Refractometers have been around for a long time and are used in a wide variety of applications, for example to measure the sugar content of fruit. Since 2008, there has been a refractometer specifically for coffee, and although there are now alternatives on the market, the VST refractometer is still the standard instrument in the coffee world.
What is a refractometer?
The refractometer shines a light through a prism onto the coffee sample, and when the light bounces back, it shows how much is transparent water and how much is solid coffee particles. This is called total dissolved solids (TDS), which essentially indicates the strength of the coffee.
The correct application explained in simple steps
To get consistent results, it is important to use the refractometer correctly. Therefore, the refractometer should be neutralized with distilled water. Make sure the glass of the refractometer is clean, and then put a few drops of distilled water on it. With the lid closed, wait 30 to 60 seconds for the temperature of the glass to adjust to the temperature of the water. Select the "Set Zero" option from the menu and press "Go".
To measure a sample, swirl the coffee, and using a pipette, place a small sample into a clean, dry cup. Swirl the sample, and allow it to cool. The ideal temperature for the measurement is between 15 and 30 °C. The temperature is important because it affects the density of the sample and thus the light transmission.
Pour the sample with the pipette into the measuring glass. Again, wait 30 seconds and press "Go" to perform the measurement.
Press "Go" several times in succession to see if there are strong fluctuations in the readings. Ideally, the same number will be displayed twice. This number is a factor we use in calculating the target extraction percentage when brewing coffee. We want to find out how much of the bean we have extracted.
Understanding the numbers
You've probably encountered the word "extraction" in the context of coffee brewing. Here's a basic definition:
"Coffee extraction is the process of dissolving soluble aroma and flavor compounds from coffee grounds in water. Brewing coffee properly requires using the right amount of coffee, ground accurately and extracted to the right degree, while controlling the right time and temperature."
The following formulas can help you find a good measure:
Extraction yield % = brewed coffee[g] x TDS[%] / ground coffee[g].
Example Espresso: 41g brewed coffee x 8.4% TDS / 18g ground coffee = extraction yield of 19.13%.
Example filter: 1700g brewed water x 1.42% TDS / 100g ground coffee = extraction yield of 21.23%.
How to understand these numbers
Using these percentages, you can quickly measure and understand what changes you need to make when adjusting a coffee. After you figure out what ranges you like your coffee in, you can brew it exactly the same way next time and enjoy fantastic coffee every time!
What is the ideal TDS range?
Deviations from these ranges mean that your coffee has been under-extracted or over-extracted and is therefore outside the standards we set.
Espresso TDS range: 8% - 10%
Filter TDS range: 1.3% - 1.55%.
What is the ideal range for extraction?
Once you get your TDS reading, you should use the equation above to determine the extraction percentage.
The range starts at the low end at 18% and the high end at 22%. If you are within this range, you know that the coffee should taste good on paper. Some coffees lend themselves to a higher extraction, others to a slightly lower one. You'll have to decide that with your taste buds and use these tools to check if you're in the right range.
The high end of the extraction percentage spectrum is 22% above this value; you will notice the characteristics of too high an extraction: Bitterness, low intensity, lower body. An example of such extraction could be that you extract with too high a yield.
The lower limit is 18%. This means that the brewed coffee is too intense and therefore you will notice higher acidity, higher intensity and thicker body.
In general, the yield is too low and must be increased.