Inside the crucible - what IS going on in there?

Seems like a while since we've discussed what we think, speculate, or "know" happens inside the portafilter basket during the various stages of extraction.  A bit of this has come up in another discussion:
http://www.baristaexchange.com/forum/topics/ideal-tamper-base-shape...
however it seems like a more thorough discussion can occur if we break away from that original question.  Some good stuff put up over there, so you should definitely read it if you haven't already.

So, what do you think is going on in there?


(note: I will ask one favor... let's approach this neutrally, scientifically, and skeptically.  I'm pretty sure that we do not actually know exactly what is happening in there... we have developed some really good theories to explain what is happening, have made observations that support them, and have made other observations that may not be fully explained.  So let's approach the subject in that way - submitting what you suspect is happening in there and sharing any supporting evidence you may have.)

Also, if you happen to have any links to other discussions that you feel have covered this well, please share them.  I don't get out as much as I should...

Ok, geek on...

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This may be too vague of a topic starter to get much of a response.

I suggest a title change to: "Does the puck swell?"

It'll get people thinking in the right direction.

That having been said, I do think that the puck swells. ;)
I think a viscous mass with convection currents is formed. The shape of the basket and the condition/size/shape of the holes would support/inhibit various convection currents that could be benificial/detremental to the extraction of the coffee.

I think someone at MIT should set up a super-computer simulation of espresso extraction.

There you go.
The puck has to swell or at least wants to. Look at how coffee reacts when you brew in a pour over or press. I'd wonder weather it could expand up against the water and if it was fully constrained by the sides and bottom of the basket. So extraction happens against the coffees natural state, it wants to swell and absorb the water but is constrained by the pressure of the water from above and the portafilter basket. My best guess would be that the puck only swells up towards the gouphead after pressure from the water stops and the coffee is able to absorb some of the water remaining in the basket.
Ding Ding Ding!

At least, that's been my theory for a number of years.

Jason Shipley said:
The puck has to swell or at least wants to. Look at how coffee reacts when you brew in a pour over or press. I'd wonder weather it could expand up against the water and if it was fully constrained by the sides and bottom of the basket. So extraction happens against the coffees natural state, it wants to swell and absorb the water but is constrained by the pressure of the water from above and the portafilter basket. My best guess would be that the puck only swells up towards the gouphead after pressure from the water stops and the coffee is able to absorb some of the water remaining in the basket.
Jason Haeger said:
This may be too vague of a topic starter to get much of a response.

I suggest a title change to: "Does the puck swell?"

It'll get people thinking in the right direction.

That having been said, I do think that the puck swells. ;)

Yes... the title on its own is pretty vague. However I thought, in the context of the discussion that people seemed to want to have on the other thread (but not wanting to derail things) I'd give it another place to go...

I may bring some of those responses over to create a bit more context. Until then...

I too believe that the puck swells. If it were "sucked up against the screen" there'd probably be a void in the spent puck... most of mine are pretty solid.

On another note...

In a discussion on extraction that we had here a couple of months ago, Lem Butler shared the idea that an important part of the extraction process was the compression of the grounds. He suggested that it was similar to pressing out oils from other seeds, olives, etc. He felt that for this to occur, your basket must be filled to at least a certain level - enough so that the growth caused by the puck would be held back and force a sort of "self compression".

I'd not heard this. Still not quite sure what to think of it. Thoughts?

The guys also discussed the impact of the processing method, roast, and other bean factors on portafilter behavior. Some coffees are more hydrophyllic than others... they'll absorb more water and as a result grow more during extraction. This effects the compression effect... in a given basket, a higher dose would be necessary of a relatively hydrophobic coffee as compared to a hydrophyllic one. We pulled identical doses of 3 different coffees - dried, washed, and natural, and observed varying degrees of puck integrity and "muddyness". We also observed the differences in the flavor of the "muddy puck" coffee once the dose had been increased to reach the "proper degree of compression". (For science's sake, this probably should have been followed by a set of shots pulled with the same dose using a deeper basket, to eliminate that variable.)

Ok... not much but its a start.
On the topic of hydrophilia, vs. hydrophobia. I'm inclined to agree.

I've often found myself frustrated at not being able to get a full extraction from SL28 beans ever, regardless of brew method.



Brady said:
Jason Haeger said:
This may be too vague of a topic starter to get much of a response.

I suggest a title change to: "Does the puck swell?"

It'll get people thinking in the right direction.

That having been said, I do think that the puck swells. ;)

Yes... the title on its own is pretty vague. However I thought, in the context of the discussion that people seemed to want to have on the other thread (but not wanting to derail things) I'd give it another place to go...

