Coffee is both art and science, but science has become increasingly predominant in recent years.
Thermometers measure brew water temperature, timers measure pours and brew times, and scales weigh grounds, water and brewed coffee/espresso.
One of the newest gadgets are refractometers, which measure total dissolved solubles.
In This Guide:
What Does the TDS of Coffee Mean?
Total dissolved solubles (TDS) measures how many coffee particles are in brewed coffee. It is used as a measure of overall extraction and strength. It doesn’t measure the types or quality of solubles, but rather the number of solubles in a sample.
You can think of TDS as quantifying how much of your brewed coffee is actually coffee (vs. water). For example, brewed coffee that has TDS of 20% is one-fifth suspended coffee particles.
How Should Baristas Use TDS When Brewing?
TDS is an objective way to quantify the total extraction and strength of coffee. Higher TDS equates to higher extraction and strength, and lower TDS to lower strength.
A good example of how TDS can be used to measure strength is cold brew, which is usually brewed as a concentrate and watered down when serving. The TDS of cold brew concentrate is quite high. When it’s watered down, the coffee becomes noticeably weaker because its TDS has been greatly reduced. Even without precise measurements, the dissolved solubles would be approximately halved if the concentrate was watered down with a 1:1 ratio of concentrate to water.
Does TDS Indicate How Good Brewed Coffee Is?
TDS doesn’t comment much on the quality of coffee, because it only measures total dissolved solubles and not what those solubles are. It says nothing about which of the 100+ compounds in coffee beans have been extracted, only measuring whether some of them have been extracted.
Because of this, the dissolved solubles don’t indicate whether brewed coffee is good in most cases. Strength shouldn’t be confused with quality (good or bad). Both an over-extracted, bitter coffee, and a properly-extracted complex coffee could have elevated TDS that are within the acceptable range. The bitter coffee has too many solubles of undesirable compounds. The complex coffee has many solubles of different compounds.
In cases of grossly bad brewing, TDS does correlate with sourness or bitterness. Sour coffee is under-extracted and will have a low TDS that’s outside of the acceptable range, while bitter coffee is over-extracted and will have a high TDS outside of the acceptable range. In these extreme cases, the taste alone is sufficient to indicate the brewing error though.
How To Measure Coffee TDS
The total dissolved solubles in coffee are best measured using a refractometer. There’s not an accurate way to measure TDS in coffee without this specialized instrument.
A refractometer works by sending light through a sample of the brewed coffee. The instrument measures how much light is refracted as it passes through the sample, and the measurement is referenced against an established index. The solubles are what light bounces off of, or refracts against, and thus the refraction is an indication of how many solubles are in the coffee sample.
Somewhat complex math is required to translate the refraction reading into a percentage, which is much easier to use. Comparing the refraction against an index eliminates the need to go through the calculation manually, however.
Refractometers can be digital or manual. Digital ones automatically take the reading, and have a setting to display the reading as a percentage. Manual ones require you to take the reading, which is done by looking through a viewer that has a gauge to check where light fully passes through. You can then reference the reading against a provided index.
How To Use a Refractometer to Measure TDS in Coffee
To measure the TDS in brewed coffee using a refractometer:
- Clean the Refractometer: The refractometer should be cleaned before use. Even if it was cleaned after the last use, dust may have settled on the tray. Any dust particles will give a falsely high reading.
To clean the refractometer, wipe it with a lint-free cloth (coffee filters work well). Then wipe it again with some rubbing alcohol.
- Zero the Refractometer: The refractometer also needs to be zeroed according to its instructions before beginning. Starting at an incorrect reference point will give an inaccurate reading.
Digital refractometers are zeroed by pressing a button (usually the button to take a reading). Manual refractometers might require adjusting the gauge via a minute adjustment on the instrument.
- Prepare Sample: Stir the coffee well before drawing a sample. Pipette some onto the sample tray. (Pipette by squeezing the pipette bulb, inserting the stem into the coffee, and releasing the bulb. Withdraw the pipette, and squeeze the bulb to expel the sample onto the sample tray.)
