Coffee Data Science
Extraction Yield (EY) in Dried Pucks and Espresso using Initial Coffee Moisture
Diving deeper into dried pucks, refractometers, and ristretto shots
I bought a Decent Espresso Machine (DE for shot) a few months ago, and I was excited because I could stop the shot unlike a lever machine where the shot continues. A lever machine lacks a three-way valve to allow the pressure inside the puck to release. So I was able to dry some spent coffee pucks and compare extraction yield to measurements I took with a refractometer similar to previous data but focused on the 1:1 shot.
Drying Methodology
To dry the coffee, I did the following protocol which was simple and allowed consistency:
- I placed the puck onto a tray.
- Cut the puck up
- Dried in a toaster oven on 200F for 30 minutes
Usually, you could see moisture accumulating on the window for the first ten minutes, but then it was gone.
Measuring Baseline Moisture
First, I measured the moisture of the coffee grounds I was using by drying fresh grounds. I didn’t think to do this before because I wasn’t adding moisture (humidified coffee to speed up degassing). Since I knew I was adding around 1% moisture, I wanted a groundtruth measurement of the freshly ground coffee.
This experiment involved 6 roasts, and the moisture levels varied. 1% of moisture was added by me after the roast to simulate humidity treatment that accelerates degassing.
This data is important to help measure extraction yield from drying spent pucks.
Equipment/Technique
Espresso Machine: Decent Espresso Machine
Coffee Grinder: Niche Zero
Coffee: Home Roasted Coffee, medium (First Crack + 1 Minute)
Shot Preparation: Staccato Tamped and Staccato
Pre-infusion: Long, ~25 seconds
Infusion: Pressure Pulsing
Filter Basket: 20g VST
Other Equipment: Atago TDS Meter, Acaia Pyxis Scale
Metric of Performance
The main metric of performance is Extraction Yield via dried pucks and refractometers.
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).
EY from dried pucks is determined by using the weight of the dried spent puck and dividing it by the weight of the input.
The Answer is 42
I ended up with 42 collected samples across 6 roasts. I looked at EY from the dried puck and then also the EY adjusted. I adjusted both the dried puck EY and the EY measured by TDS to account for the moisture in the coffee grounds.
Then I plotted these pairs of data, and if TDS was a perfect metric, all the points would fall on the y=x black line. The offset for the adjusted EY is about 1.5%.
We can also view this by roast, they follow a similar trend for the most part.
We can cut this data one more way by staccato tamped (closest to a regular shot) and staccato shots. The staccato tamped shows the offset while the staccato shots are much closer to the y=x line. I’m not sure why that is or if there is something fundamental in staccato shots like sugars that are more linearly extracted compared to staccato tamped.
I looked at the general statistics, and all of these distributions were statistically significant under a two tailed paired t-test (p-value < 0.05).
The fun part of this type of data is the relative novelty since not much data has been published. Certainly plenty of data has been collected, but like a lot of coffee stuff, data is kept private by the people who collect it. This keeps coffee more mysterious, and it motivates me to publish as much data as I collect.
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