Recently, I was looking at and MPE white paper /article at https://www.mpechicago.com/coffee/image ... e_dust.pdf.

MPE claim that the shearing velocity is critical and depends on the friable material being sheared.

Mahlkonig grinders rotate at very high RPM (usually above 1000 RPM) while MPE at a lower 500RPM differential speed for shearing (1000RPM for one roller and 500 RPM for the second roller). The Mahlkonig Burrs are large and the MPE rollers are huge 8 inch or 203mm.So calculating the velocity of the shearing surface at the example 500RPM at 203mm diameter would be: v=500(RPM)/60(M/Sec)*2*pi*r=5.3m/Sec. Similarly, the calculation for a Mahlkonig EK43 at 50Hz ~1500 RPM at 98mm burr size would result in a shearing velocity of 7.69m/Sec. Even higher than the MPE. At 60Hz the velocity of the shearing surface would be even higher.

For Versalab, I was told by Laura to adjust the RPM to the range of 525-550 (it used to be 100 RPM higher when they first started). For the 68mm burred Versalab, performing a similar calculation would result in a shearing velocity of 1.92m/Sec. Very slow by comparison.

If shearing speed is important to the uniformity of the grind as the MPE white paper seems to suggest - why are the smaller grinders operated at slower shearing speeds? (apart from cost saving considerations in manufacturing smaller grinders)

To minimize heat generation?

I'm speculating, but I would think the large surface area of industrial rollers dissipates heat orders of magnitude better than the much smaller surface area of counter-top flat or conical grinders. Isn't that why Mazzer runs the Robur so slow and Compak runs the K10 even slower? And isn't it why some manufacturers have added fans to their latest grinders?

And we still don't have consensus that uniform grinding is better than bimodal grinding for espresso! We could be talking about two completely different drinks.

Also if he's really talking about roller grinders, they don't look or feel much like cutting wheels and they tend to be water cooled so I'm not sure any comparison makes sense. There was a big roller grinder open at SCAA so I got to feel the rollers.

Ira

The shearing speed is variable for the flat burr grinders, and increases linearly over the radius. At the edge, you'll have maximum shearing speed. For conical burrs, the shearing speed doesn't increase as much as the grind path is at a slight incline from the rotational axis.

You also assume grind mechanics are known. And I'm not so sure this is the case. I think burr mfg's know, but they aren't telling us. Is the coffee ground by the ridges in burrs, shearing the bean apart, or do the ridges force the bean into a smaller space, effectively breaking it? Or are those effects essentially the same?
For roller grinders, this is more straighforward. The beans are forced into a smaller space, breaking them.

Peppersass wrote:To minimize heat generation?

I'm speculating, but I would think the large surface area of industrial rollers dissipates heat orders of magnitude better than the much smaller surface area of counter-top flat or conical grinders. Isn't that why Mazzer runs the Robur so slow and Compak runs the K10 even slower? And isn't it why some manufacturers have added fans to their latest grinders?

And we still don't have consensus that uniform grinding is better than bimodal grinding for espresso! We could be talking about two completely different drinks.

Would you imagine having the steam from dry ice placed so it would funnell through the burrs during grinding to be useful?

ira wrote:Also if he's really talking about roller grinders, they don't look or feel much like cutting wheels and they tend to be water cooled so I'm not sure any comparison makes sense. There was a big roller grinder open at SCAA so I got to feel the rollers.

Eventually they are all grinders. The exact process of particle attrition is technical.

Also - no grinder is really uni-modal - just the spread of the particle sizes changes. The MPE grinders mentioned in the white paper are indeed roller grinders.

Do roller grinders (due to size and/or speed and/or some other magic pixies) allow one to extract at a higher extraction yield (without being overextracted) like the EK43?

AssafL wrote:Do roller grinders (due to size and/or speed and/or some other magic pixies) allow one to extract at a higher extraction yield (without being overextracted) like the EK43?

As far as I can tell, there are no roller grinders for coffee that are intended to process less than hundreds or thousands of pounds per hour so I'm guessing we'll never know.

Ira

Surely one could dump 100 grams into one of these... I don't know of a roaster around here that has one of these...

AssafL wrote:Do roller grinders (due to size and/or speed and/or some other magic pixies) allow one to extract at a higher extraction yield (without being overextracted) like the EK43?

Some time ago I spoke with an expert in the field who feels that the answer is yes. Unfortunately, no one has solved the engineering and cost challenges required to scale a roller grinder for the cafe and/or home markets. We can only hope that someone does.

AssafL wrote:Surely one could dump 100 grams into one of these.....

Possibly, .. But the retention could well be a few hundred grams !
( I'm off to dig out the wife's Pasta mill, and start testing ! )

PS... I have worked in large coffee processors ( hundreds of tons /day !) and visited others..
But I have never seen one of these "roller" mills used for grinding coffee.
Maybe it's new technology, maybe it's just sales promotion spin ?