Will uniform coffee ground size improve espresso?

[jmatt]

Is espresso better or worse as the ground size is more regular?

The real question is: If I eliminate the oversize particles, AND eliminate much of the powdered fines, would that be a good thing?

I have an idea for a device that will do just that. I envision two purposes:

1) create a more uniform size of coffee particle in the basket.
2) by monitoring the % of particles that are too big and too small, one could get a good sense of how their grinder setting is working with their beans.

Any thoughts? I should have the parts to make my device within one week.

[keno]

The answer, according to Espresso Coffee: The Science of Quality, is NO. It is a misconception that more uniform size of coffee grounds is better. The book devotes an entire chapter to grinding (other chapters on beans, roasting, storage, percolation, etc.).

You can check out this book here: http://www.amazon.com/Espresso-Co...lity/dp/0123703719

The coffee puck serves as its own filter when making espresso. The resistance in the puck along with a pump creates water pressure that extracts the solids from the coffee. It is commonly known that if the grinds are too coarse the water from the pump just flows right through and you get weak coffee, not espresso. And that if the grinds are too fine you will choke the machine and get little or nothing.

But why isn't it good to have a uniform grind? It's because a distribution of particle sizes (and shapes) is needed to create proper resistance. Think of creating a puck made of basketballs. There would be a lot of space between them and water would easily flow through, meaning no resistance. But if you add tennis balls and ping pong balls they will help to fill the spaces in between. This is what helps create resistance in the puck. Dosing and tamping has as much to do with just properly aligning the grinds than compressing them. That's why there is a relatively minor difference between a light tamp and a hard tamp, but there is a huge difference between a light tamp and no tamp.

Hope this helps.

Cheers,
Ken

[jmatt]

. . . It is commonly known that if the grinds are too coarse the water from the pump just flows right through and you get weak coffee, not espresso. And that if the grinds are too fine you will choke the machine and get little or nothing. . . .

Thanks Ken. Would it not help though to get an even extraction if one removed the largest particles, to avoid sources for channeling, and remove the finest particles to avoid choke points?

Sure we need more than one size, but would a mix of 400 microns down to 250 microns not be better than a mix that included 800 microns down to 50 microns? After all, isn't the purpose of using very good grinders in the first place to make sure the grind is very fine (eliminate big particles) while avoiding the dust created by the whirling blades style grinders (and thus eliminating the really small particles)?

I'm open for debate, because I don't really know much, I'm just trying to learn.

[miKe mcKoffee]

The answer, according to Espresso Coffee: The Science of Quality, is NO. It is a misconception that more uniform size of coffee grounds is better. The book devotes an entire chapter to grinding (other chapters on beans, roasting, storage, percolation, etc.).
Cheers,
Ken

I re-read that chapter a couple weeks ago. My take on it is kind of like uniform variation of grind size.

[keno]

Mike,

I think your interpretation is a reasonable one. Here's the passage I was referring to:

Two apparently contradictory needs must be satisfied to prepare a good cup of espresso: on the one hand a short percolation time is required, while, on the other hand, high concentrations of soluble solids must be reached. Both requirements can only be attained if a close contact between solid particles and extraction water can be achieved. Thus, espresso percolation needs a plurimodal particle size distribution, where the finer particles enhance the exposed extraction surface (chemical need) and the coarser ones allow the water flow (physical need). [Andrea Illy and Rinantonio Viani (eds), Espresso Coffee: The Science of Quality, 2nd edition, Elsevier Academic Press, page 215, emphasis added]

Figure 5.4 on page 224 shows a typical distribution of particle sizes from 0.1 um to 2000 um by (1) frequency, (2) surface area, and (3) volume. The chapter devotes considerable discussion to how to measure grind fineness, including tactile feel (subjective method), sieving (doesn't work well), microscopic imaging, and laser diffractometry (best method).

The distribution by frequency is actually unimodal but very right skewed (so not a uniform distribution). But the distributions by surface area and volume are more uniform (because the larger particles make up a proportionally larger portion of the distribution by surface area and volume).

It would be interesting to see the distributions for different grinders (particularly comparing high quality grinders and lower quality) to see how they differ. I'm sure there is plenty of opportunity to conduct additional research in this area if anyone happens to have access to a laser diffractometer!

Cheers,
Ken