Coffee Data Science
Uneven CO2 Solubility during Espresso Extraction
Yet another variable to consider
I have been thinking a lot about how temperature differentials inside the coffee puck affect extraction, particularly from top to bottom. This issue has really come to light after working on thermal pre-infusion, and I made an observation leading me down the path of yet another variable: a lack of CO2.
Background
Thermal pre-infusion aimed to bring the temperature of the coffee puck closer to the brew water. These profiles are much higher in extraction, and I’m brewing at a lower temperature than previously. However, the profiles don’t generate the pressure one would expect.
My current profile is a ramp bloom with a 0.5 ml/s flow goal. This is the result of multiple experiments aiming for optimal extraction even though it turns out to be quite a long espresso shot in terms of time. One can observe in this sample profile that the pressure doesn’t go above 1 bar.
I didn’t have a good explanation, and I tried to study the variable. I needed a deeper explanation.
The main driving force behind pressure in espresso is CO2. Water is generally incompressible, and if you run water through spent coffee, little pressure is generated. Fresh coffee has CO2 trapped inside the pores from the coffee roasting process, and once in contact with water, this CO2 is released.
If you try to pull espresso from day old beans, there will be a lot of crema, it won’t extract well, and the coffee will taste acidic. The amount of gas goes down over time, and there are ways to degas coffee faster.
CO2 Solubility
CO2 dissolves in water in an interesting way. It dissolves inversely to the water temperature. A higher pressure will cause more CO2 to dissolve. This is a sample of some chemistry data on solubility.
This is another way to view the data based on pressure.
Now, let’s apply this to some known data (from a temperature investigation) on the temperature of the top of the puck vs the bottom of the cup. The bottom never quite reaches the top temperature.
The original authors of the temperature data didn’t mention what pressure they pulled at, only output time. For the sake of simplicity, let’s consider pressure at 1 bar. This chart shows the estimated CO2 solubility during the shot.
We could also look at 9 bars, and it causes everything to shift upwards.
So if we look at the ratio between the solubility of CO2 at the Bottom vs the Top, there is an initial spike, and it comes down. However, there is still a difference at the end of the shot.
From these charts, there is evidence that CO2 is dissolving differently during espresso. This could have an effect on flow, but I don’t know. I would like to design some experiments to explore the impact of this variable.
For my shots, I have observed bubbles during the bloom phase indicating that some CO2 is coming out of the coffee.
CO2 solubility could impact taste as well because CO2 in solution will create carbonic acid. I suspect that a shot pulled at higher pressure will taste more acidic.
Hopefully, time will reveal more as the area of CO2 solubility is not well explored in espresso.
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Further readings of mine:
My Second Book: Advanced Espresso
My First Book: Engineering Better Espresso