I may bring some of those responses over to create a bit more context. Until then...

I too believe that the puck swells. If it were "sucked up against the screen" there'd probably be a void in the spent puck... most of mine are pretty solid.

On another note...

In a discussion on extraction that we had here a couple of months ago, Lem Butler shared the idea that an important part of the extraction process was the compression of the grounds. He suggested that it was similar to pressing out oils from other seeds, olives, etc. He felt that for this to occur, your basket must be filled to at least a certain level - enough so that the growth caused by the puck would be held back and force a sort of "self compression".

I'd not heard this. Still not quite sure what to think of it. Thoughts?

The guys also discussed the impact of the processing method, roast, and other bean factors on portafilter behavior. Some coffees are more hydrophyllic than others... they'll absorb more water and as a result grow more during extraction. This effects the compression effect... in a given basket, a higher dose would be necessary of a relatively hydrophobic coffee as compared to a hydrophyllic one. We pulled identical doses of 3 different coffees - dried, washed, and natural, and observed varying degrees of puck integrity and "muddyness". We also observed the differences in the flavor of the "muddy puck" coffee once the dose had been increased to reach the "proper degree of compression". (For science's sake, this probably should have been followed by a set of shots pulled with the same dose using a deeper basket, to eliminate that variable.)

Ok... not much but its a start.
Jason Shipley said:
The puck has to swell or at least wants to. Look at how coffee reacts when you brew in a pour over or press. I'd wonder weather it could expand up against the water and if it was fully constrained by the sides and bottom of the basket. So extraction happens against the coffees natural state, it wants to swell and absorb the water but is constrained by the pressure of the water from above and the portafilter basket. My best guess would be that the puck only swells up towards the gouphead after pressure from the water stops and the coffee is able to absorb some of the water remaining in the basket.

Interesting thought.

The thing I think is relevant for that question is that, once the basket contains only water and grounds (no air), the water pressure will act omni-directionally. (Assuming I remember Fluid Dynamics correctly). This means that if you were to measure the pressure at any point in the middle of the puck, the pressure at that point acts from all directions. It is the "static pressure" of the fluid.

The water is clearly being moved from top to bottom, but the speed is so slow that both the "dynamic pressure" and drag force pulling the grinds down are both minimal. I guess what I'm saying is that I don't believe that there is actually anything pushing the puck down. This leads me to think that it will attempt to expand in all directions equally, even during extraction.

Thoughts?
I don't really have anything to add that hasn't been mentioned already, but I'm really going to enjoy reading this thread. Thanks Brady.

-bry
I've always beieved that when hot water enters the basket, the aroma and warmth attracts tiny angels who then begin massaging the coffee granules, coaxing the oils and flavours out before being carried through the basket and into the cup with the flow of water.

Nobody knows for sure why the angels do it; I think they may be junkies who are doing nothing but trying to get at and bath in the caffiene, but nobody has seen these angels, let-alone study them, so it really is a matter of pure speculation.

Bryan Wray said:
I don't really have anything to add that hasn't been mentioned already, but I'm really going to enjoy reading this thread. Thanks Brady. -bry
Mike Sabol said:
I think a viscous mass with convection currents is formed. The shape of the basket and the condition/size/shape of the holes would support/inhibit various convection currents that could be benificial/detremental to the extraction of the coffee.

I think someone at MIT should set up a super-computer simulation of espresso extraction.

There you go.

That's an interesting thought - that the puck could be a viscous fluid. I guess I'd always pictured the puck as mostly fused together and stationary. That could be happening, though I think it'd be more likely in a basket that was slightly under-filled, where there was more room for things to move around a bit.

I'd think that in a basket where the puck had swelled to meet the screen the whole puck would almost lock together, but have nothing concrete to base this on. Is reducing this effect one of the benefits of minimizing headspace?

Anybody have any connections in the fluid dynamics department of a local university?
Mike Sabol said:
... I think someone at MIT should set up a super-computer simulation of espresso extraction.

You know, we could probably get a little bit of a look into what is happening by mixing some ground-size "marker particles" (glitter?) in with the coffee. See where they end up once the whole thing is done?

Though this would be a great excuse to play with nano-bots, so I'm going to recommend that route instead.
My guess: They would end up towards the top.

I have a lot that I want to add, but not enough time to write it out. I hope to be back with more time later.

Brady said:
Mike Sabol said:
... I think someone at MIT should set up a super-computer simulation of espresso extraction.

You know, we could probably get a little bit of a look into what is happening by mixing some ground-size "marker particles" (glitter?) in with the coffee. See where they end up once the whole thing is done?

Though this would be a great excuse to play with nano-bots, so I'm going to recommend that route instead.

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