Espresso Measurement: Solubility

Seeing the bigger picture with data, Part 3

Robert McKeon Aloe

Coffee solubility turns out to be the easiest and best metric to differentiate between shots using a qualitative method. Luckily, a few groups like Socratic Coffee and Compound Coffee have compared refractometers which is equally important given their cost. This is an collection of data from Socratic Coffee, Compound Coffee, and my data to discuss refractometers. This is the third part in a series, the first focusing on Pre-Infusion, Pressure, and Water Temperature, and the second focusing on Grinding, Distribution, and Tamping.

To measure solubility, one uses a refractometer. This measures Total Dissolved Solids (TDS), and if you have the weight of the beverage, it determines how much coffee is in your cup. Once you know how much coffee is in your cup, you can compute the Extraction Yield (EY) by dividing by the input coffee weight. EY is the percentage of coffee you extracted. Typically, 30% of coffee is capable of being extracted, and good espresso extraction is between 18% and 22%, roughly.

The standard VST Digital Refractometer costs ~$700, and even a lower priced one is still $300 (Atago). Eventually, I bought a $20 Brix meter, and later, I finally decided to get a more expensive one (Atago). I also collected data to help understand how well the Atago works and how it compares to a cheaper tool brix meter.

Any refractometer needs calibration, and typically, one uses distilled water. Socratic Coffee investigated the different between the VST and Atago meters using Distilled Water and the Brew Water.

They found a downward trend related to the temperature of coffee, but when comparing the two meters, there did not seem to be much of a trend.


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