Coffee Data Science

RDT over Coffee Roast Development

Exploring Espresso in a Controlled Experiment

5 min readMay 14, 2024

Coffee grinding often produces a lot of static electricity and sticky fines. This has been a challenge for espresso in particular, which has been mitigated by the Ross Droplet Technique (RDT). There has been some discussion on the optimal amount of water to add in order to get this mitigation as well as what level of roast is more affected, so I decided to run a few experiments and try to isolate some variables.

Note: I made this design of experiment with Andrew Caldwell.

I used spent grounds in combination with the roasted coffee to remove the gas variable in the coffees. These are the tentative conclusions based on a small amount of data:

1% added water may be enough to mitigate most of the static during grinding.
In gas controlled conditions, darker roasts extract ~1% higher.
Adding water may increase extraction.
Adding more water may increase extraction more.

All of these are subject to change, and I welcome people putting more data to these questions using the same or different Design of Experiment (DOE).

Coffee Roasting

All coffee was roasted on the Roest using a BT/IT profile (Inlet Temperature (IT) controlled by Bean Temperature (BT)).

In terms of post-roast metrics, these are all expected trends in terms of loss of weight, moisture, color, and density.

Moisture is higher for lighter roasts. A higher Agtron number is a lighter roast.

I aimed to have a 10 point difference in Agtron number, and I was able to get pretty close.

Darker roasts have lower density.

Coffee Grinding

For these tests, I started by examining coffee grinding. I measured static by the retention stuck in the chute of the grinder. I used a Zerno grinder. Then I bumped the grinder to get the rest out into the cup. From measuring these two numbers, I could measure the percentage stuck. The lighter roasts seemed to have a little more stuck.

However, once a small amount of water was added, barely anything got stuck. I did not repeat this for 4% added water.

Shot Performance Metric

I focused only on coffee extraction. Taste is king, but coffee extraction can still tell us something about efficiency.

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).

Shot Data

I prepared a mix of 50% fresh grounds and 50% of spent coffee. I also pulled a shot of just spent coffee to be able to correct these numbers for what extraction is caused by the fresh coffee compared to the spent coffee. The spent coffee was used so that gas inside the coffee doesn’t block flow, and we can focus just on the coffee solubles. I used a flat 2 ml/s flow profile at 90C on the Decent Espresso machine.

Then I pulled a salami shot by cutting a shot into three cups. I initially aimed for 23g, but that was too much. I dropped to 20g. My initial aim was for the 1:1, 1.5:1 and 2:1, but I had 23g on my mind, so all of these were a little shifted in output ratio to something more non-standard.

The baseline across agtron aligns roughly with the expectation that a darker roast has a higher extraction efficiency.

Adding 1% water equalizes most performance except gives the darker roast a bump.

Going to 4% water gives every roast a bump and allows a little more distinction between the lighter roasts.

We can look at the individual Agtron performances. Here is 48:

58 Agtron shows a similar trend.

Same with 70.

As well as the lightest roast at 79 Agtron.

Edit: Someone asked for shot times, which show a decrease in shot time for 4%. I wonder if you could adjust the grind to still get the same shot time.

This data was very interesting. The results of 4% support my previous data on adding moisture to coffee post roast, and I typically add 4% moisture to all my roasts. This moisture is then absorbed after a day, so then I don’t need RDT when I grind.

Studying coffee across development is difficult considering the mixing of variables, but we have simple tools to be able to do such experiments even in a kitchen laboratory.

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.