Beyond the TDS number - Geeking with Meris

If you've ever watched me dial in a coffee, you've probably seen me pull out the refractometer after every shot. That little device has become such a part of my routine that our team jokes it's basically glued to my hand. But here's what I've been rethinking lately: TDS alone doesn't tell you nearly as much as you think it does.

For years, my workflow was simple. Pull a shot, measure the TDS, hit somewhere around 8-10% for espresso or 1.3-1.5% for filter, and move on. Good number-good coffee. Right? Not exactly.

TDS Is Just a Snapshot

Total Dissolved Solids tells you how much stuff ended up in your cup. That's it. It doesn't tell you *what* was extracted, or *when* during the brew it came out. Two cups can show identical TDS readings and taste completely different. One might be balanced and sweet, the other harsh and hollow. The number is the same, but the extraction journey was totally different.

This is where extraction yield (EY) becomes the more interesting metric. EY tells you what percentage of the dry coffee mass actually dissolved into the water. You calculate it from TDS, dose, and beverage weight:

EY% = (Beverage Weight × TDS%) / Dose

The SCA's "gold cup" window sits around 18-22% EY. But that range is wide, and the magic is in understanding where inside that window your specific coffee tastes best.

The Curve, Not the Number

What changed my approach was when I stopped chasing a single target number and started mapping extraction yield curves. The idea is simple: brew the same coffee multiple times, changing only one variable (grind size, for example), and plot the EY against your tasting notes.

What you get is something like a flavor arc:

• Under 18% EY: Sour, grassy, underdeveloped. The water didn't have enough contact or energy to pull the good stuff out.
• 18-20% EY: The acids and sugars start showing up. Bright, fruity, sometimes a bit thin depending on the coffee.
• 20-22% EY: This is where most specialty coffees hit their sweet spot. Sweetness peaks, body fills in, acidity balances out.
• Above 22% EY: Conventional wisdom says this is "over-extracted." But here's the thing I've been discovering: with certain coffees and certain brew methods, pushing past 22% can actually work. Especially with light roasts and Turbo Shot profiles (see my last newsletter), you can hit 23-24% and get incredible transparency without bitterness.

The curve shows you where sweetness peaks and where bitterness creeps in for *that specific coffee*. It's not universal. A washed Ethiopian Heirloom will peak at a totally different point than a natural Brazilian Catuaí.

How I Use This at TARABA

When a new coffee lands on our bar, I don't just "dial it in" until the shot looks right. I run what's basically a mini experiment:

• I pull 5-6 shots across a grind range, keeping dose, temperature, and ratio constant.
• I measure TDS on each one and calculate the EY.
• I cup all of them blind and score sweetness, acidity, bitterness, and body on a simple 1-5 scale.
• Then I overlay the tasting scores with the EY numbers.

What you see is a curve. Sweetness rises, peaks, and then drops off as bitterness takes over. The peak of that sweetness curve? That's your recipe. Not a TDS number. Not a generic EY target. The actual point where *this coffee* tastes the best with *this setup*.

It takes maybe 20 extra minutes compared to the old "pull, taste, adjust" method. But the precision is on a different level.

Why This Matters for Filter Too

This isn't just an espresso thing. On filter, the same logic applies. I've been testing our UFO Dripper recipes by adjusting pour rate and total brew time while keeping the ratio locked. Mapping the EY curve on filter has helped me realize that some of our coffees were sitting at 19.5% and tasting "fine" when they actually peaked at 21.2%. That difference of 1.7% EY sounds tiny on paper, but in the cup it's the difference between "pleasant" and "wow, what is this?"

My Take: The refractometer is not a scoreboard. It's a compass. TDS gives you a coordinate, but EY curves give you the map. If you're serious about getting the most out of your coffee, stop optimizing for a number and start optimizing for a curve. The sweet spot is different for every coffee, and the only way to find it is to plot it out.