Coffee Data Science
The Usefulness of Conductivity to Measure Coffee Extraction
Does electrical conductivity correlate to the amount of solubles?
I recently got a water TDS meter, and I wanted to know if there was any correlation between TDS read by a conductivity meter and by a refractometer. There has been some interest in using conductivity to continuously read TDS during an espresso shot.
Total Dissolved Solids (TDS) is measured using a refractometer, and this number combined with the output weight of the shot and the input weight of the coffee is used to determine the percentage of coffee extracted into the cup, called Extraction Yield (EY).
Intensity Radius (IR) is defined as the radius from the origin on a control chart for TDS vs EY, so IR = sqrt( TDS² + EY²). This metric helps normalize shot performance across output yield or brew ratio.
A conductive meter reports TDS as parts per million (ppm). These are helpful in understand brew water composition.
I collected data on 116 shots over a few weeks, and I plotted the data. The results vary quite a bit.
In terms of general correlation, PPM (TDS by conductivity) did not correlate as well to taste as TDS, EY, and IR.
If you split the correlations by roast, the results vary. In this chart, I split the beans by arabica and robusta. There isn’t much of a difference and only low sample roasts have a high correlation.
Electrical conductivity is an interesting modality for measuring TDS. However, electrical conductivity doesn’t seem to be a great indicator of coffee extraction at least in this setup. More would have to be done to prove out conductivity’s usefulness outside of measuring water TDS.
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