Recap
- 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
Glossary
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,