Pre-infusion

Pre-infusion is a brief stage before full brewing pressure kicks in, where water is applied to the coffee puck at low pressure for a few seconds, gently saturating the grounds before the machine ramps up to full pressure. Some machines build this in automatically, while others rely on manual technique to approximate it.

The idea is similar to blooming in pour-over brewing; wetting the coffee gently first lets the puck swell and settle evenly before it is forced to withstand full pressure, which reduces the chance that water finds and blasts through a weak spot. This can noticeably calm down channeling on pucks that are not perfectly prepped.

Pre-infusion is a common feature on dual boiler and higher end machines, and it is worth checking whether pre-infusion time and pressure are adjustable if this matters to you. A common mistake is expecting pre-infusion alone to fix poor puck prep, when it reduces the risk of channeling rather than replacing the need for even distribution and tamping.