What the refractometer is good for
Now the refractometer still has its uses, don't get me wrong. In circumstances where we need objective measures and statistically significant numbers, a refractometer is very handy and is definitely useful. This can come in the form of tabulating research data or even measuring equipment performance and consistency.
It can give also us an idea on the strength of the extracted coffee. But outside of this, there is very little use for a coffee refractometer in a barista or coffee roaster's repertoire.
What the refractometer is terrible at
Tasting. The refractometer doesn't differentiate between sour, salty, sweet, bitter, savoury, astringent, floral, fruity, grassy, vegetal etc etc. It doesn't give information on any of this.
You do. Your tongue, olfractory system and brain does the real analysis of coffee. It doesn't matter if it is a 15 % extraction or 25% extraction, your customers certainly do not care. All they care about is how it tastes.
I've had 16% extractions that tasted better than 23% extractions. I've also had multiple instances where I've had 21-23% extractions and customers and well-known professionals alike gave feedback that it was sour and underextracted.
But how can it be underextracted if it is 21+% extraction? That is well within, if not even beyond the golden extraction percentage of 18-20% taught by the Specialty Coffee Association (SCA). If you've ever been in this situation, I've got news for you. You're overrelying on the refractometer and its senseless numbers.
What training with barista champions can teach you
During every training session I've had with barista champions from Malaysia, England, Japan, Switzerland, not once has the idea of a refractometer come up. It did not positively impact the creation of the final coffee experience and it certainly did not help espresso/filter calibration.
To them, the numbers meant nothing. It was always taste, taste and more tasting. They understood that whether it was a judging panel or a customer at the shop, they were going to assess the coffee based on how it tasted, not it's extraction numbers. And so they calibrated the coffee using different methods that, if you went solely by the number reflected on your refractometer, you would never have achieved the same deliciousness.
Of course, me being overreliant on the numbers at that time, still had a burning curiousity to know what the numbers were. So I tested it anyway. What I found was
a) The numbers had such a wide range of extraction % where it tasted good that the numbers barely provided much guidance. You would almost always fall within this range just by following standard recipes.
b) the optimal extraction percentage changed every time a variable changed. Different water? Different ideal extraction. Different rest date? Different ideal extraction. Different basket size? Different.. you get the idea.
Trying to calibrate based on numbers was a mediocre effort and showcased a lack of training and understanding of your own palette.
How this can make you a better coffee roaster
Well at this point I just sound like a broken record. You simply need to learn how to taste. Especially if you are a coffee roaster, the "ideal extraction" number on a commercial cafe machine and home espresso machine is very different even for the same coffee. The brewing recipe is also different because of the different equipment setups.
How are you going to account for these differences in your roast with a refractometer? You can't. The only way is testing the same roasted coffee through different environments. Understanding how your coffee reacts and optimizing your coffee for that setup.
A coffee that brews well as a 22g puck in a cafe doesnt brew the same way in a 7g basket. If you optimized your roast for a cafe, are you going to tell your home brewer to buy a refractometer and extract to the same percentage? No of course not. Neither will you roast to fit a home machine and get subpar results in a cafe setting.
To be a good coffee roaster requires you to test on these different machines and understand not just taste differences but brewing differences and how to cater to them not just in your roast but also in catering to customers expectations and equipment. What kind of flavour profile should home brewers expect from your coffee vs cafes? How can you keep it consistent across all your coffee lineups?