Have you ever heard someone at a café, cupping, or brewing competition throw out the acronym TDS and felt completely lost? The coffee jargon lexicon is lengthy and confusing at times, but we’re here to help breakdown the acronyms and fill in the blanks, starting with TDS. TDS stands for total dissolved solids, and in short, is our way to measure how much “coffee” is in our coffee.
To start, dissolved solids are the soluble parts of roasted and ground coffee that are dissolved and extracted by hot water and thus, make their way into our brew. Since water is a polar molecule, meaning it displays both negative and positive charges, it is particularly easy to bond with and dissolve into. This polarity allows water to easily attach to the various chemical compounds in coffee and extract them. Having two hydrogen atoms and one oxygen atom make the molecule uneven and ready to attach to another compound (coffee!) that might even out its molecular weight.
Using TDS as a guide, we can determine how much of the coffee has been extracted. Then, with taste as our method, we can decide if we want more or less extraction during the next brew. While coffee is approximately 30% soluble, for a long time, it was believed that coffee tastes best somewhere between 18%-22% percent. As the quality of the world’s coffees continues to improve, we are able to extract higher percentages with the flavor of the coffee still in the desirable range. Approximately 25% seems to be where the cut off point starts for specialty coffees. Each coffee is different and all coffees are constantly changing especially from the moment they leave the roaster.
How To Measure TDS
Unfortunately for your wallet, a device called a refractometer is required to measure the amount of dissolved solids in your brew. Refractometers for coffee can be somewhat expensive ($200-$1,000), but they are considerably cheaper and much smaller than a gas chromatograph coupled to a mass spectrometer, the other VERY fancy and scientific way of measuring dissolved solid. This magic tool works by measuring the deflection of light as is passes through the coffee. (Barista Hustle has a great article explaining this light bending property further if you’re interested in nerding out a bit more.)
Along with your refractometer, you’ll need these items:
- Distilled water
- Alcohol wipes
- Paper towel
- Small bowls or cups
- Filtered syringe*
*Optional: If you are brewing using a metal filter like a Prismo or espresso shot, there will be suspended solids that are not water soluble. You will also want to use a filtered syringe to eliminate the suspended solids to get an accurate TDS reading. If you are using a paper filter, then your brew will be entirely dissolved solids.
Step 1: Set refractometer to zero using distilled water
- Turn on your refractometer.
- Add 3-4 drops of room temperature distilled water.
- Take a zero set reading.
- Wipe down refractometer lens with an alcohol swab.
Step 2: Put a small sample (around 2 oz) of well-integrated brewed coffee into a small bowl or cup
- Make sure to stir the coffee to unionize the liquid. Stratified coffee will give you an inaccurate measurement.
- Be sure to also allow the brewed coffee to cool to approximately 100°F or below. If you don’t have a thermometer, aim for room temperature or cooler.
- If you’re using a non-paper filter, this is where you use the filtered syringe
Step 3: Take a small sample of brewed coffee with a dropper or syringe
- Place 3-4 drops of brewed coffee on the eye of the refractometer.
- Close the lid and wait 10 seconds for the sample to reach a temperature close to the refractometer.
- Press go and take a reading. Do this 2-3 times to ensure a proper reading.
Step 4: Use a clean paper towel to remove the sample
- Wipe clean with an alcohol swab to reset for the next coffee sample.
Optimal TDS Readings For Different Brew Methods
For a pour-over dripper like Stagg [X] or Stagg [XF], the typical TDS measurement will be somewhere between 1.2-1.7 TDS. That’s right, your pour-over coffee is actually about +/- 98.5% water! For espresso-style shots with Prismo, we love the flavor most with TDS readings of 3.5-5.
AeroPress® Coffee Maker: 1.4-1.7
French Press: 1.4-1.7
Once you know the measured TDS, you can use a simple equation to determine the extraction percentage, which is the amount of coffee that it into your coffee.
Ex: 320 grams of brewed coffee x 1.43 TDS = 457.6
457.6 / 20 grams ground coffee weight = 22.88
22.88 = your extraction percentage
How To Improve Your Brew Using TDS
Think of TDS as a tape measure for your brewed coffee. Did you take it as far as you wanted to? Did you take it too far? Using taste/flavor as the primary gauge, and then determining what level TDS tastes the best will allow you to know when you have reached the desired extraction level for a given coffee. Try your favorite pour-over using the same dose of ground coffee, brew style, and coffee-to-water ratio with three different grind sizes and see which you prefer. You can also try three different ratios (1:15, 1:16, 1:17) and see which one tastes the best to you. Remember, different coffees will taste best with different levels of TDS.
Are you ready to start using TDS measurements to fine tune your brews? Check out these great options: VST LAB Coffee III Refractometer or ATAGO Coffee Pal III. If you’re in San Francisco, you can also stop by the Playground for a brew demo. I can measure the TDS of your brew, and we can discuss how it may be improved. I also teach a Home Brewing class once a month. We’ll measure the dissolved solids in our brews and use taste/flavor to find the overall best version of the pour-over we are brewing.