The big test of the grinding quality of electric grinders

How can the quality of grinding vary with electric coffee grinders? For example, find out the differences between the German Mahlkönig and Graef grinders or how the popular Mignon grinder fares in terms of quality.

How to measure differences in ground coffee

First, a little introduction to the issues we are about to discuss. Relax, it's not rocket science. Or rather, it's already been done. Specifically, I'm drawing on information posted by astrophysicist (yes, he's interested in space and coffee at the same time) Jonathan Gagné on his blog Coffee ad Astra.

This "lord of the stars and coffee" describes the resulting data from measurements of PSDs, or particle size distributions, from grinding samples on 24 coffee grinders. The PSD research was carried out at the University of Zurich using special Camise X2 equipment from Retsch Technology (if you want to test it too ????).

What does PSD mean for coffee grinders?

So back to PSD. It is a value that describes how large/small the ground coffee particles are and in what quantity they are in the grind samples. Jonathan describes a lot of contextual issues with the calculations in his post, but the result is the determination of a reference point of 340 microns. The PSD at a grind of this value is then compared between grinders.

What do boulders do in ground coffee?

When people talk about "boulders" i.e. so-called boulders, they mean that larger particles of coffee arepresent in the grind (relative to the set and expected fineness). It would also be fair to say the opposite is the case. Ground coffee contains "fines" i.e. dust particles.

PSD comparison of 24 coffee grinders

In practice it looks like this:

You take samples of ground coffee from 24 coffee grinders and put them into an expensive scientific machine.
You determine a reference point to assess the PSD of the grinds between each other and calculate the values for the boulder range and dust particle ratio.
You display the results graphically to keep you in the loop. Therefore, I am now attaching a graph of the average values from the grind samples of 24 different coffee grinders tested.

graf - test kvality mleti u 24 elektrických mlýnků

Explanation of the grind quality chart - PSD and speed

In the above graph you can see:

points whose number refers to the type of grinder tested
a point in a circle indicates that the grinder has flat grinding stones
a point in a triangle indicates that the grinder has conical grinding stones
the colour of the point indicates the number of revolutions per minute
the horizontal axis is a scale for the proportion of dust particles expressed in %
on the vertical axis are the values of the boulder range in microns

What does this measurement of grinding mills show?

Let's talk some more about what grinding characteristics are directly related to the quality of the ground coffee. That is, those objectively measurable. In other words, what we want to look for in a graph and consider as a sign of a quality grind.

The most commonly cited advantage of a quality grinder is the consistency of the grind. I'm sure you already know this. A grinder whose grind is as uniform as possible (PSD ...there it is again) is taken to be of higher quality. Yes, there are more and more influences on the grind such as stone material, mill heating, etc. But we'd be analyzing that all week here.

So let's just stay with the fact that are looking for a grind where there will be no boulders or dust particles. Such a perfectionist among grinders. We can also call it as a mill that is uniform and unimodal at the same time. The advantage is that these values are correlated.

unimodal - the grind has a minimum of dust particles
uniform - the grinding has a minimum of boulders

On the need for finesse in the grind of an espresso grinder

It should be said that a grinder with 100% consistency of ground particles might seem perfect, but it doesn't have to be. And that's specifically with espresso grinding. The fine coffee dust particles in the grind greatly affect espresso extraction.

Although we said a moment ago that a unimodal grinder should be better quality, there should always be a percentage of those finesse in espressocoffee. They affect the parameters of the espresso in the cup, specifically the extraction yield, purity and body of the coffee.

graf - test kvality mleti u 24 elektrických mlýnků

Interesting facts from the analysis of grinders

Conical grinding stones exhibit lower unimodality and uniformity than flat
Grinding speed has no significant correlation with unimodality and uniformity
Unimodal grinders may not exhibit the necessary amount of finesse for espresso
The extent of boulders in ground coffee can be a useful measure of grinder quality
The geometry of the grinding stones should have more influence on quality than their size
The percentage of dust particles increases with lower average grind coarseness

graf - test kvality mleti u 24 elektrických mlýnků

My choice of grinder according to the grinding quality test

Finally, let me add my own feelings about the results of this test. A pleasant finding was how the Rocket Fausto performed. As you can see, it is a grinder of both design and very capable. Grinders from Mahlkönig are always sure of their quality, here too they proved to be kings of consistent grinding. Then there's the Eureka Mignon XL, which shines as an example of a handy grinder at a good price.