Picking up the Pace

[Andy Schecter]

I've seen a 53mm one, and am still trying to get him to send me the video

We're dying to see it. (Really)

What struck me as odd in the perspex portafilter extraction is that the crema only became visible as the liquid seeped through the base of the basket. I am going to go right ahead and assume that crema forms as CO2 that was in the water due to the brew pressure supersaturating it is released once the atmospheric pressure is too low to keep it in there. (go go excellent grammar!)

If, and I assume we do, we have a declining pressure as we go down through the puck shouldn't there have been some signs of crema nearer the bottom?

Illy observes, through various indirect means, that the fines migrate to the bottom and provide the major portion of hydraulic resistance. He says, "Simulations of espresso percolation obtained by powerful computers demonstrate that particles with a given possibility of movement tend to migrate and to accumulate in a specific critical section simulating a metallic filter."

In other words, most of the pressure drop occurs in a narrow band near the bottom of the basket. That's why you don't see crema up higher.

[James Hoffmann]

I wonder how much is fines and how much is the base of the basket. Thinking out loud I wonder what would happen if you replaced the bottom of the basket with a very, very fine sieve. I just bought a 90 micron sieve today (It is useful for lots of stuff - honest!) so maybe that is what has gotten me thinking.

[barry]

I wonder how much is fines and how much is the base of the basket.

almost all fines.

Thinking out loud I wonder what would happen if you replaced the bottom of the basket with a very, very fine sieve.

gusher.



back when we (alt.coffee folks) were first looking for ways of restricting the flow through a portafilter for temperature measurement purposes, one of my first efforts involved using a hole drilled through a blind portafilter insert. there was some discussion about flow rates and calculations involved and the result was a very very small hole that was prone to plugging up by a single espresso ground. i, too, had considered all manner of filters & substances to simulate the restriction offered by the coffee cake, but to no avail.

this topic reared its ugly head again when greg put out version 1.0 of the thermofilter. the flow restricting orifice is bloody small, .013" diameter iirc, and was prone to plugging, so a filter was installed upstream from the orifice. a .013" diameter hole has an area of .000133 sq inches. i'm not positive as i haven't measured it, but i think the .013" orifice is smaller than a single hole in the bottom of a portafilter basket.

[Andy Schecter]

I wonder how much is fines and how much is the base of the basket.

I think it's mostly the fines themselves. The average size of an espresso fine is 0.2 microns, so they can really pack tightly.

Perhaps some of the resistance is also caused by the perforations in the basket becoming partially clogged with medium-sized pieces.

[James Hoffmann]

Right then - tomorrow, for my own childish interest I shall set a grind and dose for a coffee, and then take a dose and sieve out everything under 90microns and then brew the show and see what happens.

I am sure it will gush, but I am sort of interested to see how dramatic it will be...

[Scott Rao]

i shouldn't have quoted so much of what that guy said, since i realize it's common sense that the forces are not equal in all directions or else the water wouldn't travel down through the puck. i should have been more careful. sorry.

i just got my illy book today i'd like to quote a few lines about puck expansion:

"an important consideration to be taken into account when considering the shape of the cake is the expansion of the bed due to the swelling of coffee particles when wetted"...

(ok, so he thinks there is expansion)

"during expansion wet coffee grounds exert a pressure comparable to that of the wooden wedge used in the past to cleave marble blocks, whose force of expansion, once it is driven into the block and wetted, prevails over the cohesive forces of the stone and splits the block into slabs. On account of this behavior, an empty space is left over the ground coffee cake inside the extraction chamber. The actual expansion of the cake varies with blend, roasting degree, and dose, and determines the exact headspace (around 5-6mm) needed to prevent over-compacting"

so i apologize for getting in over my head with a layman's understanding of physics, but i would like to still posit that heather's espresso (remember the start of all this?) was suffering from channeling brought on by the "over-compacting" of a swolen updosed puck pressing the screen.

i've been enjoying this exchange. thanks to all.

scott

[barry]

so i apologize for getting in over my head with a layman's understanding of physics, but i would like to still posit that heather's espresso (remember the start of all this?) was suffering from channeling brought on by the "over-compacting" of a swolen updosed puck pressing the screen.

i was reading the illy book today, too, and recalled that passage from the previous edition.


if heather's problem was due to channeling brought on by the "over-compacting" of a swollen updosed puck pressing the screen, then how do we account for all the successful shots pulled from hugely updosed pucks, where the barista is challenged just to mount the pf in the group (a la paul bassett)?

