Grinding - why the variations?

[gregpullman]

About 8 months before I got married I convinced my wife-to-be that it was the perfect time to get a coffee machine given we probably wouldnt be able to justify it once we had a mortgage etc. Having grown dissatisfied with the eBay special of a Kenwood thermoblock even before taking delivery of the thing, I had my sights set on a Quaha / Imat unit. One of the justifications for getting the Napoletana with the built-in grinder was the ability to grind to whatever granularity required by the bean (and have never looked back incidentally!)

Ive worked out that for the beans I get from Alan Frew, I need a grind of about 2.75-3.0, depending on how long Ive had them. Decaf needs to be more like 2.0 or it flows straight through. I recently was given some Cibo beans and found these had to be more like 3.5 to prevent choking. Im sure you all have similar magic numbers for your own blends and grinders.

But why the variation? Why is it that some beans, or blends, need to be ground coarser or finer to prevent choking or fast-flow respectively? Is it that the drier beans absorb more water, therefore expanding more, therefore need to start coarser? Is it the roast level? The caffeine content? What actually happens? All that is clear to me is that for the same blend, the same grind density is required, sometimes slightly finer as the batch ages.

Greg

[TC]

Re: Grinding - why the variations?

Hi Greg- one bit of this is easy. Decaf requires a finer grind as the swiss water process removes most of the oil- therefore less resistance to the flow of water. I find I dose and collapse at the cafe, tamp like crazy and still the grind has to be so fine as to blow a set of flat blades in perhaps 50kg...at best...

As for age and grind change, your observations are correct. I guess that the answer must go something along the line that as beans age/degas they undergo a a change in density? Can any other add to this?

Chris

[gregpullman]

Re: Grinding - why the variations?

Thanks re decaf, that makes sense.

Agreed about what *probably* happens as they age; my guess was that the beans absorb moisture from the air (as part of the oxidisation process), thus theyre already a bit damp and have expanded a bit, hence dont have as much capacity for further moisture absorption and resultant expansion. But just a guess, Im sure an expert will have a better idea.

Still lost as to why some blends consistently require finer / coarser though.

[MattG]

Re: Grinding - why the variations?

A most interesting question which took me back to Dr Illys "Espresso Coffee" book looking for an answer. My understanding of this discussion was that assuming machinary is constant (where different grinding mechanisms will produce a different result), grinding variables include combination of variability in a coffee blend, roast profile, and moisture content.
Different beans in a blend provide variability in homogeneity. Roast profiles will vary the internal composition of the bean (degree of brittleness). A higher moisture content in roasted beans results requires more energy to grind, resulting in "overheating" the coffee.

From observations Ive noted that darker roasts tend to require a less course grind setting for a similar flow rate to ligher roasts. From the above "theory" Id assume it is due to the fracture mechanics as a result of the brittleness of the internal structure of the bean as a result of the bean variety and roast level. Or maybe it is simply due to faster degassing in darker roasts.