Burr geometry

Time to return to my favourite piece of coffee kit/equipment/machinery… the coffee grinder.

Today we’ll discuss grinder burr geometry as well as coffee grind particle distribution.  I promise that that all sounds way more intimidating than it actually is.

There are two dominant types of coffee grinder burr shapes: flat and conical.

A crazy cadre of people who represent the Venn diagram intersection of coffee nerd and engineer have looked at coffee grounds using electron microscopy (really).  The main finding is that the two burr geometries generally produce different grind size distributions.  One might imagine that a coffee grinder would turn coffee beans into smaller entirely uniform particles ready for extraction by hot water and pressure.  The truth is, naturally, more complicated.  Grinders produce a range of grind sizes… the catch is that conical burrs display a bimodal distribution (there are two grind sizes that dominate the overall distribution) whereas flat burrs tend to produce a unimodal distribution (the grinds tend to be more uniform).

The small bump to the left of the bimodal distribution coming out of conical burr grinders is often referred to as the “fines”.  Fine particles tend to migrate down towards the bottom of the portafilter basket during extraction and can also move into the cup increasing viscosity and mouthfeel.  The differences in flavour are interesting.  Conical burrs tend to blend flavours a little more whilst flat burrs tend to produce discreet flavours with greater separation and delineation.  For a while I had both burr geometries sitting on my coffee bench and I found that I preferred the flavours produced by the flat burr grinder.  It all depends on your preference!

So next time you think about purchasing a grinder, you might consider sampling both burr geometries to see if it makes a big enough difference to influence your decision!