Æ Grinding

Æ 1.07 Recap & Glossary


  • 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



Erosion  Washing of solubles from the coffee surface

Plurimodal  Referring to a grind size distribution that has more than one peak

Diffusion  The movement of molecules from a region of high concentration to an area of low concentration

Compression  The stress applied when a material is pressed against a surface

Shear  The stress applied when a material is subjected to forces from opposite directions on either side of a plane, causing stress along that plane

Grind Size Distribution  Also called particle size distribution or PSD, a representation of the relative amounts of different sized particles within ground coffee

Artifacts  An error in a measurement or experiment resulting from the methods used, such as a defect in an image

Logarithmic  A nonlinear scale in a graph, in which each interval increases by a factor of 10. The numbers on a logarithmic axis will read 1,