[Jim Schulman]

The accelaration in flow is due to the fines dissolving, since it happens to a greater or lessr extent in all shots, channeled or not. It also seems unlikely that Heather's shots channel.

However, the rate at which the flow accelarates varies by basket (LMs less than Faemas), by dose (usually more with lower doses) and probably a bunch of other things.

I, for one, have no clue why this is. Fortunately, there seems to somewhat less variation in the taste of shots than in the way they flow.

[tonx]

I do have a suggestion (getting back to Heather's original question). It might help to try a harder tamp with a coarser grind and see if the same phenomenon still occurs. That could mitigate to some degree problems with either worn out grinder burrs (to a small extent) or the particular blend/roast tending to let go its "easy" solubles at the early part of the extraction. I think a lot more of those fast-into-the-cup solubles get front-loaded as preinfusion time increases, leading to shots that start out dramatically darker than they finish. I feel this becomes more of an issue with certain roasting styles. In which case it might be more accurate to say that your shots are starting too dark rather than they are finishing too light.

(But I could have it all backwards - I haven't pulled any good shots in years.)

[Brent]

i would love to see the vid if you acquire it.

You and everyone else

[James Hoffmann]

I tried my very unscientific experiment where I took a dose that would give a nice 25 second shot and then before brewing I sieved it to remove as much below 90 microns as possible, and unsurprisingly the shot time hurried in at 16 seconds though what caught me off guard was the amount of mottling on my pale crema. Very odd. Where did those fines come from?

[Scott Rao]

my understanding is that pf holes are about .25mm; 90 microns is .09mm. my guess is that you filtered out enough fines to help prevent clogging the holes but there were some (bigger) fines left which then had an easier time getting through the holes into the cup, forming the mottling.
scott

[Andy Schecter]

I tried my very unscientific experiment where I took a dose that would give a nice 25 second shot and then before brewing I sieved it to remove as much below 90 microns as possible, and unsurprisingly the shot time hurried in at 16 seconds

How much of the original dose was sieved out? IOW, was the dose that you used for the shot significantly less (in mass) than the standard dose that givers the nice 25 sec shot? Or did you replace the sieved-out fines with more coarse stuff?

[Scott Rao]

another thought: would you mind pulling a couple of more sieved shots? adjust the grind so the first is 25 seconds, or whatever time you look for, and if that one is underextracted, adjust the next one slower, and if it was overextracted ,adjust the next one faster? my thinking is:
1. you get to taste a 25 second shot which is fines-free, see if it has more clarity, still has the mottling, etc...
2.it's possible 25 seconds/no fines will lead to an unusual extraction since the particle distribution is not what you're used to; i'm curious which way it will go.

don't mean to burden you; i currently have no espresso machine, not to mention no sieves. will certainly try it myself one day when i can.
thanks for the post, great idea.
scott

[Heather Perry]

I am starting to get confused. But from what I can gather are we pretty much saying that the finer grinds create a floor in the portafilter which controls the pace, and older burrs would not give me thowse fine grinds? If that is the case, then why would it happen with the smaller gigglers? And why would it occur on competition equipment like Deferio experienced?

[Mark Prince]

My guess is that the timing of coffee cake swelling is very easy to learn. Lock and load a shot's worth of coffee. Activate brewing. Stop the process after a few seconds and look at the cake. If it's swollen, then you know. If it ain't, knock out the cake, reload and try again, waiting for longer before you examine the cake.

i don't think this would be an effective test due to the pressure relief mechanism of the solenoid.

Jumping in late, so apologies for that....

but here's a situation where one could use a non-solenoid equipped machine (read: consumer machines that "sneeze") to see if any swelling occurs. I recently revisited this using a Gaggia and a Pavoni pump machine to see if any swelling took place (as per James' insistence to me last year that it didn't), and from what I could tell, not very much does take place, but some does. Very unscientific though.

Mark

[Andy Schecter]

Our old machine was a stock linea. Why would smaller gigglers result in an extraction that picks up, and more than that, why would it go away like your case Chris? Or is it still there you just don't notice it as much?

