Coffee Bean Degassing

A review of some wonderful coffee research

Published in

Towards Data Science

7 min readSep 11, 2020

In July 2020, I started thinking more about degassing and coffee. I had been experimenting with letting my coffee roasts rest for 3 or 4 weeks before using them, and I noticed an improvement in taste and extraction level. I suspected part of the extraction was due to less carbon dioxide.

So I started an experiment to see if I could degas beans faster using a vacuum jar with the hope of using them sooner. In the process, I started see what research was out there, and I ran across a great paper and video presentation of that research by Samo Smrke and others.

I was so excited, I wanted to review that information here. Often, I think research is confined to those who do research and inaccessible to those who don’t. None of this work is mine. My hope is that maybe I could present the most interesting pieces of the work and a few graphs to look at the data in a different way to help others understand. One can always look at the video or read the paper, but maybe you just want the graphs.

For each plot in the presentation, I wrote a script to pull the data out of the original plots so I could re-plot it. I have added an additional plot showing the degassing as a percentage of the total, which I have found to be a helpful normalization. All of these graphs contain the original data, but some times, I have changed the x-axis tick marks to something I think makes more sense or mixed some data that wasn’t mixed before.

Different Beans

Different beans degas at different rates. For espresso, this is particularly important because the amount of gas still left in the bean is related to how a shot will flow. I suspect this means that different beans should be used in different time windows rather than a generic window like 2 to 4 weeks in the case of espresso.

Roast Level

The darker the roast, the more gas is created! What’s fun for this data is that as a percentage, most of the roast levels have a similar rate and pattern. As the authors of the study pointed out, most degassing (70%) occurs in the first 24 hours.

Roast Level vs Speed of the Roast

They looked at dark, medium, and light roasts at three speeds. As percentages, they follow a similar trend no matter the speed.

I mixed some of these for Fast and Slow to give an idea across the three roast levels.

Again, in terms of percentages, they have a similar pattern for degassing no matter the roast level or speed.

Roast Speed

The study also looked at roast speed, and a faster roast had more gas to come out of the beans.

Whole Bean vs Roast and Grind (R&G)

Immediate grinding after roasting drastically changes the gas coming out due to the gas loss during grinding.

Grind Date vs Roast

They looked at two roasts over time, and sampled the roasts to grind them at regular intervals. The most interesting thing they found was that at 4 weeks, the amount of gas coming out of the coffee after grinding is more than what will come out of the whole bean. They theorize that this is due to some gases being trapped in the coffee, which would explain why coffee that is months old can still have some crema during espresso brewing.

As expected, the light roast has less gas in the beans and the grinds at the various stages.

We can line up the different weeks past roast for the grounds, and it’s a fun to see how quickly the grounds degas.

Freezing Beans!

Their study was very informative to understand what happens when you freeze coffee. The author in the presentation pointed out that they don’t think of the process as freezing coffee beans because most roasted beans have very little water in them. So they consider the beans are cooled instead which slows down degassing. As a result, freezing is an effective method to elongate shelf life. You can also freeze and re-freeze beans which you can’t typically for other food.

They looked at beans that were roasted and split between fresh, frozen for 1 week, and frozen for 2. They were ground at the designated time, and the amount of degassing for all three follow a same pattern with a slight dip for 2 weeks vs fresh.

They looked at 70 days in the freezer, and they found the frozen beans degas almost exactly the same as the fresh beans with an offset.

Extra Bonus: Coffee Aromas

Samo also did some work on different aromas that come out of the beans in order to look at coffee freshness. The specific chemicals have attributed tastes below. Each one contributes to different components of taste as aromas are complex.

The original plot shows a drop, and I adjusted the X-axis to be one week intervals. My aim is to get a graph and some tables that help clearly show how much aroma is left by week. So I also renormalized the data for their maximum and minimum.

What this chart is missing is CO2. So I took one of the degassing curves from the section above titled “Roast Level vs Speed of the Roast.” I used the medium roast at the medium speed.

CO2 degasses very quickly relative to aroma, which probably explains why the best time to brew espresso is 7 to 21 days post-roast. I have been hitting the range of 21 to 35 days because I can get a higher extraction level (~22% Extraction Yield).

Here are some tables to bring a more concise summary. In the first, you can see that CO2 depletes the quickest while Methanethiol is the slowest. If you look at the second table, that is done with respect to each week. After 2 weeks, 3 of the 4 aroma parameters are still near 50% of the starting point while CO2 is very low. After 6 weeks, most of these aromas are at their lowest point. It doesn’t mean they are gone because these were normalized values with respect to their maximum.

Overall, I was very surprised to run across this research, and I was very excited to play around with the data. It doesn’t change how I’m looking at espresso, but it does help provide extra explanation to the amount of off-gassing I’ve measured using a scale that’s accurate to 0.03g.

I’m hopeful further research will help improve the way I roast and store coffee beans. Of course, the trouble in the field is the wide variety of coffee beans, roasters, and brew methods.

If you like, follow me on Twitter and YouTube where I post videos of espresso shots on different machines and espresso related stuff. You can also find me on LinkedIn.