An espresso requires a short extraction/percolation time, yet with high concentration of soluble solids. To achieve this contradictory chemical and physical requirement, a varying particle size distribution of coffee grounds is needed, so that the finer particles will improve the extraction rate while the coaser grounds will allow water flow.
There are 2 phases to grinding: first, the beans are crushed into fragments, and the second phase sees the fragments being submitted to shearing forces that produce small particles with few larger ones of different sizes.
Factors affecting a consistent grind:
Grinding motor design can affect the temperature build-up during grinding. This can adversely damage the grounds by speeding up the Maillard and oxidation process, thus losing the aromatic compounds.
Roast degree. Coffee oil, released of lipids from dark roasted beans, on the bean surface can be very viscous and sticky at semi-solid state at room temperature. And this can result in over- heating due to loss of contact between the grinding disk and the oily beans.
Cooling method after roasting. Air-cooled roasted beans have larger quantity of gas trapped in the cells that may affect the grinding process stability. A common practice to degass naturally is to allow the beans to rest for couple of hours in silos before grinding. Water-quenched roasted beans, on the other hand, have degassed substantially from the cracking of cell-walls due to sudden drop in temperature during water quenching. Water-quenched beans also contains more moisture that may consume more grinding energy, resulting in over-heating of the ground coffee.
Feeding mechanism can affect the grind consistency when the load is full as compared to when the hopper is almost empty. That is why it is common to see barista keeping their grinder hopper at a certain level to maintain a specific load for consistency.
Cleanliness of the grinder. This is especially important after successive grinds of dark roasted beans when coffee oil build-up on the grinder blades/disks can result in over-heating of the grounds.
Sharpness of grinding disks can either produce consistent particles size from shearing or mere crushing effect resulting in particles in various sizes.
Blend of different variety from different countries can have different hardness.
