Grinding is the fracturing of coffee beans into many smaller particles. The primary purpose of grinding is to facilitate the extraction of soluble solids by exposing more particle surface to the brewing liquid.
Grind Size and Contact Time
A finer grind creates smaller particles with more specific surface area, causing coffe esolids to extract more quickly. Therefore, in order to achieve a given extraction quantity, a finer grind requires less contact time with the brewin gliquid than does a coarser grind.
Particle Size Distribution
Due to the irregular of coffee beans and the unequal distribution of forces on the structure during grinding, coffee does not shear into equally sized or identically shaped pieces. In a typical size distribution, most of the coffee mass is of a narrow range of sizes clustered around the "grind setting". In addition, a distribution contains a small number, but moderate mass, of larger particles called boulders and a large number, but tiny mass, of very small particles called fines.
The narrower the particle size distribution, the more equally the particles will extract. If thedistribution of particle size is too dispersed, with too many very small andvery large particles, the resulting brew will have too many overextracted and underextracted taste.
A narrower particle size distribution is created by grinders with:
1. Sharper burrs
2. Longer burr grinding paths
3. Less heat generation during grinding
4. Multiple grinding stages (such as a roller mill with two or three pairs of successively close rollers.