COFFEE DATA SCIENCE
What does WDT do exactly for espresso?
Confusing Experiments
Coffee grounds distribution is one of the more straight forward and cheap ways to improve espresso. The most popular and effective technique is the Weiss Distribution Technique (WDT). Available data suggests WDT improves extraction yield, and I use the technique myself. However, I wanted to better understand why it works.
What does it do for espresso, exactly? I went through a few experiments with mixed results, and I would like to test WDT more across more variations.
WDT is supposed to declump coffee and more evenly distribute the groups. Prior experiments show that coffee grounds distribution changes during the grind, so it is best to mix coffee. I wanted to separate some variables, so I started with clumps.
Clumps
To understand how a clump could impact flow, I coarsely ground coffee. I sifted out the larger parts.
Then I mixed these very large boulders with regular coffee to observe the effects.
The caveat: the boulders were evenly distributed.
I didn’t see a particular issue nor was there anything noticeable in the extraction yield measurements. Another potential reason is that the coffee was 6 months old, so it is quite possible that gas plays a more important role in blocking flow than large boulders.
Oil
To make more clumps, I turned to oil. I added olive oil to the grounds which did create more clumps, but it didn’t seem as good as clumps from a grinder.
In terms of extraction yield, the coarse plus fines was low, but that was relative to the input. I then removed the coarse particles (Just 14g Fine), and the extraction yield was what I would normally expect. Even with the added olive oil, I got similar EY.
WDT vs Mixing in Cup
I thought part of the issue with grinding is going directly into the basket. So I compared dosing directly in the basket, mixing in a cup after grinding, and WDT in the basket.
WDT was only slightly better and definitely not statistically significant. More data would be needed.
New Roast
I thought to do a quick test on a new roast, thinking the increase in CO2 would make things different. I theorized that WDT would make the CO2 more evenly spread.
I was wrong though in this test, and I was very surprised. WDT performed worse throughout the shot.
Previous Data
One previous dataset showed WDT improved performance significantly.
Over time, these results seem to show better performance at the beginning of the coffee usage. At least, WDT seemed to show better consistency.
Another dataset from Sprometheus was not statistically significant between WDT and direct dosing:
All of these tests convinced me that there is not enough data across roasts and age for WDT. I wish there was more, but I will continue using it in the meantime.
Equipment/Technique
Espresso Machine: Decent Espresso Machine, Thermal Pre-infusion
Coffee: Home Roasted Coffee, medium (First Crack + 1 Minute)
Shot Preparation: Staccato Tamped
Pre-infusion: Long, ~25 seconds, 30 second ramp bloom, 0.5 ml/s flow during infusion
Filter Basket: 20 Wafo Spirit
Other Equipment: Acaia Pyxis Scale, DiFluid R2 TDS Meter
Metrics of Performance
I used two sets of metrics for evaluating the differences between techniques: Final Score and Coffee Extraction.
Final score is the average of a scorecard of 7 metrics (Sharp, Rich, Syrup, Sweet, Sour, Bitter, and Aftertaste). These scores were subjective, of course, but they were calibrated to my tastes and helped me improve my shots. There is some variation in the scores. My aim was to be consistent for each metric, but some times the granularity was difficult.
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.
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
