Brew pressure gradient of espresso - Page 2

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#11: Post by shadowfax »

Indeed, I don't see the need to take offense. You asked a question at first, and you got a number of good answers, including a nicely detailed one from Jim Schulman and a focusing reminder of what this is all about from Chris-trust your taste.

Now you're having a semi-related discussion about puck pressure gradients and how to eliminate them by restricting the flow downstream of the basket. You haven't demonstrated or suggested any knowledge of doing this beyond the purely hypothetical, nor do you appear to intend to study this yourself and post your own thoughts. I've told you that I think what you're suggesting has been done and is a bad idea. I don't know that to be the case, and I'm not offering my comment as proof that you're wrong and I'm right. It's just my perspective. If you would like to pursue the science of puck pressure normalization through documented experimentation, this is your forum. There is, IMO, no better one for that.

But from my perspective, you're looking at something that has been tried and failed. To convince me (and, I suspect, others on this board), you'll need to go much further than taking offense at a criticism of, disbelief in, or even snickering at your idea. If you're looking for people to agree with your idea or do the experimentation for you just based on the thoughts you've expressed so far in this thread, you're right that this is not the place.

If you're wanting to read more about pressure profiling as covered on Home-Barista, there are a number of threads on this board that you'll find very handily with a Google search of the site. Discussion ranges from your original question of what pressure to set and how pressure impacts taste to making pressure-profiling equipment and manufacturers that produce pressure-profiling machines... and beyond.
Nicholas Lundgaard

Decent Espresso: espresso equipment for serious baristas
Sponsored by Decent Espresso
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endlesscycles (original poster)

#12: Post by endlesscycles (original poster) »

Thank you for your response (shadow). I think you are completely correct, except I didn't have a question regarding what pressure to set at. I merely asked if the needle valve had been used to brew, and I am curious the effect different full puck pressures has on flavor.

sorry brief, roasting...
-Marshall Hance
Asheville, NC

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Team HB

#13: Post by another_jim »

The pressure gradient through the puck varies over the course of the shot.
  • Initially, prior to seeing any drops, as the fines migrate downwards, the biggest pressure drop is at the air/water boundary (as this travels down the puck), which is where most of the fines and therefore flow resistance is located. Ramping up the pressure slowly during this phase clearly makes the shot more fault tolerant, but I have no clear intuition or knowledge as to what it does to the fines migration.
  • In the first phase of the flowing shot, most of the pressure drop is at the bottom of the puck, because the layer of fines is still intact. The flow is slow, very black, and without much crema.
  • As the fines partially escape and partially dissolve into the cup, the flow increases, and more importantly, the pressure drop throughout the puck becomes more equal. This is when most of the crema is formed.
  • But the pressure drop is never uniform, rather it is always biased towards the bottom. The coarse particles brew from the top down, and the top of the puck always resists flow less than the bottom. In other words, it is not just the fines that migrate down the puck, this is followed by a second wave of coffee solubles migrating down the puck. They are partially reabsorbed by the ground coffee lower down in the puck, and therefore move more slowly than the water itself.
  • As the coarse particles brew and lose substance even at the bottom of the basket, the shot turns into a gush, and it is time to stop.
The point of this lengthy description is that an espresso puck does not have an even pressure gradient at any time in the shot, and that the major pressure drop is usually below the point where the current extraction takes place. In a restrictor basket, the entire dose floats around and brews/steeps over the entire course of the shot. In a proper espresso puck, the extraction is much more top down, and proceeds much more quickly within each horizontal layer -- for instance, the top layers of the puck are fully extracted before there is even a first drop, while the bottom layers actually increase in coffee solubles until the second half of the shot.

Clearly this complex dynamic is required to get good crema. But more importantly, it is required to be able to shape the flavors of the shot -- changing dose, grind, temperature or pressure with a restrictor basket doesn't do much of anything.
Jim Schulman