Cracking into the Extraction: when and where the espresso puck brews.

Beginner and pro baristas share tips and tricks.
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another_jim
Team HB

#1: Post by another_jim »

Andy Schecter and I have been studying the details of espresso extraction. Andy has worked on overall brewing ratios; I've been trying to find out how the puck extracts over time. We're also working on how much of the puck's weight extracts at different doses and basket geometries -- this is a work in progress. We're hoping when all this is combined, we'll have a model of the espresso extraction process that will aid in designing better baskets, groups, and grinders.

My work has been the rather thankless job of brewing and tasting partially spent pucks and measuring their TDS, which is a rough measure of how much brewable coffee they still hold. After a lot of trial and error, I hit on the following procedure:
1. I ground a fresh coffee sample
2. With the same coffee, I made six shots, stopped at the first drop (time 0), and 6, 12, 18, 24 and 30 seconds beyond that point.
3. I took the spent pucks, and divided it into a top layer, middle layer, and bottom layer.
4. The fresh sample and the puck sections were oven dried for 2 hours at 225F, then brewed at a coffee to water ratio of 1:20. In addition, the fresh sample was brewed at 1:40 (50% strength) and 1:80 (25% strength). I measured the TDS of all the samples, and tasted them, rating their strength by comparing them to the fresh samples.

There was a very close correlation between my taste strength ratings and the TDS measures, so I'm only reporting the TDS data at this point. The graph is illustrated below:

Image

(Click on the image to view it at full size)

The horizontal axis shows the shot time. The vertical axis the TDS readings. The three pairs of dashed horizontals lines shows the range of TDS readings at 25%, 50%, and 100%. The pair of red graphs show the high and low TDS reading for the puck bottom, the green pair for the puck center, and the blue pair for the puck top.

Notice that the bottom of the puck brews stronger than the full strength fresh coffee, even at 15 seconds into the shot.
-- The puck basically brews from the top down. This is to be expected, since the coffee picks up in strength as it goes down the puck and has less ability to absorb solutes at the bottom until the top layers exhaust.
-- What is surprising is that the bottom gets stronger. What is happening is that the ground coffee at the bottom is absorbing the overbrewed coffee from the top, and retaining this until the second half of the shot.
-- Blonding only occurs when this absorbed, super-concentrated coffee is released in the last 5 to 7 seconds of the shot.
-- In essence, the puck bottom is acts like a coffee battery.

The result implies a revision of the conventional wisdom, at least to me. The usual idea is that poor grinders produce lots of fines. These clog the bottom, brew too fast, and are responsible for the exaggerated flavors in the first third of the shot. However, it looks like the fines at the bottom of the puck absorb the overbrewed coffee from the top and release it slowly throughout the shot. In addition, we are just beginning to see laser analyses of grinder particles, The scant data so far does not support the idea that good grinders produce less fines. My guess is that the fines required by espresso extraction are being confused for the dust produced by whirly blades. Based on this, I'm guessing they are quite different in their size.

I find these results somewhat surprising. I've offered my take; but I would appreciate everyone elses' ideas -- this is new, and my interpretation is likely to be off base.
Jim Schulman

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hbuchtel

#2: Post by hbuchtel »

A quick question- does the machine you are using have a 3-way valve? If yes, how do you think it would affect the measured TDS of the puck? (especially the top layer)
another_jim wrote:My work has been the rather thankless job of brewing and tasting partially spent pucks and measuring their TDS
Thanks! :)

Henry

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another_jim
Team HB

#3: Post by another_jim »

hbuchtel wrote:A quick question- does the machine you are using have a 3-way valve? If yes, how do you think it would affect the TDS of the puck? (especially the top layer)
Yes, it does; like all pump machines except budget home models.

You have a point; it could affect the result. The liquid coffee left behind in the puck gets dried out and leaves the solubles behind, like instant coffee. If the top is dryer than the bottom, fewer of these instant coffee solubles get deposited. However, I sit the basket down upside down before knocking out the puck, and let it sit a few minutes. I was thinking this equalized the remaining moisture; at least it seemed that way when I was handling the pucks.
Jim Schulman

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Jepy

#4: Post by Jepy »

I wonder how different your findings would be with a falling pressure at the end, lever-like, since flavor and mouthfeel can be very different from a straight pressure machine.

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jesawdy

#5: Post by jesawdy »

another_jim wrote:The result implies a revision of the conventional wisdom, at least to me. The usual idea is that poor grinders produce lots of fines. These clog the bottom, brew too fast, and are responsible for the exaggerated flavors in the first third of the shot. However, it looks like the fines at the bottom of the puck absorb the overbrewed coffee from the top and release it slowly throughout the shot.
Jim-

Just an idea. It may not be that the fines at the bottom of the puck absorb the overbrewed coffee at all. It may be that the other (non fine or super fine) particles control the migration of fines from the top of the puck to the bottom of the puck. These other sized particles may slow the movement of fines from the top of the puck to the bottom and even entrain them for a short while. If these fines or superfines migrate from the top of the puck to the bottom over the course of the shot, your TDS will be higher in the lower puck sections when you go to do the analysis. Now, can the coffee in the lower half of the puck actually reabsorb dissolved solids, or just impede or entrain them? I dunno.

