The Tamp or No Tamp Discussion

An interesting topic has come up with growing frequency on espresso forums that I find very interesting.   Is tamping the espresso grounds into the portafiter really necessary?  Some people have proposed that tamping is not needed.  The reason being that 9 bars of pressure is certainly more than anyone ever tamps by hand.  A simple 30 pound tamp is no match for the powerful 9 bars (126 pounds per square inch, give or take) delivered by a good espresso machine.  Seems simple enough.   You can just let the pump do the tamping for you, right?  However, at the same time, I find this discussion a little annoying.  The annoyance comes from the basic flaw in logic of the premise of the debate, if it can even be considered a debate.   I don’t think it is a real debate based on the fact that I don’t think anyone out there is really pulling shots without tamping.  The flaw that I find disturbing about this line of reasoning is the good old mixture of apples and oranges.  The Tamp/No Tamp issue, to me, sounds a lot like saying,

“You should eat an apple every day, because oranges are so full of vitamin C, and you body needs lots of vitamin C”

I do not believe that the pump pressure delivered by the espresso machine is any substitute at all for the force applied by tamping the ground coffee.  The reason is because the tamping pressure of your arm is a directional force, and the pump pressure creates an artificial atmosphere pressure inside the portafiter.   Let’s look at these two kinds of pressure.

-Atmospheric pressure

Atmospheric pressure is the force that our air exerts because it is carrying the weight of all the other air on top of it.  As the upper layers of air squeeze in on our layer of air, everything in our layer becomes squeezed also.   So if you fill a balloon to the brink of explosion while standing at the top of a mountain, what will happen if you bring it down to sea level?  The balloon will shrink as the thicker layer of upper air squeezes in on it.   Likewise, a balloon filled at sea level will pop if you try to take it up the mountain.

-Directional pressure

When you tamp you espresso, you press down with what is called foot-pounds. You apply a force in a particular direction.  This action moves the particles of coffee closer to one another, displacing the air that is trapped between the particles of you fluffy coffee grinds.   You can push a car up a slight incline by applying enough foot-pounds in the direction of the incline. But if you increase the atmospheric pressure around the car, it will not move up the hill.


The purpose of tamping is to get the air space out the coffee grounds, and hopefully to create an even thickness of coffee with an even density throughout.   If you do not tamp, the pump pressure will raise the atmospheric pressure inside the portafiter to something near 9 bars, but that will not remove space between the fluffy grinds, it will simply push water into that space.   If you have uneven density when the grounds are dry, you will have uneven density when they are wet.  This causes uneven extraction, which in turn, causes bitterness and bad after taste in espresso.  Perhaps you have one of those grinders that creates a near perfect distribution of even density coffee in the portafiter.  How nice for you, (no seriously, that is nice for you), but that is a whole nuther topic.

Another factor that I have heard brought into this subject, inevitably, is the expansion of the coffee in the portafiter.  The case is sometimes made that the expansion of the coffee in hot water provides the force needed to “seal off” the puck in the portafiter.  If the puck is swelling, the theory goes, and the compartment inside the portafiter is now relatively smaller than it was when the coffee was dry, doesn’t that give you the squeeze on the puck to remove space between the fluffy grounds?  If we refer to Illy’s glass portafiter tests  we find that the puck actually shrinks under pump pressure.  This is for the same reason that the balloon shrinks when you take it down the mountain. Let’s look at two kinds of coffee expansion.

-Gaseous expansion

When you make a pour over with fresh coffee you may have noticed that the grounds seems to almost double in size when hot water is added.   This is mostly caused by the sudden release of lots of CO2 bubbles.  In the portafiter environment, however, this gas is not allowed to bubble outwardly, and becomes dissolved in the liquid coffee to be release later as the foamy cream, so no puck expansion there.

-Expansion by absorption

There is a physical growth of each individual coffee particle due to fact that coffee is porous.   The cell structure of the bean is such that the coffee can absorb water into each cell, thereby expanding a tiny bit as the water takes up a little space, and the cell walls soften and become flexible.  If you have ever done any fine woodworking, you know you have to compensate in many ways for the fact that wood will always swell in humidity and shrink when it gets dry.  Don’t you have a drawer at home that is always sticking in the summer, but opens more easily in the winter?  Well, again, according to the Illy experiment with transparent portafiters, the coffee is not allowed to swell until the pump is turned off.  So, no expansion there either.

So the conclusion that I have drawn is: the physical compaction that we induce by tamping to increase our chances of creating even extraction is something that we can only get from physically tamping with a tamper, and a push from either a mechanical device or one’s arm.  The pump pressure provides an entirely different kind of pressure that serves it’s own purpose, but it is no substitute for tamping.

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    • nick
    • December 16th, 2009

    Sorry, but your logic is wrong, Phil.

    First of all, it is not an “atmospheric pressure” that is created in the puck. It is absolutely directional (btw, I think you mean “static pressure,” not “atmospheric.” Atmospheric pressure involves, well, the atmosphere). It is directional because the bottom of the basket is porous, and the resulting pressure-drop out the bottom of the basket creates a pressure gradient, and a direction to the water pressure. In the top, out the bottom.

    It would be static pressure if there were no holes in the bottom of the basket. It would be an essentially direction-less pressure.

    There are four main “real” reasons for tamping:
    1) keeping things (relatively) clean
    2) protecting the grinds-distribution from being jarred on the portafilter’s way to the group
    3) reducing channeling by pre-compressing the puck
    4) protecting the puck from being blown apart

    The first three are still viable. The fourth is pretty much obsolete with flow-restricted brew water.

