Extraction in espresso occurs only at the surface of particles — this is called erosion
This means a fine grind is required, and the smallest particles are especially important in extraction
Grinders exert two kinds of force on coffee: compression and shear
Different grinder types apply these forces in different ways
The grind size from any grinder isn’t uniform, but a distribution of different sized particles
Small particles contribute a disproportionate amount of the surface area for extraction, while larger particles play an important role in allowing sufficient flow through the puck
Grind shape also plays a role in extraction that is not yet well understood
Grind size distributions are measured in the laboratory using laser diffractometry
Baristas can measure grind size distributions directly using the new Barista Hustle app