Coffee Data Science
Revisiting Pressure for Espresso
9 Bars vs 6 Bars: Espresso
Espresso has often defined by 9 bars of pressure. This definition doesn’t fit with modern espresso as people have been experimenting with different pressures as well as methods to reduce required pressure by decreasing the gas contained in the coffee.
Caveat: While this data is interesting from a data perspective, as a result of bean processing, thermal pre-infusion, and profile design, my profiles max out at 2 bars of pressure while extracting between 18% and 22% in the 1:1 ratio shot. So I have a bias, but my bias is so far from 9 bars that it doesn’t make much of a difference when talking about high pressure experiments.
The amount of data on extraction compared to pressure is small, but let’s take a look and see what it says.
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.
Compound Coffee
Compound Coffee looked at varying the pressure of the machine during extraction, and they found between 7 and 8 bars to be optimal instead of 9 bars. They looked at a multiple shots over a single coffee roast.
All samples sorted all lined up:
The story I see once the data is sorted is that extraction at 7 bars is slightly better than at 8 bars. It’s clear from this chart that 9 bars is not the optimal pressure for higher extraction as it is the lowest trend. 6 Bars is also better than 9 bars for these experiments.
Lance’s Data
Lance Hedrick collected some data comparing 9 bars to 6 bars across a few roasts. I pulled his data, and I plotted it as a scatter plot. First, the data is plotted unsorted.
Then I sorted the shots by EY within each coffee to compare the best to the best. This shows a slightly different picture.
Over 17 sorted shots, there is a 0.44 difference in EY which is statistically significant using a two-tailed paired t-test.
Two different datasets tell two different stories. There is a challenge in producing a good dataset for shot pressure. I suspect this also requires a measurement of pressure as well as shots over a few weeks as roasts rest.
The bigger challenge is whether one can taste that difference in EY or not, so it may not matter much to most people.
If you like, follow me on Twitter, YouTube, and Instagram where I post videos of espresso shots on different machines and espresso related stuff. You can also find me on LinkedIn. You can also follow me on Medium and Subscribe.
Further readings of mine:
My Second Book: Advanced Espresso
My First Book: Engineering Better Espresso
