that's around 30–36 ml of water.
How fast 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. With reduced flow (about 3 ml/s) it takes even longer.
Vibratory pump: Your machine is already doing this
If you have a vibratory pump—and that applies to the vast majority of home machines under €2,000—then your machine is already automatically doing a gentle pre-wetting. The vibratory 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 vibratory pump machine additionally offers a "3-second pre-infusion", during those three seconds exactly what the pump does anyway happens. In the worst case, the existing pressure is even relieved by the water in the portafilter. Your additional pause has no positive effect. The effect is zero. You're just extending your 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 rotary pump machines that can additionally reduce flow. The Lelit Bianca is a good example: using the paddle you can throttle flow at the start and pre-wet the puck slowly—9 to 10 seconds with reduced flow, then full pressure. With many coffees that produces measurably more even extraction.
Summing up the prerequisites: You need a machine that can actually reduce 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 overall brew time
If your machine does pre-wetting, that naturally extends the overall contact time. How do you handle it when you have a recipe from a roastery?
Starting point: The recipe says 27 seconds, made with a rotary pump.
You have a vibratory pump: Add about +3 seconds, so around 30 seconds. That's because the vibratory pump builds pressure more slowly and the first phase extracts less intensively. (We explain this in detail in the article Espresso extraction time: When does brew time really start?)
Your machine additionally has pre-brewing with a pause: Then the pause time goes on top. If the machine takes two to three seconds to "breathe" after pre-wetting before full pressure builds up, you also add this time. In the example: 27 + 3 (vibratory pump) + 3 (pause) = roughly 33 seconds.
But—and this is important—don't go crazy with second-precise recipes. These times give you a framework, an orientation. Maintain your brew ratio (e.g., 18 g in, 36 g out), land approximately in your time window, and then decide by taste. Your palate will get better from espresso to espresso.
More on brew time and why the first drop isn't the right starting point: Espresso extraction time: When does brew time really start?
This article is also available as a video on our YouTube channel.
