Three factors – sharpness, size, and alignment – have a big effect on grind size distribution. Worn, blunt burrs tend to ‘crush’ the beans with compression stress rather than ‘cut’ them with shear stress. As discussed in Lesson 1.02, compression forces cause the particles to break up into randomly sized fragments, so blunt burrs produce widely spread particle size distributions, resulting in less even extraction.
Larger burrs seem to generally produce more even grind size distributions, and they grind faster and are less prone to getting hot.
Aligning burrs so that they are perfectly parallel also has a dramatic effect on grind size distribution and coffee quality. If the burrs are not properly aligned, then particles of different sizes can escape from different parts of the burrs and the grind size will be much less uniform.
The effect on grind size distribution can be seen in this graph, which shows the grind size distributions produced by two different Mahlkoenig EK43 grinders. The line in blue is the distribution from a grinder with properly aligned burrs, while the distribution in grey, from a less well aligned grinder, shows two distinct peaks.
A comparison of grind size distributions of two EK43 grinders. Source: https://www.nature.com
End Æ 2.01