What is pre-infusion and why use it?

Pre-infusion is a gentle wetting phase applied to ground coffee before full-pressure extraction. It lasts 2-10 seconds in espresso (1-3 bar) and 30-45 seconds in filter brewing (the bloom). Its job: degas CO₂, saturate the bed evenly, and prevent channeling and uneven extraction.

Pre-infusion answers a simple physical problem. Freshly roasted coffee holds CO₂ trapped in its cell structure — about 4-8 mL per gram depending on roast level and age post-roast. When hot water hits the grind, that CO₂ expands fast and carves preferential paths (channeling): water takes the easiest route and leaves the rest of the bed under-extracted. A slow wetting phase lets gas escape before pressure rises.

In espresso, pre-infusion has several flavours. Manual lever machines (Londinium) deliver gravity pre-infusion at 0.9-1 bar from the spring. Vibratory-pump e61 machines (Rocket, ECM, Bezzera) generate hydraulic pre-infusion via the expansion chamber. Flow-controlled machines (La Marzocco Linea Mini, Slayer, Decent) let you program duration and pressure electronically. Matt Perger's Barista Hustle tests show that a 5-second pre-infusion at 2 bar improves TDS repeatability by about 30 % on a light roast compared to a straight 9-bar start.

In filter brewing, pre-infusion is called the 'bloom'. Classic rule popularised by James Hoffmann: pour 2 to 3 times the dry coffee weight in water (36-54 g for 18 g), wait 30-45 seconds. Some protocols — Tetsu Kasuya's one-pour V60 (2016 world champion) or Scott Rao's spin — add a swirl during the bloom to saturate the bed more evenly. Jonathan Gagné in 'The Physics of Filter Coffee' recommends extending the bloom to 60 s for very fresh coffees (< 10 days post-roast) and shortening it to 20 s for coffees past four weeks.

Belgium picked up bloom technique in the Brussels and Ghent specialty scene from around 2013, carried in by V60 and Chemex gear in third-wave shops. At espresso bars in Antwerp or Liège, pre-infusion-capable machines are now a professional standard — the historical anchor being the mechanical e61, imported since the 1960s through the Italian espresso tradition. At home, using a bloom on filter is one of the two or three habits that separate a 'fine' coffee from one that actually honours the origin.

Pre-infusion by method