Well, I can't taste colors or shapes yet, so my input here will be slightly muted in comparison to some
But I do have some thoughts on what TDS and EY actually tell us regarding the extraction and why it is that some grinders need more extraction than others to taste good, so I'll give it a go. I'm going to use "well-aligned" or "precision" to describe grinders with a tight particle distribution spread and "misaligned" or "poorly-aligned" to describe grinders with a wider spread. Really, the grind distribution is all that matters, whether it be burr geometry or alignment that gets you to one extreme or the other.
Below is a random bunch of plotted numbers that vaguely represent a particle distribution plot of a very well aligned flat burr grinder, with a little pictogram of "perfectly aligned" burrs to drive the point home.
In this perfect world, the burrs are 400 um apart and they generate relatively few fines in the 50 um range. You build a puck with these grinds and pull a shot. You get some TDS at some brew ratio and it corresponds to some EY%. All clear?
Then we have some misaligned burrs.
With these burrs, we have the lower burr cocked off to one side in the rotating burr mount.
Looking at our pictogram, the "center" of the lower burr is at 400 um and the left side is maybe 300 and the right side 500. This widens the particle distribution and greatly decreases the amount of 400 um particles. Even so, on average there are twice as many 400 um particles as there are 300 or 500. This is because if we were to look at the burrs from a top-down view, the front (6 o'clock) and the back (12 o'clock) of the burrs (the center in our pictogram) would both be 400 um apart. The left (9 o'clock) and the right (3 o'clock) would be at 300 and 500 um, respectively. Two points of contact at 400, one each at 300 and 500. Make sense so far? I left the fines alone because I'm not sure how alignment affects fines so I won't make assumptions suggesting that I do...
Brewing a shot with this batch will result in some TDS at some brew ratio and another corresponding EY%. These values may or may not equal the values we got above. But here's why they will taste different even if the TDS and EY% are the same...
Extraction is all about contact time, concentration gradients and surface area. You create a puck, with soluble compounds clinging to the surface area of the ground coffee particles. You introduce a solvent into the puck and it strips the soluble compounds off of the exposed surfaces. The process is similar to heat transfer through a heat exchanger. Too fast, not enough contact time to transfer the compounds into the solvent, so the concentration gradient remains high (in an HX, this means the cooling water doesn't get hot and the thing being cooled doesn't cool down). Too slow, and the concentration gradient gets too low and the solvent is just as concentrated as the puck and it can't extract any more (in an HX, the cooling water gets hot, but the thing being cooled doesn't cool off...).
The trick in both cases is to find the flow rate where the contact time is sufficient to strip the desired compounds from the available surface area, and the replenishment rate is high enough to bring new lower concentration solvent into the puck to keep the extraction progressing (in the HX like an engine radiator, the goal would be to cool the engine coolant by optimizing this rate).
Why do I bring up this?
Well, if you look again at the particle analysis, it's clear that since smaller particles have a higher surface area to volume ratio than larger particles, the better aligned grinder has a more uniform amount of surface area available for extraction than the misaligned grinder. So what does that mean?
Well, the smaller particles in the misaligned grinder will extract more solids
than the target size and the larger particles because they (the smaller ones) have more surface area. The target particles will extract roughly the same
as the target particles coming from the well aligned grinder. Finally, the larger particles will extract less solids
than the target and smaller particles.
Begin clarifying edit:
So, when the flow rate is optimized for the average particles, you get a mix of extraction yields from the various particle sizes.
•The smallest particles, which have the most surface area, release the most solids per unit volume, but do so without being over-extracted
, because they have a lower contact time per surface area than any of the other particles. This is because the flow rate is faster than what would be ideal for particles with this much surface area since it is optimized for the average particles.
•The target particles behave as expected. They have a medium surface area per unit volume, and a medium contact time per surface area. So, fewer solids per unit volume are extracted, but what is extracted is extracted more
than the smaller particles.
•Finally we have the large guys, or boulders. They have the lowest surface area per unit volume, and the highest contact time per unit surface area. The boulders contribute the least solids per unit volume, but because the flow rate is optimized for the average particle size, the contact time on each particle is huge relative to its surface area. Because of this, the boulders give you over-extracted flavors first.
•All together you have an average extraction yield that is based on some portion of under-extracted small particles and over extracted boulders mixed with the target particles that extracted at the level that you are shooting for.
End clarifying edit:
What does this mean?
Well, I think it means that over-extracted flavors will show up faster with misaligned grinders and they will taste better at lower extraction yields
than their well-aligned brethren. Conversely, with fewer small sized particles and boulders (for a given target grind size) the well-aligned grinder will taste better at higher extraction yields
than the misaligned grinder.
Put all of this together and even though the higher extraction of smaller particles and lower extraction of larger particles may even out with the extraction of target particles on a well-aligned grinder, they will taste different. Likewise, a burr geometry that yields a tight distribution (such as "unimodal" burrs) will taste better at high extraction yields than a burr geometry that yields a wider distribution (or "bimodal" burrs). Conversley, the wider spread will taste better than the tight spread at low extraction yields and the one constant is that they will taste different from each other at the same extraction yield...