The Coffee Data Science

TDS Filtering for Espresso: More Data

Another data set looking at sample filtering

3 min readMar 12, 2024

TDS or Total Dissolved Solids is the best metric currently to assess coffee strength and extraction efficiency in a quantitative way. There has been some debate on using syringe filters, and I have previously looked at data to show syringe filters don’t necessarily give more accurate results as filtered samples have a linear correlation to unfiltered samples.

Recently, more data has become available across a larger variety of espresso shots and grinders, so I took a look. In particular, the author collected 57 shot pairs across two grinders, two filter baskets, two groupheads, and two shot profile types with multiple shots for each variant.

Definitions

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). Typically, one aims for 18% to 22% extraction or some times higher, but it is difficult to get more than 30% EY.

Data

For each shot, TDS was measured with and without a syringe filter. I plotted those data points, and they have a high correlation coefficient (0.93).

These results match previous results where I showed unfiltered samples have a linear correlation to filtered samples. I have not seen evidence that filtering brings the values closer to groundtruth; I keep an open mind to data.

Dried Measurements

To try to understand how these measurements connect to groundtruth, we can look at another piece of information recorded to determine extraction yield: drying the puck. This assumes no solubles went through into the cup, and that is difficult to measure. My previous measurements found the amount of fines that make it into the cup is less than 0.01g, but he also used a different shot profile. So I take these results with a grain of sand because they were not the main aim of his data collection.

I took these dried puck measurements to calculate extraction yield because they have the amount of the coffee left over after extraction, and then I compared the samples. One more caveat is that these samples were dried at 250F for 8 hours. In the past, I have let my samples dry for 1 hour at 200F, and I am not sure the effects of drying for that long at that temperature.

I also did this another time assuming the coffee had 1.7% moisture, which is typical for medium roasts. Unfiltered results seem a lot closer to dried puck measurements, but they were still statistically different.

These results were very interesting, and the data collected was very rich in other variables. I’m very curious what can be learned as a result, and I absolutely love when people publish coffee data.

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