another_jim wrote:This is what has me scratching my head about what exactly a refractometer is measuring, especially for espresso.
%TDS. That is all. I don't agree that %TDS measurements are inconsistent for a particular coffee, grinder, basket, prep technique, etc. In my experience they're quite consistent, though they might not agree precisely with calibration solutions or drying done under lab conditions. Sure, %TDS isn't necessarily consistent across grinders and baskets, but that's exactly what I'd expect -- different particle distribution, different extraction and so forth. Why should refractometer readings be consistent when those things change?
And like I said, %TDS only tells you about concentration, which is roughly correlated with strength. IMHO, strength plays a lesser role in taste, and strength preferences vary wildly from one person to the next.
We can use %TDS, dose weight and beverage weight to compute %EY. I'm not sure what softness/angularity mean, but I don't agree that %EY computed from scale and refractometer readings don't correlate with acidity/bitterness. In my experience, they most certainly do. However, the measurement is only useful as a relative
measure for a particular combination of coffee, grinder, basket, prep, etc., etc. I think you can use it to compare the level of extraction when grinder, basket or prep is changed, but it's not meaningful at all if you change the coffee. And, most important, the level of extraction, and the associated level of sourness/bitterness, don't describe the full gamut of taste.
This is why the current practice of evaluating grinders by %EY seems odd to me. It's simply not the case that the more you extract, the better the taste. It may be true that grinders that average higher %EY might be better for light roasts, but does that mean they're better for medium and dark roasts?
So what does the refractometer measure? Well, if you compare %EY for two different extractions, it'll give you a good idea which one extracted more and which one extracted less, which one is likely to taste more sour and which one is likely to taste more bitter. When you're not a super-taster, or when your tongue is exhausted by tasting many cups, or when you're troubleshooting or evaluating the change of one variable (like the grinder), it can be quite helpful as an adjunct to taste, which is of course the final arbiter.
And again, let's not blame the messenger (i.e., the refractometer.) We have the same lack of correlation with taste if we get the %TDS measurement by drying pucks.
While I normally don't use the refractometer to dial-in, %EY comparative readings can be quite helpful for newbies learning how to dial in a balanced shot. I wish I'd had the refractometer when I was struggling with that, as well as inconsistent equipment. I'd have gotten a much better understanding of the dynamics of extraction, and would have upgraded the grinder much sooner.
another_jim wrote:In other words, we can work the taste much more easily and predictably than we can measure it. So what gives?
You can. Not all of us are able to do that as well. Measurement tools benefit some, but not all.