Preinfusion is the brief wetting of the coffee grounds at a low pressure prior to engaging consistent, full pressure in espresso preparation. This initial phase allows the grounds to swell, redistribute themselves and become more adhesive before full pressure is applied. There are two important benefits of preinfusion:
- Decrease the frequency of channeling
- Decrease fines migration
To be clear, preinfusion will not guarantee that you will pull a great shot of espresso, nor will it raise the ceiling for the kind of greatness your espresso can achieve. But, it will almost certainly result in a much higher frequency of great shots. The following are two common types of preinfusion found on commercial espresso machines.
Manual Preinfusion
With manual preinfusion, you begin infusion at a low pressure (roughly 3.5-4.5 bar), commonly by moving a paddle attached to the group head. Once you’re satisfied with the time and pressure, you move the paddle once again to fully engage the pump. This method provides greater access to control, but it requires that the barista understand the correct preinfusion time and pressure.
Progressive Preinfusion
In progressive preinfusion, you engage the pump – either through an on/off button or a paddle that performs an on/off function – and water fills a spring-loaded preinfusion chamber attached to the group head. Once the water has filled the empty spaces in the group head and preinfusion chamber, the spring is extended, allowing the pressure applied to the coffee bed to increase.
