A short initial water contact phase before the main extraction begins. The grounds are wetted with a small amount of water, then allowed to sit briefly so they can absorb water, swell, and release CO₂ before the bulk of the brew water arrives.
In pour-over, pre-infusion is just the bloom by another name — usually 30 to 45 seconds where you pour twice the dose weight of water and pause. In espresso the term is more technical: a low-pressure pre-wetting phase (often 1–4 bar) lasting a few seconds before the machine ramps to nine bars.
What it does to the brew
Pre-infusion solves two problems at once.
First: gas expulsion. Fresh roasted coffee is full of CO₂. If full-pressure water hits a dry, gassy bed, the gas escapes violently and creates channels. A short low-energy contact lets the gas escape in advance, so the main brew flows through a calmer bed.
Second: even saturation. Dry coffee particles repel water for a moment because of surface tension; pre-infusion gives time for capillary action to wet every particle uniformly. Without it, the first part of the bed extracts while the rest is still dry.
How long is enough
Specialty filter brewing typically uses 30 s pre-infusion for fresh beans (within 14 days off-roast), bumping to 45–60 s for very fresh beans (under 7 days) or pushing down to 20 s for older coffees that have less gas to release. Espresso pre-infusion is usually 3 to 8 seconds, sometimes longer for light roasts.
If your bloom phase is going completely flat with no visible bubbles, the beans are old. If it's still erupting after 45 seconds, give them another week.