  1. Phil-

    Far be it for me to declare that your logic is “wrong” – especially without testing your science and relying completely on anecdotal evidence. I’m constantly amused by the amount of pseudo-science proposed by lay people in our world of coffee.

    Tamp or no tamp? I’ve heard many advocates for both sides. I remain on the “tamp” side of the battlefield strictly due to my own un-scientific tests behind the bar, i.e. making coffee on a daily basis for customers.

    Your comments do cause me to wonder about the expansion thing and your assertion that the pucks do not expand under pressure. Since I’ve been doing non-stop training for a couple of months now, I’ve seen more examples in the past few weeks of puck aberrations by barista trainees than in my entire career (that’s how much training we’ve been doing). Sometimes, the puck, when examined in the portafilter after the shot, has “ballooned” with the “top” of the puck elevated like a dome above the rest of the coffee. Is this from wet expansion, brew pressure or the release from brew pressure back to atmospheric pressure I cannot determine, but I think it could be a fascinating study with the right equipment.

    Your article underscores a problem within our community – as though someone must be “right” and others “wrong.” Why a debate/argument over tamping or not tamping exists is just ludicrous to me. We (as in me and my company) have a standard and that’s what we pursue and advocate. However, those with different standards are not “wrong” merely because their different.

    Perhaps when our community can release itself from this kind of petty argument we can start to be recognized as professionals instead of smelly, unshowered hipsters with chips on our shoulder at the ready to punch anyone in the dick who may disagree.

  2. When I use the term “atmospheric” I am trying to convey, in a metaphorical sense, the kind of pressure that pushes from all sides inward, not from top to bottom, or left to right. Technically speaking, I am referring to “total pressure”, or we could just say the “pounds per square inch” type of pressure.

    As far as the portafilter pressure being directional because of the flow of water, my claim is that a flow rate of 3 ounces of water per minute will not exert enough directional pressure to knock down a Jenga tower, let alone “pre-compress” a puck enough to ward off channeling. If you look to the inflatable Santa on the front lawn, you see that the fan constantly pumps new air into the body. The fabric is porous and lets out air all the time. But the addition of new air is greater than the amount of air leaking out, so there is a surplus in the Santa, creating positive total pressure inside. A balloon inside the Santa will be slightly smaller that it would be on the out side of the Santa. If you suddenly cut the Santa open, the balloon will suddenly expand a little bit. That is why, I believe, that Illy found that the puck does not expand under the force of the espresso pump. When the pump disengages, extra pressure is released via the 3 way valve (in machines so equipped) and then the puck expands.

    I have seen the puffy pucks, buckled out in the middle with a hollow in the core. Does this happen only to under extracted pucks that still have a litte CO2 to give off? I am not sure, I have not keep track of this data.

    I was careful to say in the title that this is a discussion, and not a debate or argument. I don’t actually know of anyone not tamping, even among those who say pump pressure will essentially tamp for you. Probably because everyone acknowledges the list of benefits listed in the first comment. All the opposing view points in the industry, however, are part of the growing pains that the “coffee body of knowledge” must go through as it grows. We must examine bits of data and determine what to throw out and what to keep.

    As my boss likes to say,
    “it’s just coffee, there is no need to yell or cry about it. We are not saving anyone’s life here”

    • nick
    • December 16th, 2009

    I get you, Phil, but the pressure is absolutely directional. You’d be right if there were either no holes in the bottom of the basket, or if the holes were so small that they caused the primary resistance to the flow out the bottom of the basket. However, it is the coffee that causes the main resistance.

    However the coffee puck has some depth to it, and there’s a pressure gradient. If you assume that the pressure out the bottom of the basket is 1 atm., and the pressure above the coffee is about 9 atm, then there’s a progressive (but not evenly gradated) pressure drop through the puck as you go down. The drop will be more drastic towards the bottom of the puck. The “directionality” of the pressure follows the vector of the pressure drop.

    Note I wrote “reducing channeling by pre-compressing the puck,” not eliminate channeling. If the water moves through an un-tamped puck, it can more readily exploit an area of lower lateral density than if tamped. If all else were equal and you DID tamp that before extraction, there would still be channeling, but theoretically less.

    As you wrote, I don’t know anyone who isn’t tamping either. However, the value of discussing tamping, at least to me, is most relevant in discussions about the relationship (or lack thereof) between proper tamping and proper dosing/distribution, and what tamping is really achieving (or not achieving) at all.

    Anyway, it’s nice to see a blog post from you, Phil! Keep em comin!

  3. My beef is with the notion that you can let the water pressure do the tamping for you. Judging by the pucks you get if you don’t tamp, it seems like the water fills in the spaces between the coffee grinds rather than compressing them against the bottom like a Barista’s arm does. That doesn’t mean you can’t get a good shot this way, it is just less likely.

    The reduction of channeling you mention; I believe that most all of what we do form drying the basket, to distributing, to tamping and what not (soft and pre infusion included) are all to decrease our chances of uneven extraction. But it happens. It is much like golf. You play your highest percentage shot and hope for the best. The more practice and the more loose ends you tie off, the higher your percentages.

    I appreciate y’alls thoughts, thanks.

    • Mahala
    • December 23rd, 2009

    Phil,
    Did you get my email? I don’t have much to say about atmospheric vs. water pressure (although I do agree that ultimately it comes down to making a good shot for the customer, science ought facilitate and not hinder that…) but I’d love to catch up with you.

    mahala@mail.utexas.edu

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