Why are conical burrs inherently bimodal?

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#1: Post by DavidMLewis »

In one of the LIDO 2 discussions, Doug Garrott mentioned that conical burrs gave an inherently bimodal distribution. Is this something that is well-known? And what causes it?


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#2: Post by another_jim »

Roasted coffee is inherently bimodal. If anyone can snap a cracker into two parts without any crumbs, I'll buy the claims for unimodal grinders.

Obviously, if you treat the cracker so the surface isn't brittle, i.e. wet it, you can crack it without crumbs. Sumatra beans are less brittle, so need finer grinds, as do stale beans. But every time you break a bean with a certain degree of brittleness, you get a roughly fixed amount of fines.

Finally, fines make up a very small amount of the mass in ground coffee. If you contaminate a regular cup of coffee with 1% to 2% instant coffee and do a triangle test, how many people will be able to distinguish the result? I certainly can't. Therefore, my belief is that everything that has been said about fines and taste is almost certainly rubbish, based neither on blind testing nor on accurate measurement. We know some grinders are better than others for any given purpose; but we do not have much of an idea why this is so.
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#3: Post by MWJB »

Generally the finer you grind coffee, the more very small particles are created as a side effect of trying to break the beans into ever smaller pieces...at coarser settings grinders typically make a more unimodal grind (one peak, or mode), even the Lidos. Fines are still produced but make up a smaller ratio of the overall grind distribution.

Burr grinders all make grinds in a range of particle sizes.

Bimodal grinds are still typically produced by flat burr grinders at fine settings too.

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#4: Post by rpavlis »

It is interesting to make espresso with essentially no fines at all. The fines, as pointed out earlier in this thread, are a rather small fraction of the ground coffee, and they result from pieces of bean that break off. With the aid of a proper sieve on can eliminate the fines entirely. The small particles tend to decrease flow rate, and do so dramatically. One needs to grind the remaining particles substantially finer to get the proper "back pressure" to make decent espresso. I used a 300 micron sieve, slightly smaller would be better, but this size worked fairly well, but it begins to remove some particles that were ground rather than broken off during the grinding. (I used this sieve only because it was the only one I had that was available.)

I ground 20 grams and sieved the result and shook the sieve until I got 5 grams of fines. I made Turkish coffee with the fines. I have used both the Pharos and Hg one for doing this, results were essentially identical. I discovered both needed similar grind adjustments.

The espresso that was sieved vs. that which was not sieved was noticibly different, and I suspect that was due to smaller particle size for the bulk of the particles brought about by the required finer grind, not by the removal of the small fraction of fines itself. (I made the espresso with a La Pavoni Europiccola with the 1975-2000 group type.) The espresso made with sieved beans, to me, was better. However, it also is not practical to sieve the ground espresso every time one makes espresso! It is far too time consuming.

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#5: Post by another_jim »

Fines are sub-100 micron, you removed the fines and the lower 3rd of the regular ground distribution. See the collective graph of 26 grind samples below.


The better taste you got from sieving is probably due to the higher extraction. The result would probably be the same if you ground finer, dosed lower, and drew less water
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#6: Post by rpavlis »

When I sieved the ground coffee, it was apparent that the 300 micron sieve was somewhat too large, but it was what I had available. Clearly the advantage of less fines is that the fines tend to plug up the spaces between the particles, and to avoid choking espresso machines, one normally ends up with the main particles that are larger than they really should be. Perhaps a 150 micron one might be better. When I sieved the material, I had the grinder set substantially finer. I suspect irregular shaped particles are best too, because they have more surface area than spherical ones.


#7: Post by spencerwebb »

I tried sieving espresso grinds with a home-made 125 micron sieve, I didn't even manage to get 0.01g of fines in a couple of minutes of shaking. I was going to try a larger mesh but am interested to hear about other people's experiences before stumping up the cash for it.

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#8: Post by another_jim »

spencerwebb wrote:I tried sieving espresso grinds with a home-made 125 micron sieve, I didn't even manage to get 0.01g of fines in a couple of minutes of shaking.
There may be about 0.3 grams sub 125 micron fines in a double shot of espresso, at least according to the laser particle counters. To get all that you'll need a shaker, since the fines clump tightly into the nooks and crannies in the coarse ground particles.

But unless you happen to be a bloodhound or the princess on a pea, I don't see much point -- the amount of fines is too small to affect the taste. The taste differences people get by sieving ground coffee are those that Robert observed, due not the missing fines, but by compensating for their absence with coarse particles that are ground finer.

But Robert's observations also show there is a missing piece of knowledge here -- how much of the flow control in an espresso puck comes from the fines (i.e. the sub 100 micron particles), and how much comes from the heterogeneity of the coarse particles (150 - 1000 microns), i.e. from their absolute size differences along with their shape differences
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#9: Post by spencerwebb »

I'm with you Jim, it's one of those things that I wanted to try after watching this years WBC performances. I have an EK43 which produces more fines (and less boulders) than many other grinders including the Robur and the shots from that generally run much faster than on the conical thus adding even more credence to your statements about fines vs boulders.

I find it really interesting I just wish I had access to a particle size analyser to carry out my own experiments :)

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#10: Post by Terranova »

another_jim wrote:There may be about 0.3 grams sub 125 micron fines in a double shot of espresso, at least according to the laser particle counters.
I would have interpreted the amount of particles less than 125 micron in the graphic to ~ 15%, but I might be wrong...
Grind distribution graphs can look hugely different depending on what axis have been used (i.e. log scales, volume %, total count, etc) and because a lot is going on at once the interpretations can be troublesome. As mentioned in the last post, changing the axis from volume % (where fines are minimal contributors because they are very small) to surface area % (where fines have a huge surface are to volume ratio and thus contribute massively) completely changes what we see as significant. The fines bump which looks so small and uninteresting in the volume % presentation, becomes the most important part of the graph. They contribute the most to surface area and extraction. Not by a little bit. By A LOT. They are 70% of the grind, minimum.
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