Espresso crema and flow profiling - Page 2

Beginner and pro baristas share tips and tricks for making espresso.
Molina (original poster)
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#11: Post by Molina (original poster) »

Sorry for the late response. I was high on my grinding and knee deep into other coffee topics.

But wow, thanks for this. I understand the relation between the different profiles (their output pressure) and what it does to crema now. Was able to produce thick layers as well as slim cups of foam. Surprised that I didn't see that Hoffmann video about crema yet, thanks for the tip and everyone else for sharing your knowledge.

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#12: Post by Mike-R »

Jeff wrote:Before parroting Darcy's law, you should catch up on a few years of discussion. There was quite some discussion over if the term should be squared or not in relation to espresso extraction. JoeD has done some interesting work that suggests that neither holds well. The "puck resistance" line has been accepted as pretty much bogus for much more than watching its general trends during the shot.
Can you provide a link? I'm interested to understand more about JoeD's work in this area.

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#13: Post by Jeff »

I think RapidCoffee published some of his tests here with the early DE1 that showed the effects of "secondary compression" -- where increasing pressure doesn't increase the flow rate as one would expect from Darcy's law.

Most of the meaningful discussion has been on among JoeD, Collin, and Rohan. As it's not my work, I'm not comfortable extracting it publicly in convincing detail. At least my understanding is that there wasn't a good fit between experimental data and p/f^k for any k. I recall some information that suggested that the bulk flow rate was not very dependent at all on pressure across a relatively wide range, at least with 98 HU burrs.

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#14: Post by Mike-R »

Thanks, Jeff. I haven't looked at the Decent forums before, I'll try to do a search over there and see if I can find it.

If I understand you correctly, you are pointing out that the flow rate is not proportional (or even related by a power factor) to the pressure drop across the puck throughout the extraction phase. I completely agree.

But just a semantics nitpick, Darcy's law doesn't say that flowrate will be proportional to pressure through out the extraction phase. Darcy's law just describes the relationship of the flow rate to the pressure drop, permeability and dynamic viscosity at that moment in time. There is no requirement in Darcy's law that the permeability and dynamic viscosity must be constant over time.

I read a published study which determined that Darcy's law applies pretty well starting about 30 seconds into the extraction. They seem to also think that Darcy's law also applies in the first 30 seconds after saturation, and the next step of their research is to develop models to predict viscosity and permeability to go along with Darcy's law. If they succeed, they will be able to use Darcy's law together with other models to better understand what is going on in the puck.