Stone-mill RPM and particle distribution: the real cause of grit on the tongue
Faster rotation raises the fine-powder ratio, but excess friction heat blows off aroma. We examine the groove depth of the granite mills used on site and the meaning of the 'mill's growl' that masters listen for, from the perspective of particle-size distribution.
Raising the stone mill's rotation increases the fine-powder ratio, but friction heat loses volatile compounds. The 'growl' a master listens for is the sound of the mill's teeth meshing with the leaf — you could say they correct distortions in the particle distribution by ear.
Under a laser-diffraction particle sizer, an ideal usucha grade often shows a distribution where a peak below a few microns coexists with a sub-peak in the low tens of microns. For lattes, by contrast, the sub-peak is intentionally suppressed to prioritize a creamy mouthfeel.