Heather, to try and find out what's happening for you, I ran four shots using a moderately updosed 18g in a Faema double basket. Two used a simulated "small gicleur" (~0.6mm), and two used a simulated "big gicleur" (~1.0mm). I say "simulated," because instead of using an actual gicleur I use a needle valve that can be easily readjusted.

For each shot, I separately captured and weighed the espresso from seconds 0-5, 6-10, 11-15, 16-20, 21-25, and 26-30. These were not ristrettos, they were more or less "traditional double" style (35-42 grams of espresso from 18 grams dry coffee, brewing ratios were 42-50%).

I think the chart may show what Kyle and Chris B are talking about. The small gicleur shots start out very slow and rapidly pick up. The big gicleur shots start faster, so they don't seem to speed up as much later. If your original machine had no gicleur at all, perhaps the effect would be more dramatic.

As usual, the big gicleur shots required a little coarser grind to get a reasonable flow rate.

I'm not sure if this is what you're noticing, but perhaps it helps. Even if it doesn't help, oh well, at least it's a pretty graph.

[Andy Schecter]

but here's a situation where one could use a non-solenoid equipped machine (read: consumer machines that "sneeze") to see if any swelling occurs. I recently revisited this using a Gaggia and a Pavoni pump machine to see if any swelling took place (as per James' insistence to me last year that it didn't), and from what I could tell, not very much does take place, but some does. Very unscientific though.

Hi Mark, it just so happens that I've been fooling around with this lately. I've been experimenting with blocking off the 3-way valve drain port in order to test the hair brain theories I floated back in the 3-way valve abomination thread.

I've tried using this technique as you suggest to ascertain whether the puck expansion is due to normal happening during the shot, or else due to the explosive expansion that occurs when the 3-way releases pressure. I'm using a 14.5 gram dose in a Faema double basket, which is a mild down dose (to make the effect clearer to observe).

So far it's pretty hard to make accurate measurements. If you unlock the pf right after the shot, you get an explosive sneeze that defeats the purpose of the test. If you wait 1/2 hour or so, the puck seems to stick to the screen, destroying the ability to make an accurate measurement. But the fact that it sticks to the screen is evidence that it's the circumstances of the extraction, not the 3-way, that makes it swell.

I could easily be mistaken on this, though; it's hard to know what's really going on using this simple setup. We really could use Illy's transparent group to find out.

As an alternative, I am trying to find some X-Krytonite. According to Wikipedia, X-Kryptonite "has no effect on Kryptonians, but bestows temporary superpowers on Earth lifeforms." If I can source this (I'm told small fragments show up on eBay regularly), I will be able to use x-ray vision to examine the contents of the pf in real time.

Judging from past experience, I think I'll snag the kryptonite long before the mysterious Illy video ever surfaces.

[nick]

I called Heather right after her original post. Here's what I told her:

The main issue is the grinder. In conjunction with a smaller gicleur'ed machine, you'll see different things than you might be used to.

In a dry (pre-extracted) puck, I can identify three distinct elements that restrict the flow:
- the insoluble solids
- the solubles
- and a subset of both: the fines.

Remember that the coffee is the final flow-restrictor in the system. In an unscientific sort of way, I like to say that an in-line flow-restrictor (gicleur) helps espresso quality by relieving some of the flow-restriction burden from the coffee itself, letting the coffee focus more on extraction instead of flow-restriction. A bit of anthropomorphism here for sure, but if the coffee can speak, the coffee can focus.

Based on burr design and RPM's, I'd hypothesize that flat-burr grinders (super jolly) create more fines than conical-burr (robur) grinders. Also, newer/sharper burrs would create more fines than older/duller burrs as well (Heather told me that she was using a Robur, and that the burrs were almost a year old).

Generally speaking, as the extraction progresses, a number of things happen, with a couple of them relevant to our discussion: solubles are extracted from the puck, and fines are pushed downward (often into what I like to call a "floor").

In an extraction environment with a sufficient quantity of fines, the fines will be pushed downward (through the matrix of larger particles), and with the bottleneck created by the basket holes, grid-lock into that "floor." As the extraction progresses, the flow will actually slow down as that grid-lock gets more packed and more backed-up.