- Espresso: A special filter must be used if measuring TDS in espresso. The filter removes the high amounts of carbon dioxide that are in espresso (e.g. forming the crema). Not filtering will cause the CO2 to give a falsely high reading. Because there isn’t a standard amount of carbon dioxide in shots (think of different amounts of crema), there isn’t a way to consistently reference unfiltered espresso. The filters used are for this express purpose, and usually available only in larger quantities.
- Metal Filters: If measuring TDS when using a metal filter, wait a moment after stirring and pipette the sample from the upper third of the brewed coffee. Metal filters allow fines (small grounds) through, but these settle toward the bottom of the cup soon after the coffee is stirred. The fines settle as a sludge that’s usually not drunk, and thus shouldn’t be included in the refracted sample. Delaying the pipetting a moment and taking from the upper third of the sample will prevent the fines from giving a false reading.
This applies to French presses, pour overs and automatic drips with metal filters. It also should be done for ceramic filtered pour overs.
- Take Reading: The reading should be taken 10 to 20 seconds after preparing the sample. Simply press the button if using a digital refractometer.
If using a manual refractometer, look through the viewer. You’ll see a gauge (likely with little lines). You should see a bright area on the bottom majority of the gauge, and a darker area at the top of the gauge. The line where the darker area meets the bright, clear area is the reading.
- Reference Reading: A digital refractometer should automatically reference your reading against an index, and convert the reading to a percentage. There might be a button that toggles between an initial reading and a percent.
If using a manual refractometer, reference your reading against the provided index to get an accurate percent.
- Clean Glass: Clean the glass with a lint-free cloth and isopropyl alcohol (rubbing alcohol) as described in step 1.
Factors That Affect Coffee TDS
Since TDS is a measure of total extraction, and factors that affect extraction will impact TDS:
- Water Quality: Any particulate matter in water will increase a TDS reading. The effect will be greater with hard water than soft, since hard water has more minerals.
Using distilled water (which has no particulate matter) isn’t good for brewing coffee. A slightly alkaline water that has magnesium and calcium provides more comprehensive and even extraction. This water will increase TDS some, but the effect shouldn’t be taken into account when reading because the minerals bind to coffee compounds during extraction.
- Time: Extraction continues for the entire brewing time. Longer brew times increase TDS, and shorter times result in lower TDS.
- Coffee/Water Ratio: The more coffee grounds used, the more compounds can be extracted. The coffee-to-water ratio has a direct correlation to TDS. A higher ratio (more coffee, less water) yields higher TDS. A lower ratio (less coffee, more water) yields lower TDS.
- Grind Size: Finer grinds have more surface area, and thus increase extraction. A finer grind causes a higher TDS, and a coarser grind a lower TDS.
- Water Temperature: Hotter water temperatures cause faster extraction, and extract more compounds (especially those that create finer notes). Hotter water creates higher TDS, and cooler water lower TDS.
- Brew Method: Brew method impacts TDS in certain situations (see below).
An undesirable TDS reading doesn’t tell you which of these variables to change. Even a desirable TDS doesn’t tell you if all of the variables are correct. TDS isn’t a measure of quality in the brewing process or final cup.
What is a Good TDS for Coffee?
The ideal range for TDS is 18 to 22 percent for most brew methods. Some methods are outside of this range:
- Pour Over: 18-22%
- Automatic Drip: 18-22%
- French Press: 18-22%
- Cold Brew: 30-36% (concentrate), 18-22% (diluted)
- Espresso: 18-22% (after special filtering)
- Espresso Lungo: 22-24%
- Espresso Ristretto: 16-18%
- Turkish Coffee: 24%+ (without sugar)
- Moka Pot Coffee: 24%+ (without sugar)
Scott M. Brodie has over 20 years of professional experience working in coffee shops and writing about coffee (including selling superautomatic machines). When not writing, he can usually be found roasting a new African single origin or composing a fictional work.