Grind Mechanics and Uniformity
Lance Hedrick claims in one of his latest videos that a coarser grind setting generally produces more uniform distribution. Why is that?
Every time a piece of coffee hits the burrs to get ground, it makes some fines as the whole bean or larger ground essentially explodes or shatters kind of like glass when it hits the burrs. You get some pieces that are about the size of the gap between the burrs, and a lot of pieces that are very, very small (the fines). The finer your grind setting is, the more times you have pieces of coffee hitting the burrs (as bigger pieces that have already been created from the whole beans will hit the burrs again to get made smaller), so more fines are produced. So the finer you grind, the more fines you get.
However, "uniformity" is more complicated than just how many fines a grinder is producing. There is also a distribution of particle sizes right around the actual distance between your burrs. Let's say you put your burrs 700 microns apart. You get a whole bunch of fines that are 50-100 microns or whatever, and then you also get a spread of particles that are 600-800 microns (blue curve). If you put your burrs 650 microns apart, you get a bunch of fines that are 50-100 microns (more than you did when grinder was set to 700 microns) but the spread of the rest of the particles might be (and often is) tighter, maybe only 575-725 microns (red curve). In this case the fines are closer in size to the rest of the grounds than they are with the coarser grind setting - does that mean it is more uniform? Those 600-800 and 575-725 micron spreads aren't going to be even either. One could have more on the finer side and one could have more on the coarser side (not shown in diagram). The 600-800 distribution might have way more particles right at 700 and actually very few down near 600 and up near 800 (but enough that we aren't calling it zero) (also not shown). The point is that there are LOTS of different ways that a particle distribution curve might look, so “uniformity” is not really a very well-defined concept. At some point, the more uniform a curve looks (very few fines, very narrow spread near the peak), the worse the coffee can taste too.
He glosses over this very quickly just saying "largely."
We'd argue it isn't always true that coarser is better as while it will likely make the water flow through your grounds more evenly in a percolation brew, it also severely restricts your overall extraction. Some people are going to like a brew that has a lot more sweetness with a little bit of astringency (finer grind) over a brew that has a lot less sweetness but less astringency (coarser grind).
The best thing to do is simply experiment with a grind that is a bit finer than your normal grind size and also with a grind that is a bit coarser than your normal grind size. One of them may taste better (or not!).
