Grind particle size is undoubtedly one of the most critical factors in a good grinder. The distribution of particle sizes and (to some extent) shapes may be determined by measuring the scattering of a beam of coherent light as it passes through the sample. This measurement is done in an instrument known as a laser diffraction particle size analyzer. Beam scattering measurements may be done wet (with the sample particles suspended in water) or dry (in air).
With the assistance of Russ Lingenfelter, a colleague at SDSM&T, I recently ran particle size analyses on three of the Titan grinders: the Macap MXK conical burr grinder, the Mazzer Robur conical burr grinder, and the Mazzer Super Jolly flat burr grinder.
To eliminate as many variables as possible, I prepared all the samples from one batch of roasted coffee in the same session. The coffee was a Kenya, homeroasted on an AeroRost electric drum roaster:
The samples were prepared by adjusting each grinder to yield a specific type of shot on one espresso machine. In this case, a ridgeless double basket was dosed with 19g coffee, and the grind adjusted to yield a 60ml shot in 30 seconds on a QuickMill Vetrano. After adjustment, three samples were ground on each grinder. Sample size ranges from 10g (1/8 cup) for wet analysis to 40g (1/2 cup) for dry analysis. Samples were placed in airtight containers (plastic ziplock baggies) for analysis later that day:
Yours truly in the lab:
Here's a full frontal view of the MicroTrac S3000 laser diffractometer:
You have to add the samples and flush the sample chamber manually, but the analyses are computer controlled:
On this day the machine was set up for wet analyses. For a wet analysis, you drop a couple of pinches (pardon the technical jargon) of grinds into a small chamber filled with water, and allow the grinds to circulate through the system for particle sizing. Somewhat surprisingly, the particle distribution shifts to slightly smaller sizes over time. My explanation: the ground coffee particles absorb water and swell when they hit the water. Circulation in water gradually breaks up clumps, resulting in smaller sized particles. This was a minor effect, but I decided to give each sample two minutes of circulation before running the analysis.
The MicroTrac software allows you to extract the number of particles, particle surface area, and particle volume distributions as a function of particle diameter in microns. Here are the results from my samples:
As you may recall, I ground three samples from each grinder. The results were very consistent, indicating a low degree of inter-shot variability. The Mazzers in particular showed virtually no differences between samples. I used the average of the three samples in the plots presented above.
Volume distribution appears to be the standard for most particle size analyses. These grinders all produce bimodal volume distributions. Presumably the larger particle size peak (around 400-500um) is indicative of the grinder setting, and the smaller particle size peak (around 40um) represents fines. If that's true, these results indicate that flat burr grinders tend to produce more uniform grinds with fewer fines than conical burr grinders. The flat burr Super Jolly peak is the tallest and narrowest, followed by the conical burr Robur. The conical burr MXK has lowest and widest peak, showing the greatest spread of grind sizes. The MXK also has the largest fines peak.
According to Jim Schulman, the volume peaks are quite close to the values quoted by Illy (although surface area and number of particles are not):
another_jim wrote:The Illy book gives three graphs. The number of particle one is unimodal and peaks at 0.2 microns. The surface area and volume ones are bimodal peaking at 1 and 20, and 40 and 500 respectively.
The volume curves also resemble Teme's particle size distribution plots.
It's interesting to note that the volume curves correlate with my taste impressions. I've consistently placed the MXK at one end of the taste spectrum, the Super Jolly at the other, and the Robur in between. More on this later.
Two days later I was finally able to run a dry particle size analysis on the same samples. The particle size distribution results are as follows:
You'll notice that I added another flat burr grinder: a Bunn ES-1G (actually a rebadged Cunill Columbia, with the same 60mm flat burrs and 1/3hp motor as the Cunill Tranquilo). Why bother? I needed another 2 cups of grinds to calibrate the dry analysis setup, and thought it would be interesting to compare another flat burr grinder. Indeed, the Bunn curves follow the Super Jolly quite closely, even though the Bunn sample came from another batch of Kenya (roasted just the day before).
The wet and dry volume-based size distributions are reasonably consistent. The wet large peak and the dry small peak have shifted to the right (towards larger particle sizes), presumably due to swelling and clumping, respectively. There are some interesting changes in curve shapes as well. But these effects are relatively minor, and could be partially attributed to the two-day lag between wet and dry sample analyses. I will probably opt for wet analyses in the future, unless someone can make a convincing argument in favor of dry analyses.