Coffee Data Science
Who is the reigning champion
I bought a Decent Espresso (DE) in January and have been working on creating the best profile I could. I bought the DE1Pro. I thought it was time to compare it to my beloved Kim Express.
We can come back to this question another time, but today let’s focus on how the results differed greatly and some initial thoughts on the subject.
This Kim Express is a manual lever machine produced from the 1960s to the 1980s, and then the company went bankrupt. There are still a few of them around, but there is no other information about them. The Kim is a spring-loaded lever machine, and the key difference between the Kim and other lever machines is that the group head is inside a closed boiler. Other levers have the group head inside the boiler, but the boiler is open.
This design allows for incredible temperature control between the boiler and the group. The temperature is still not controlled in the control loop, so some temperature surfing is necessary by manually turning the switch off and on. The water temperature can be higher than the pump machine because most pump machines such as E61 or DE have a 105°C limiter.
My Kim has a slight modification to the safety valve so I can set it to 126°C. I usually shoot between 116°C and 120°C.
This is one of the biggest differences compared to DE. I tried changing the temperature calibration on DE to get closer to 120°C, but the best I could do was 113°C. The company doesn’t recommend this, and I don’t recommend it either. I used this temperature for these experiments because I was trying to close that gap, but I theorize that some other key difference was missing.
Coffee Maker: A Decent Coffee Maker and Kim Express
Coffee grinder: Zero niche
Coffee: Home Roasted Coffeemedium (First crack + 1 minute)
Shot Preparation: Normal Shots
Preinfusion: long, ~25 seconds
Infusion: pressure pulsation
Filter basket: 7g VST packed into 14g
Other equipment: Atago TDS meter, Acaia Pyxis scale
I use two sets of metrics to evaluate the differences between techniques: Final Score and Coffee Extraction.
Final result is the average of a scorecard of 7 metrics (sharp, rich, syrupy, sweet, sour, bitter, and aftertaste). These ratings were of course subjective, but they were calibrated to my tastes and helped me refine my shots. There is some variation in the results. My goal was to be consistent across each metric, but sometimes the granularity was difficult.
Total Dissolved Solids (TDS) is measured using a refractometer and this number, combined with the output weight and input weight of the coffee, is used to determine the percentage of coffee in the cup, called Extraction Efficiency (EY).
Intensity Radius (IR) is defined as the radius from the origin on the control chart for TDS vs EY, so IR = sqrt( TDS² + EY²). This metric helps normalize shot efficiency across the entire output efficiency or brewing ratio.
I looked at 10 pairs of shots with similar output ratios in 7 smokes. With one exception, all of Kim’s shots tasted better.
Kim’s shots were tastier because they had higher extraction and TDS.
When it comes to individual indicators, we noticed large discrepancies, with the exception of bitter tastes.
I used a two-sided t-test, which showed that the differences were statistically significant. This seemed obvious from the data, but a t-test wouldn’t hurt. This data set is small, but interesting given the large gap.
