some aspects of espresso extraction - 4

puck extractıon during the shot

summary of results :

the yield does not depend a lot on shot time or weight, because the puck is extracted almost as far as it will go within the first 20 seconds of the shot. i'm not sure why the total amount extracted is nearly done by twenty seconds, or why it varies by dose. that doesn't stop me from speculating about it at the end of this section

the taste of the early, middle and late part of an espresso shot :

there's an old alt.coffee exercise: brew an espresso shot into three cups; the first 10 seconds into the first cup, the next 10 seconds into the second cup, the rest into the third cup. then taste ( 8 ).

i. the first cup will taste extremely sharp, sour, and strong.

ii. the second cup will taste sweet and creamy, but bland.

iii. the third cup  will taste watery and slightly bitter.

when we discussed this, we assumed that the puck was extracting evenly throughout the shot; so that the first cup represented the 0% to 7% solubles fraction, the next cup the 8% to 14% solubles fraction, and the third cup the 15% to 21% fraction. however, the last section shows that the degree of extraction is relatively immune to shot time and weight. this leads to a, in hindsight, much simpler interpretation of the three cups data. the first cup represents the early extraction, the second cup the late extraction, and the third cup a diluting of the shot. if the puck loses almost all its solubles into the first 20 seconds and 50% weight of the shot; then the near immunity of the yield data to shot timing is explained. all the shots in the extraction data set were within normal espresso parameters. so the only thing that varied is by how much the full extraction, present in the cup after the first 20 seconds, was diluted by the rest of the shot.

is there any way to confirm this supposition ?

intra-shot solubles yields 
the obvious solution is to check the solubles yield of the puck at various stages over the course of a single shot. this can't be done with just one shot, but it can be done with six shots stopped short at staggered intervals which use the same coffee, dose and grind. i did two of these sets, stopping the first shot in each set at the first drop, and the subsequent ones at 6 second intervals after that, out to 30 seconds (actually 35, since the dwell time is around 5 seconds). i used a high dose, because ı was also disecting the puck (see below), and the final yield is, as predicted, on the low side. the graph shows the averaged readings of the two sets (9).

this confirms that the bulk of the solids extraction takes place in the first 2/3rd of the shot, and begins to explain why shot time and weight play such a minor role. what it doesn't explain is:

i. why does the extraction stop after 20 seconds, even in the case of a high dose, low yield shot?

ii. why do low dosed shots extract more than high dosed shots?

getting inside the puck while it is brewing ı haven't gotten very far with these questions; and ı may be looking in the wrong direction. but, ı think the answer lies in what happens inside the puck during the shot. this is pretty much a "black box," but with some theory and some tricks, one can begin to crack into it.

the theory is based on the idea of percolation. this has nothing to do with those accursed coffee percolators from the 50s. rather, it looks at the brewing process as water flowing through a column of ground coffee. since the puck is, sort of, a column of ground coffee, and since the solubles yield depends on how high this "column" is, it seems an appropriate way to think about the problem.

discussions of percolation ( 10 ) can get confusing very fast. the ground stuff is "coffee," the liquid coming out of the bottom is "coffee," the stuff going from the grinds to the liquid is "coffee." one needs to come up with a new vocabulary to prevent total confusion. here goes :

i. grinds : the ground coffee.

ii. liquid : the water going through the grinds and becoming coffee.

iii. solubles : the stuff going from the grinds into the water, turning it into coffee.

the mental model percolation is real simple. the liquid goes into the top, picks up solubles from the grinds at the top, and becomes saturated. once it's saturated, it can't pick up any more solubles, so the solubles stay inside the grinds at the bottom until the top grinds are exhausted, and the liquid reaching the bottom is less saturated. ın other words, the percolation column brews from the top down.

the problem is that this mental model doesn't fit our facts. in this model, it doesn't matter how high the column is, or how coarse the grinds. it brews from the top down. send through enough water, and it will extract as far as you desire. but the intra-shot graph shows that the extraction basically levels off after 20 seconds. the dose by extraction data shows that the amount of extraction in normal shots varies almost entirely by puck height, and very little by time or volume.

time to gather some data and make the model more complicated. the data is a series of shots stopped at the 6 second intervals; with each puck divided into three horizontal slices; and each slice's brew strength measured. the more brew strength, the less coffee has been extracted. this provides a picture of how the top, middle and bottom of the puck extract over the course of a shot.

the puck sections were oven dried, and brewed at exactly 4 grams powder to 80 grams of water. these brews were compared to the fresh coffee, also oven dried, brewed at 4 grams ( 100 % ), 2 grams ( 50 % ), and 1 gram ( 25 % ) per 80 grams water. the comparison was by tds meter. i did two series of measures.

the three pairs of horizontal lines show the tds of the fresh coffee at 100%, 50% and 25% brew strength. they provide a rough measure of the extraction levels of the puck sections, with the 25% line indicating a full extraction. the top of the puck is shown by the blue lines, the middle by the green lines, the bottom by the red lines.

the measure at time zero is at the first drop. it shows the puck state when it gets completely soaked. and it leads to out first real world revision of the simple percolation model : grinds absorb liquid. the liquid absorbed at the top is water, while the liquid at the bottom has all the solubles it picked up at the top. so, when the percolation column becomes completely soaked, but before any liquid comes out, solubles have been transferred from the grinds at the top to the grinds below. below a certain depth, the grinds actually are charged with solubles, rather than losing them (the same trick is used to recharge decaf coffee after the caffeine has been removed)

the next revision comes from looking at the shape of the lines. they are roughly straight, rather than the asymptotic curves one expects from the simple model (the intra-shot extraction graph in the previous section is an asymptotic curve, it reaches a limit). the water is not marching down the puck column in a straight line. the puck has become soaked, and has turned into a slurry. the fresh water enters this slurry and mixes with it. the mixed water is pushed out of the bottom. the proper mental model is of a big pail filled with liquid and grinds with a filtered hole in the bottom where liquid but not grinds flows out. the water in the pail is both brewing and being diluted by the in flowing water. ıt's the mixed up result that emerges from the bottom.

do these two revisions to the percolation model explain the extraction results ?

a bit; but not like a slam dunk. ın the soaking phase, the thinner the puck, the less overloaded with solubles the bottom layer becomes. since the bottom layers can't extract their own solubles until they've got rid of the excess ones from the top, thinner pucks help along the extraction. in the progressive dilution phase, the bottom of a thinner puck will get more fresh water and extract further. these two factors together may explain the results.

there's a lot more to the story that i don't know or can't document. thinner pucks require finer grinds, so the extraction is probably accelerated by that. the fines, and finer particles in general, migrate toward the bottom of the puck, so when the extraction at the bottom gets going, it probably proceeds faster than at the top. top down extraction, soaking and dilution phases, thinner and thicker pucks, finer and coarser grinds: there's a lot happening during an espresso shot. these intra-shot measures are a first peek, but there's a lot more looking required.

writen by jim schulman