Too many fines may still be a bad thing, but the ratios of different particle sizes may have more of a bearing on how well the coffee (and grinder) perform.

Out of curiosity, what machine and grinder(s) are you using for the above experiment so far
Jeff Sawdy

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HB
Admin

#6: Post by HB »

another_jim wrote:However, it looks like the fines at the bottom of the puck absorb the overbrewed coffee from the top and release it slowly throughout the shot.
I'm no food scientist, but perhaps the bottom section absorption was higher because the coffee was initially exposed to lower temperature water. The speed of full pressurization / length of preinfusion affects not only how quickly water progresses through the puck during the critical early seconds, but the speed of the temperature rise:

Image

This in turn is affected by the temperature profile (a "humped" profile would close the top-to-bottom temperature gap more quickly). Many believe that a ironing board flat temperature profile is superior, but measured mid-puck, there's little or no difference between the highest hump HX and the flattest flat double boiler temperature profile. The trend in thinking is that super tight stability of temperature and pressure is a good thing, despite that the difference is immeasurable beyond the noise of the first half of the puck; your data suggests the extraction experience of the top affects the extraction experience of the bottom more dramatically than I expected. That's good news, because it's a lot easier to precisely manipulate the temperature and pressure of the top than the bottom!
Dan Kehn

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jesawdy

#7: Post by jesawdy »

HB wrote:I'm no food scientist, but perhaps the bottom section absorption was higher because the coffee was initially exposed to lower temperature water.
Dan, that's a very good point, the bottom of the puck sees a lower temperature AND the water that passes through the bottom of the puck is less likely to extract more solubles as well. Once the water reaches the bottom of the puck, it is less likely to extract anything because it is already somewhat saturated and the concentration gradient is less. The steeper the gradient, the greater the extraction. The concentration gradient is always steeper at the top of the puck. This results in less extraction of solids, oils, whatever.

With that being said, and if I understand what Jim has shown, neither temperature gradient nor concentration gradient explain the "supersaturation" that Jim is seeing at time zero at the bottom of the puck.

One things is for sure.... there is A LOT going on..... thermodynamics, fluid dynamics, extraction chemistry, packed fluid bed chemical processing, etc, etc. Explaining it all away is the work of a lifetime.
Jeff Sawdy

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another_jim
Team HB

#8: Post by another_jim »

Jes: I was using Ethiopian DP Lekempti coffee roasted just past the first pops of the second, the Versalab grinder and Elektra Semiautomatica for the experiments. The '0' time on the graph is when the first drop came out of the naked PF.

Dan: The water dispersion on the Semi is excellent, so the '0' puck was fully saturated, despite the short dwell time. This means the dropping temperature could be a factor in the supersaturation. I'm no chemist, but presumably, if the coffee is fully saturated at the top, and cools off, it will initially precipitate out solubles at the bottom. As the solution gets less saturated, and warmer at the bottom, the precipitation will cease, and eventually it'll start absorbing again.

Also, with the fines at the bottom, the pressure in most of the puck will be near 9 bar, and it'll drop fast as it passes through the fines at the bottom. I have no idea how this affects the way the solubles absorb. Is the high pressure only a device for forcing water through more finely ground coffee, or does it also effect the extraction in other ways?

Basically, we are quite close to creating a detailed simulation of the extraction. Dan has the temperature map over the course of the shot at various puck depths. I have (if the data holds) a map of the extraction patterns. Finally, once we organize the laser counter, we can get the particle distribution of the puck over the course of the shot the same way I did the extraction map. This particle map will allow allow the pressure gradients to be computed; and add detail to the extraction dynamics.

In other words, the three maps together will give a fairly complete physical model of the espresso extraction process. In theory, such a model would allow the grinder's particle distribution, the machine's temperature and pressure profile, and the puck geometry to be used as boundary conditions, so one could predict how they affect the extraction at the nitty-gritty level.
Jim Schulman

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HB
Admin

#9: Post by HB »

FYI, this topic is cross-posted on alt.coffee. The responses so far have varied from "Who cares?" (Ken Fox) and "What's TDS?" (JuleG) to "Anybody got an SEM handy?" (David Lewis).
Dan Kehn

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another_jim
Team HB

#10: Post by another_jim »

Yeah, not much action.

I've had a bit of serendipity today, and want to suggest an easy experiment for people inclined to try it. Do several shots, trading off grind and dose, that is, smaller doses at finer grinds versus larger doses at coarser grinds, so you get roughly the same shot size in the same time. Report your findings. I'll start a poll thread to gather the results.
Jim Schulman