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Permalink Reply by Kevin Ayers on June 1, 2009 at 12:18pm

It seems that although the flow rate may stay consistent, a few of us have pointed out that they didn't taste as good with no tamp pressure. Seems to at least make the case that SOME pressure is necessary, however the 30-40lbs rule may be in some jeopardy here. I would really like to look into the research that lead to the 30lbs number, as there must be some minimum it takes to form a decent puck (and maybe that is 30lbs?!). However I think we can ALL agree that even and consistent dosing is far more crucial than tamp pressure.

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Permalink Reply by Phil Proteau on June 1, 2009 at 12:45pm

On the issue of the tamp, it's purpose is to squeeze out air pockets in the coffee bed to help create an even density distribution. This is to reduce the possibility of "path of least resistance" that may exist where there is less density. Even density = even distribution = even extraction. So it is not really fair to make the argument that the water applies more pressure than the tamp. The water will percolate through the particles, not to mention passing though the physical structure of the coffee itself. The tamp is a solid and will come to rest on the surface. The dynamics of the two types of pressure is very different, not to mention the effect they both have on the coffee.

It is true that restriction of the water through the coffee is mainly a function of particle size. Here we use the "a bucket of sand holds off water longer than a bucket of rocks" reasoning.

So no, we do not need the tamping pounds as a flow restrictor regulator, but we do need tamping as a density distributor technique.

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Permalink Reply by Jonathan Jarrow on June 1, 2009 at 1:51pm

Correction:

Over 500 lbs of pressure, not 150 lbs.

Jonathan Jarrow said:

According to The Professional Barista Handbook by Scott Rao, when a puck is hit with 9 bars of atmospheric pressure, it is equivalent to around 150 lbs. of pressure across the surface of a 58mm+ puck(I don't have the book on me at the moment to give the exact #, but I know it's around 150). He also has the math in the book explaining how he got that #. Because of this, it doesn't really matter if you tamp the hell out of that shot, it's nothing compared to what the machine is putting the puck through.

The purpose of the tamp is to remove "pockets" of air from inside the coffee bed to prevent channeling.

This said, I have a theory about tamping pressure which I have yet to prove involving reduction in crema due to over tamping. If crema is the result of oils in the coffee wrapping themselves around the air in the coffee bed, then it would seem that a tamp with excessive pressure would remove a lot of the air from the bed as the particles compress together, creating less available material for the creation of crema. I have seen hands on evidence of this, especially when working with a much finer grind.
Any feedback on this in particular would be much appreciated.

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Permalink Reply by Jonathan Jarrow on June 1, 2009 at 2:05pm

Yes, the gases do come from the bean, but this doesn't negate the fact that there is still ambient air between and around the particles themselves. (The puck is definitely not a solid piece of material even when compacted at a fine grind) This ambient air is increased by not fully compressing the grounds in the basket.
\
Whether or not this is actually affecting the amount of crema is really the question. As water hits the grounds, does it go straight from top to bottom forcing the air out of the bottom of the basket?? Or does it encapsulate some of the air and force it into the finished product?? This is what I'm curious about.

It's not so much squeezing a bean to get the gases out of it, as it is squeezing a puck to remove air. Hope that makes more sense.





Jesse -D-> said:

"This said, I have a theory about tamping pressure which I have yet to prove involving reduction in crema due to over tamping. If crema is the result of oils in the coffee wrapping themselves around the air in the coffee bed, then it would seem that a tamp with excessive pressure would remove a lot of the air from the bed as the particles compress together, creating less available material for the creation of crema. I have seen hands on evidence of this, especially when working with a much finer grind.
Any feedback on this in particular would be much appreciated."

I don't think that I agree, but I'm not sure I understand fully. The gases that create crema are coming from inside the grounds, so the fact that you tamp harder really would have no effect on this. You are essentially saying that if you squeeze a roasted coffee bean you can get CO2 to come out, and that you cannot do.

Again if I misunderstood your post please correct me and accept my apologies.

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Permalink Reply by Phil Proteau on June 1, 2009 at 3:16pm

The first few drops of coffee are the darkest and the least infused with crema bubbles, so based on that, we might rightly assume that the air held in the spaces between the coffee particles has been pushed out the bottom by the time most of the crema is created.

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Permalink Reply by Jonathan Jarrow on June 1, 2009 at 3:44pm

Phil, respectfully...I disagree entirely. The first drops of espresso are incredibly viscous due to the emulsification of the gases and oils being pushed out along with other insoluble material. Just because a shot pulls dark in the beginning does not mean that it isn't carrying the bubbles which create the crema. I can clearly see it, even at the start of a shot.

By the way, couldn't agree with you more about the massive importance of distribution and density.

Phil Proteau said:

The first few drops of coffee are the darkest and the least infused with crema bubbles, so based on that, we might rightly assume that the air held in the spaces between the coffee particles has been pushed out the bottom by the time most of the crema is created.

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Permalink Reply by Chandler Rentz on June 1, 2009 at 5:40pm

killer... glad to see so many responses... and differing viewpoints. Good food for thought ; )

I was experimenting today with different tamping pressures (mostly from 10-50lbs) and oddly enough sometimes @ 50lbs the shot would pour seconds faster than a 20lb tamp for example... perhaps due to crappy distro=channeling?

