Burrs are a factor in a grinder's coffee retention

Grinders are one of the keys to exceptional espresso. Discuss them here.
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another_jim
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#1: Post by another_jim »

We usually think of grinder retention, i.e. the amount of coffee that remains in the burrs, exit chute and grind chamber after grinding, and which comes out in subsequent grinds, as determined by the design of the grinder itself, rather than what kind of burrs are used. This is not entirely true.

I have a Vario which I use for brew grinding with the Ditting brew burrs. These are finely designed burrs that produce a very high standard of grind in the pour over and immersion range. However, they have a lot of grind retention, on the order of 1 to 2 grams. The ceramic burrs, which are "all-purpose" but shine for espresso, have basically no grind retention at all, even at espresso fineness.

It would be interesting to see if this is also true when using different OEM burrs in a Jolly, since they come in a wide variety of burr geometries. At the moment I do not know if this is peculiar to the Vario or a more general thing.

So what gives? Here's one theory:

The inner/upper section of a coffee burr has a widely spaced, "bean grabbing and bean crushing" burr geometry. These vary widely in efficiency. For instance, on the Vario, the Ditting burrs are a lot slower and more popcorning than the ceramic burrs, while on the Jolly, the Duranium OEM burr is a speed demon. I think the better a burr grabs, the less the grind retention. In effect, the grabbing section is key in creating the pressure that forces the bean fragments through the system. The more pressure; the less grind retention.

Note: this does not mean a faster grinding burr set produces better quality. For instance, the Ditting burr does better for brewed coffee than the ceramic burr; the Duranium burr received mixed reports in terms of the Jolly's grind quality. However, a better bean grabbing section may make for easier and cleaner operation.
Jim Schulman

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TomC
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#2: Post by TomC »

I don't think a faster burr contributes to "better quality" but I do think it contributes to less retention.
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DanoM
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#3: Post by DanoM »

Jim, Interesting post.

You're going to kick off a whole new class of grinder burr discussions. The next thing will be "Clean Path Burrs" or something to that effect! I can hear it now, "I want a titan-flat-conical-clean-path-ceramic-metal-burr grinder and I'll settle for nothing less!" :lol: :D :lol:

When you see the grind retention on the Vario is it more in the center of those burrs or the outer edges? The center, since it travels at a slower speed than the outer edges, might retain more? (My Vario has gone to another home, so I can't look at my ceramic burrs there anymore.)
I also wonder if the ceramic burrs are slicker, with less grab on the coffee and oils than the metal burrs?
So if you had a little stickiness causing extra drag that could keep beans from moving into the grinding path as easily due to a bit of retention buildup. Maybe?

Of course your theory on the initial grab, and crush into the grinding path is likely a huge factor in the retention. I also wonder if the gap vertical opening is as large on the steel as the ceramic burrs? Could this effect popcorning?

We don't have any RDT going on here do we?
LMWDP #445

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another_jim (original poster)
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#4: Post by another_jim (original poster) »

Good tips, Dan. I have both burrs, so I can look for that over the next few days.
Jim Schulman

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Jofari
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#5: Post by Jofari »

DanoM wrote:I also wonder if the ceramic burrs are slicker, with less grab on the coffee and oils than the metal burrs? So if you had a little stickiness causing extra drag that could keep beans from moving into the grinding path as easily due to a bit of retention buildup. Maybe?
I too wonder if the burr material is what is causing the difference in grind retention, rather than geometry. Or at least in addition to geometry.

Mrboots2u
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#6: Post by Mrboots2u replying to Jofari »

If you look at something like a NS mythos,this has very little retention ( around 0.2g for a large flat burr ).
The burrs are not set horizontal though which i suspect along with the grind path is what allows this.

bmb
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#7: Post by bmb »

That' also my feeling and that sleeker, and oil free surfaces could lessen retention.

After using my HG-One for almost a year, it appears to me that grind retention in the lower funnel and under the burrs, is caused mainly by oils and the slightly "roughness" of the funnel and burrs.

Rougher surfaces (in contact with coffee grinds and dust) may retain more grinds, and coffee oils may also add to the problem acting as a "glue", and that this kind of retention could be mistaken with static.

I tried to explain my point in this comment, in a HG-One post:

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bobdeomega
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#8: Post by bobdeomega »

Believe that 200%. First thing I tell whole bean buyers is to SPEND THE MONEY ON THE GRINDER. It's worth it! Breaking down the idiosyncrasies of coffee retention would be like speaking Latin to them! Good post.

Beenbag
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#9: Post by Beenbag »

Mrboots2u wrote: If you look at something like a NS mythos,this has very little retention ( around 0.2g for a large flat burr ).
The burrs are not set horizontal though which i suspect along with the grind path is what allows this.
Ahh ! :o :P That gives me inspiration to carry out a mod i was considering on a Compak K3 by making a "cradle" to support it at 45 degrees, such that the exit port from the grind chamber is pointing down rather than horizontal as it is normally.
since i single dose beans, a simple curved (45 deg) feed pipe will provide an easy "loading" chamber.

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cannonfodder
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#10: Post by cannonfodder »

Conicals have a huge bean crushing zone which will hold a lot of beans. My grinder of choice has to have the largest bean purgatory zone of any. The hybrid DRM burr set which uses a conical crushing burr set that feeds into a medium/fine cutting flat burr has a huge no mans land in the burr transition space. You have to purge the grinder for about 1 second to clean it out.
Dave Stephens

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