When coffee professionals talk about the success or failure of a particular batch, you might hear them throwing around terms like "TDS" or "extraction percentage." What they're talking about is the necessary and related principles of how much coffee flavor is being drawn (or "extracted") from the coffee grounds, and then how concentrated that soluble coffee material is in the finished drink, which is also known as the beverage's "strength."
Extraction and strength are the two most important physical properties of a finished coffee drink as far as a barista is concerned. Sure, as taste-obsessed professionals, any coffee-making craftsperson needs to be able to detect extraction or strength mishaps using their palates, but luckily, as nerds of the modern age, we're also equipped with some tools to do the hard math for us, as well.
TDS, as mentioned above, stands for "total dissolved solids," and in coffee brewing this term refers to the amount of the soluble flavor material from the coffee bean that is extracted as a part of the brewing process. While coffee beans (and, therefore, coffee grounds) are comprised of about 70% nonsoluble material—essentially the cell material that makes the beans solid in the first place—there remains about 30% of compounds that do dissolve in water. These are not only the things that make your coffee taste like, well, coffee, but they're also what makes the brewing water turn from clear to brown—and from nonstimulating to caffeinated.
Unfortunately for everyone, not all of what can be dissolved from coffee grounds necessarily should be: A lot of it doesn't taste very good, and can overwhelm the finished product with bitter flavors. Conversely, extracting too little of what the coffee has to offer leaves a sour taste and a cup devoid of sweetness. Hitting the sweet spot, which is typically considered anywhere from 18% to 22% is what makes the act of brewing coffee so darned hard: It's much too easy to over- or underextract this stuff.
Once the desired amount of solubles have been dissolved in water, there needs to be a balance of that coffee flavor to the liquid that completes the drink. Coffee is an incredibly strong-tasting substance, and so it needs to be pretty heavily diluted in most instances in order to be drinkable. (Espresso is a notable exception: Its concentration is part of what makes it so appealing.) In most nonconcentrated instances, the cup you sip will ideally be less than 2% coffee solubles, and more than 98% clean hot water. Too much water to coffee in your mug makes a weak, papery-tasting brew; too little water can make coffee taste terribly intense, almost ashy.
Tasting is a great way to diagnose and troubleshoot when either strength or extraction—or both!—has gone haywire, but it can also be a pretty subjective or impressionable skill for a barista. (Palate fatigue, people: It's a real thing.)
Enter the dynamic duo of a refractometer and the ExtractMoJo program, courtesy of the tech company VST.
Refractometers are used for myriad things, from testing urine samples for medical purposes to reading the relative salinity of water in an aquarium, and even to determining the potential conversion of sugars in alcohol before the process of fermentation. The tool is used by measuring the refraction of light shone into a substance as it sits on a lens. A few years ago, VST developed a universal beverage refractometer, which can be used to measure the concentration of coffee solubles in both brewed coffee and espresso. In the case of coffee, the refraction can tell us pretty precisely how much soluble coffee material a sample of the liquid contains, and the tool then displays a TDS reading on its LED screen.
Plugging in that TDS percentage to VST's complementary ExtractMoJo software, along with data regarding the coffee dose and brewing-water weight, will yield the percentage of the extraction which was achieved during the brewing process. (Note: While ExtractMoJo can be used without a refractometer, the two together make a super user-friendly (if really expensive) combination.)
Of course, the tools can only tell us what percentage of the material was dissolved, not what type: Even when the extraction shows smack dab in the perfect 18–22%, some of what makes up that tally might not taste especially good. This is part of the reason that this kind of geekery is best left to the labs and training rooms of coffee professionals.
I'll grant you that this all sounds like a head-scratchingly complex way of brewing a simple cup of coffee, and wouldn't be surprised if some readers wonder why the fuss. The tools are hecka useful, however, not just for training and quality control purposes at a café or for a coffee roaster, but they're also functional for calculating things like the least amount of coffee necessary in order to achieve a good-tasting coffee (which can help manage a company's bottom line), and to track preferences among groups of customers or potential customers along the way.
Plus, come on—who doesn't love a neat toy?