If, however, there is a relatively low-quantity of fines, that floor won't be estabished sufficiently, in which case the flow rate through the coffee will increase as the solubles are carried away by extraction into the water.

We'd always been a flat-burr (super jolly) shop, only experiencing Roburs at competition. It was always weird adjusting to the different flow that the Roburs created (that, and the unrestricted flow on the machines prior to Charlotte USBC). Now we have a Robur in-house (as well as a Kony), and it's interesting to compare the conical vs. flat side-by-side.

BTW, hand-in-hand with this line of thinking is my belief that the proliferation of conical-burr grinders encouraged people to up-dose more and more. I (again) hypothesize that flat-burr grinders create more fines. With fewer fines (proportionally) from a conical-burr grinder, you make up for it by grinding more coffee. Just a theory.

[nick]

Remember that the coffee is the final flow-restrictor in the system. In an unscientific sort of way, I like to say that an in-line flow-restrictor (gicleur) helps espresso quality by relieving some of the flow-restriction burden from the coffee itself, letting the coffee focus more on extraction instead of flow-restriction. A bit of anthropomorphism here for sure, but if the coffee can speak, the coffee can focus.

Mr. Schecter just called me and chastised me for my non-sequitur rambling about conical vs. flat (Heather didn't switch grinders, so AndyS told me that stuff wasn't relevant), as well as my total oversimplification of the effect of gicleurs that I quoted above.

Guilty as charged.

[Marshall]

I don't know if this will add anything useful to this discussion, but it involves Michael Teahan, so shouldn't be a total waste of your time.

I have an Isomac generic (but non-HX) E-61 home machine. When I switched to a bottomless portafilter, I saw that it always channeled at the front of the puck. Made no difference how I ground or packed, it poured first from the front and went blond there within 10 seconds, while the rear half stayed dark.

After confirming every part of the head was squeaky clean, I took it over to Michael's shop for an educated analysis. We both noted that water entered the head from the rear, shooting toward the front. Michael suggested restricting the flow by dropping gicleurs of different sizes into the head.

We found, first of all, that it came with a 1.0 mm gicleur, which Michael thought was large for an E-61. After a couple of experiments, we settled on the 0.5. The improvement was dramatic.

Since then I have gotten extremely even flows, which often take as long as 30 seconds before going blonde. Very little comes out in the first 10 seconds. I presume that the gentler flow allows the puck to saturate more evenly. I don't know if there is also a migrating fines effect.

I don't know how much of this applies to a large commercial machine. But, there you are.

[Jim Schulman]

I think the chart may show what Kyle and Chris B are talking about. The small gicleur shots start out very slow and rapidly pick up. The big gicleur shots start faster, so they don't seem to speed up as much later. If your original machine had no gicleur at all, perhaps the effect would be more dramatic.

They seem to show what I thought was the obvious explanation, the small gicelur shots start slower and end up at the same flow rate.

[Andy Schecter]

They seem to show what I thought was the obvious explanation, the small gicelur shots start slower and end up at the same flow rate.

Yup. The small gicleur shots:
1. start slower
2. ramp up really fast
3. briefly flow faster than an equal-volume big gicleur shot
4. end up at about the same rate.

[Andy Schecter]

Remember that the coffee is the final flow-restrictor in the system. In an unscientific sort of way, I like to say that an in-line flow-restrictor (gicleur) helps espresso quality by relieving some of the flow-restriction burden from the coffee itself, letting the coffee focus more on extraction instead of flow-restriction. A bit of anthropomorphism here for sure, but if the coffee can speak, the coffee can focus.

Flow restriction from the coffee isn't a "burden." Quite to the contrary, it's a necessary condition if you want to make espresso.

In an espresso extraction, energy from the pump produces 9 bar of water pressure. The 9 bar is put to good use forcing water through the coffee's flow restriction in the filter basket. That force is what strips insolubles from the coffee bed and emulsifies oils. It's what makes espresso espresso!

If you relieve the coffee of its "flow restriction burden," you can make good coffee, but it'll be of the drip or french press kind, not espresso.

As far as gicleurs are concerned, they serve a bunch of functions. One function is, through flow restriction, to modify the pressure profile delivered by the pump. This can result in a better extraction, or a worse one, or maybe just a different one.

But to make espresso, you still need that big-time flow restriction in the puck!