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From the Niche Zero review (and corroborated with my past experience), I notice a trend/observation which I thought would be interesting to share/discuss. I have been noticing that although the extraction yield (EY) can be similar (within the deviation of my repeated runs), the taste profile or quality can be consistently different.

You can almost confidently say that different level of extraction yields will taste different but can you say the inverse is true (same extraction yield = same taste)?

In my opinion, the refractometry measurement can give you an inkling where the process sits on the extraction yield spectrum. But I argue that there is no substitute for tasting. As subjective as our palate may be, you will still need to taste the shot to evaluate the taste. For example, for what it's worth, my Helor 101 hand grinder can achieve similar extraction level with my big conicals, but they clearly don't taste the same to me.

So I am trying to rationalize why I don't see a statistically significant difference in TDS/EY yet the tasting suggests there may be subtle differences. I think there are three main reasons:

The nature of refractometry measurement: Refractometer works by passing light through a liquid phase and depending on how much the light is scatteredrefracted/bent, it predicts the amount of dissolved solids based on what it was calibrated on. There lies the shortcoming of refractometry measurements - assuming the refractometer was calibrated properly (using the right technique and right calibration solution), it can only tell you roughly how much bulk dissolved solids you have, but it has zero information on the chemical/flavor molecules composition. It's akin to telling you how many apples you have in the promotion bag you just purchased in the supermarket clearance last night, but doesn't let you know how many of them were bad apples.

Limit of detection : Human palate have a taste sensitivity of around 5-400 ppm (that's 0.005- 0.4 mg/ml or 0.0005-0.04% TDS)*. The accuracy of of VST TDS meter starts at ± 0.05% TDS (or 0.5 mg/ml). That is just the instrument error alone and it is already higher than the taste sensitivity. To draw on another analogy, this time related to coffee, this is like trying to measure the coffee particle size using a regular ruler. Sure you can measure some of the larger particles, but you are definitely working at the detection limit and won't be very accurate.

*https://onlinelibrary.wiley.com/doi/pdf/10.1002/j.2050-0416.1974.tb03598.x

Human factor: One major factor, without a doubt, is the huge variation due to human factor. I am not running scientific experiments (nor I pretend to) and my instruments are not lab-certified or calibrated on a regular basis. But I also don't know anyone who can repeat the process over 5 times and get <0.05% TDS variation!

With all these factors combined, even if I (the operator) am a perfect scientist with absolute precision, my limit of detection is still above the tasting threshold. I will also have the issue of not knowing what exactly have I detected/measured in my sample. In practice, >0.3 TDS % (or more) variation is normal for me even when I do everything in the same way (same coffee, similar extraction time/ratio etc). So all hope is lost if you want to put a taste label to the resulted extraction yield %!

So in my opinion, TLDR (or if it got too technical): Refractometer measurement is not sensitive enough to account for subtle taste difference, and the measurement doesn't account for the chemical composition. Most of us are not trained enough (if it is at all possible to achieve zero variation) to do the measurement with enough precision.

This is not to say refractometer is useless (far from it, they can definitely be useful in some scenarios), but I think it should not be the only guideline to evaluate anything, especially not without a proper context.

That's just my way of rationalizing what I am seeing and by no means tested or proven, so take it with a big grain of salt! But I am keen to hear what you think about it. Have you done EY measurements across different grinders/machines using the same coffee and notice the same observation?

I thought we had settled this question long ago.

After many years of using a high-quality refractometer, my conclusion is that extraction yield is not indicative of taste in the cup, except in a gross way as a relative indication of where the extraction lies on the sour-bitter spectrum. That's it.

Extraction yield won't tell you if you've nailed the extraction and pulled the very best cup from the beans. It can help you find the range where sour and bitter are in balance enough to unmask the more subtle flavors, but the range is pretty broad and the refractometer won't tell you exactly where in that range the optimum taste lies. And that point definitely varies from one bean/roast to another, and usually varies from one taster to another. Again, it's a relative measurement that's most relevant for one coffee and one taster.

But, and this is important for me, even that gross measurement is a useful confirmation of the balance I'm tasting, especially after the sensitivity of my tongue has been reduced by tasting multiple extractions in the same session.

%TDS is an even more rudimentary measurement. It basically tells you how strong (concentrated) the coffee is, but not with much precision. For my typical Kalita Wave pourover, I know that if the %TDS is 1.20%, the cup is going to be weak, and if it's 1.55% it's going to be stronger, and if it's somewhere in the 1.3%-1.4% it'll be about the strength I like. Sure, it varies by coffee, but the correlation of the measurement with what I perceive as strength is so gross that it doesn't matter. Still, it's a quick confirmation that tells me if I need to change the brew ratio, grind finer or coarser, etc.

I very much doubt that %TDS or %EY measured from weighing evaporated grounds tells you anything more about taste. It's possible that method may be more accurate for determining absolute %TDS and %EY, but who cares about that? Maybe lab people. Forget trying to share recipes based on %TDS or %EY with friends. Doesn't work.

Yes, I've use the refractometer to compare %EY of the same beans from two different grinders (Monolith Flat versus Compak K10.) I did a bunch of measurements, which were very repeatable within the precision rating of the instrument, and I found that the %EY of the two machines didn't vary by more than 0.5%, which is not enough to detect by taste -- at least by me. However, the cups from the two grinders, while very much in the same ballpark quality-wise, and similarly balanced, tasted different. The difference was subtle, but it was quite evident to me and I'm not the greatest taster. I think that illustrates the limitations of %EY measurements.

I mainly use my refractometer to measure %EY of brewed specialty coffee, with which I have far less tasting experience than espresso. It helps me to quickly determine if I need to change the grind setting or contact time to get the kind of balance I like. I can do that by taste, but not as well, especially after tasting multiple roasts from my Quest M3.

I've also used the refractometer to troubleshoot equipment and evaluate results of the flow and pressure profiling mods I've made to my espresso machine. It's been very helpful for that. And I've used it to compare the solubility of a particular coffee roasted to lighter and darker levels, and sometimes I use it to see how the solubility of a particular coffee changes in the days and weeks after roast.

The refractometer is a useful tool with enough precision for the purposes I've described, and a heck of a lot faster and easier than drying coffee grounds.

Taste in the cup is not a function of the refractive index. The refractive index is a measure of the concentration of the solubles and possibly small particulates extracted from the coffee. The idea was to come up with a direct relationship between taste and refractive index, this might work for some roast levels and coffees but has proved to be less than reliable.