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How can CO2 influence espresso extraction?

An alternative way of extraction and flow control that is raising a question about coffee storage.


Recently I was discussing the blog post from Barista Hustles called “Temperature’s hidden effect”, and my reply - “Temperature’s not so hidden effect”. Later on Jeremy Challender from BH brought my attention to a website called DIYCoffeeGuy which I had not yet discovered at that time. An article from this website grabbed my attention and I wanted to investigate more on this topic. For some reason, Troy (the coffee guy behind DIYCoffeeGuy) is no longer active but I found this interesting article, and THIS video on his YouTube channel. The blog post was called “THE ESPRESSO RE-EXTRACTION EXPERIMENT” and in summary, he used stale old coffee for pulling an espresso shot. After the extraction, he added baking soda on the top of the coffee and extracted again the same coffee. According to his observation, the second extraction of the same coffee was a “thick syrupy pour with a light caramel color and even some crema!”. His experiment was meant to explore the influence of carbon dioxide on the flow of espresso. Please check his article for more reference.


BH observation state the more CO2 is in the coffee, the slower will be the first part of extracting the espresso shot. After the first half of the extraction, the flow rate should increase again ending the espresso shot for roughly the same time. It seems that this topic is interesting for many and I was thinking about how I can check both observations.

According to both findings, CO2 could be a way to control the flow rate and extraction process.

This could lead to a completely new trend in the coffee field – precisely controlling the amount of CO2 in the coffee bean. It seems this is something already interesting in the coffee field as more and more products like the Fellow Atmos are released these days.


How can I check this phenomenon? Adding baking soda is not a good way to go, because it will completely change the extraction process. It will dissolve in the water and will create too many variables. Baking soda will end in the espresso drink and will change its taste. I had to come up with a different solution that won’t end in the final espresso shot and will not influence other parameters during extraction. Taking into account all the above I decided to use dry ice – a solid form of carbon dioxide. Yes, the same dry ice we use to put out fires😊. It will produce plenty of CO2 gas but during extraction, it will be degassed and won’t influence the taste of the coffee.

Finding dry ice is not difficult, especially if you are a chemist who has friends and colleagues in the Faculty of Chemistry or in Bulgarian Academy of Science. I took around half a kilo in a chill box, which was more than enough to conduct all the experiments. I tried different methods to add and distribute the dry ice, but the best one was just to add a small amount on the top of the coffee puck as Troy did with the baking soda. Adding dry ice to the bottom, middle, or mixed with coffee grounds led to a huge channeling and worst results. This was expected as the dry ice evaporates quickly and creates empty pockets in the coffee puck disturbing the entire extraction.

What I needed to do for my experiment was to compare a simple extraction of old coffee with the extraction of the same old coffee using dry ice.

You can continue reading the entire article on my Patreon page HERE.



Special thanks I want to give to one of my Patrons – Rain or Shine Coffee. Go check them out, it is worth it. You can join in their live stream every day, where you can meet incredible people, and hear professionals talk about different coffee-related topics.

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