Coffee Data Science
The Impact of Flow on Espresso Extraction
Quick experiment on flow rates
We have been talking about water contact time, so I was curious what would happen if I slow down a constant flow rate to the minimum? I experimented with the Decent Espresso and varied flow rate. The slower the flow rate, the higher the extraction yield.
The main caveat is that this becomes much more like slow coffee than espresso as the express is removed. 0.5 ml/s took 4 minutes, and 0.1 ml/s was truly a labor of love. I made a salami shot of measurements for each, where I measured TDS.
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).
The slower the flow rate, the higher the extraction. This was also at 90C, which does not do so great relative to higher temperatures. However, the coffee was 4 months old, so gas did not interact with extraction.
I also looked at the peak pressure, and the higher the flow rate, the higher the peak pressure.
This short experiment gives some more data to the idea that more contact time with water will lead to higher extraction.
The one uncontrolled variable in this mix is thermal energy. We assume it is water contact time, but part of water contact time is heating. It is quite possible the coffee in the slower flow was able to come up to 90C better than at a faster flow.
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