I started at 18g then did shots at 19 and 20g (@ higher doses I had to apply at least 20lbs to get enough headspace btw the coffee and screen). One thing I noticed (and as I read now others are echoed this as well) is that when I tamped super light I wasn't happy with the results. Maybe it's just me. I have friends who swear upon tamping with 10lbs and they say they grind finer.... uh, hmmm. But, when I was tamping between 30-40lbs (of course also focusing fully on getting the coffee to land nice and even in the PF basket) i got sweeter, better tasting shots... who knows?!

'massive importance of distribution and density' I definitely vibe with that. To that I add the necessity of learning how to dose with intentional accuracy and consistency and a good level tamp.

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Permalink Reply by Jesse -D-> on June 2, 2009 at 5:52am

Jonathan:

The ambient air is O2 and the gas that creates creama is CO2. This is where I disagree with your theory. The ambient 02 would be displaced by any water that entered the puck. When the oil and H2O emulsify, only then does the liquid (now espresso) have enough suface tension to contain bubbles. Water alone does not have the power to hold gas.

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Permalink Reply by Brady on June 2, 2009 at 7:07am

Great discussion so far. I'm on vacation and away from my bar for a couple of days, so can't make direct observations to add, but have a couple of thoughts and questions...

Agreed that proper distribution and consistent dosing are critical, as is grind size.

To what extent do we think the increased headspace improves the extraction? I think some headspace is good, promoting more even water distribution and allowing room for the puck to grow. Perhaps this is another reason that the puck that is tamped a little harder (20-30 lbs) may taste different than one that was lightly tamped.

Also, for consistency of discussion, perhaps we should be clear if your machine uses pre-infusion and when you begin timing your shots. I know some out there start timing when the first drops appear. This is relevant, because I suspect that tamping harder may increase the time for the first bit of water to make its way through the puck, as well as the rate that the puck expands once its first wet. These factors both have the potential to influence the way the extraction occurs. I wonder if we time when the first drops appear if variation in extraction time drops...

I've also heard some claims that preinfusion reduces the impact of variations in tamping. Not played with this so can't verify.

What I suspect is that there is a benefit to tamping to a certain point - removing voids, making a more consistent puck, creating a little headspace, but that at some point the increased density makes it too hard for water to propagate through - causing increased variation in how the espresso at the top of the cake extracts vs that at the bottom. This is all speculation, but would love to see your thoughts on this idea.

Neat discussion that I'll be following.

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Permalink Reply by Luis Zaman on June 2, 2009 at 8:53am

I also have this book lent out at the moment, but figured I'd put my 2 cents in....

9 bar = ~130 PSI (pounds/square inch)
58 mm puck = ~2.2 inches diameter (1.1 inches radius)
PI * r^2 = ~3.8 square inches

130 pounds/square inch * 3.8 square inches = 494 pounds

This is the number I seem to remember, that its somewhere around 500lbs of force, not 150. So, same point just thought I'd share where the numbers were coming from.


Nathan Lyle Black said:

At 9 bars, a machine is pushing against the espresso with a force of 130.68 psi. Pretty close to the 150 you estimated.

Jonathan Jarrow said:

According to The Professional Barista Handbook by Scott Rao, when a puck is hit with 9 bars of atmospheric pressure, it is equivalent to around 150 lbs. of pressure across the surface of a 58mm+ puck(I don't have the book on me at the moment to give the exact #, but I know it's around 150). He also has the math in the book explaining how he got that #. Because of this, it doesn't really matter if you tamp the hell out of that shot, it's nothing compared to what the machine is putting the puck through.

The purpose of the tamp is to remove "pockets" of air from inside the coffee bed to prevent channeling.

This said, I have a theory about tamping pressure which I have yet to prove involving reduction in crema due to over tamping. If crema is the result of oils in the coffee wrapping themselves around the air in the coffee bed, then it would seem that a tamp with excessive pressure would remove a lot of the air from the bed as the particles compress together, creating less available material for the creation of crema. I have seen hands on evidence of this, especially when working with a much finer grind.
Any feedback on this in particular would be much appreciated.

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Permalink Reply by Luis Zaman on June 2, 2009 at 8:55am

Saw the correction just now, good deal, I'm glad I wasn't crazy! Still figure the raw numbers may be useful to someone!

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Permalink Reply by Jonathan Jarrow on June 2, 2009 at 9:20am

Well said Jesse. I realize that the ambient air is O2, I have just wondered about it, since it is present in the puck at the time of extraction. To be quite honest, I sincerely doubt that it affects overall flavor in comparison to many of the other variables in espresso extraction. However, I'm just curious about any small effect that it may or may not be causing.

Jesse -D-> said:

Jonathan:

The ambient air is O2 and the gas that creates creama is CO2. This is where I disagree with your theory. The ambient 02 would be displaced by any water that entered the puck. When the oil and H2O emulsify, only then does the liquid (now espresso) have enough suface tension to contain bubbles. Water alone does not have the power to hold gas.

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