Doserless, electronically timed grinders, such as the Mazzer Robur E, have gained popularity recently because they reduce wastage and improve dosing consistency. However, some baristi have noticed at least two strange behaviours during their initial period of use. The first, that the grinders appear to speed up and the second, that the adjustment of grind size and dose is both erratic and required frequently. Anecdotal evidence suggests that new burrs require around 20—30 kg to ‘wear in’.

The most obvious explanation appears to involve the burrs of the grinder. New grinders have new burrs and new burrs probably do not have perfect cutting edges. This is clearly evident in the close-up image of a new lower Robur burr.

These imperfections are most likely an artefact of the manufacturing process. The situation may be compared to that of drilling a hole in sheet metal, where a sharp and inconsistent edge is left around the rim of the hole. Another example is where a knife is sharpened with a grinding stone. A very fine and inconsistent strip of metal known as a ‘wire edge’ is produced along the knife edge and this needs to be removed with a sharpening steel or leather strop before it is considered sharp enough to use.

Because the imperfect edge on the new burrs is very fine and consistent, it starts to break off and wear away during initial coffee grinding. This affects the grind setting which means the barista must make adjustments to the grind size to compensate for the rapidly changing burr edges.

But why does the grinder speed up? While there is an imperfect edge on the burrs, the coffee is not being ground by the whole edge but rather only by the imperfections. When the imperfect edges have worn away, then grinding occurs using the whole edge and therefore more efficiently than before, producing more ground coffee for the same grinding time (see Figures 1 and 2).

Figure 1 shows a schematic representation of an upper and lower burr with imperfect edges. Only about half of the burr edge can grind to the adjusted size, so grinding will take longer. Figure 2 shows burrs with perfect edges. All of the burr edges can grind to the adjusted size, and grinding is more efficient.

Furthermore, unless the grind size is adjusted, the grind size will increase and this will result in both a greater volume of coffee per dose (since larger particles don’t pack as closely together) and even faster grinding, since it is quicker to grind large particles than small ones.

At first glance it may appear that such small imperfections in the burr edges would have a negligible effect on grinding. However, it should be noted that a single notch on a Mazzer grinder typically moves the burrs closer together by approximately 0.015 mm, and it’s routine for baristi to make adjustments of half a notch.

Stay tuned for new experimental data on the ‘popcorn effect’ in grinders.

Author: Graeme Burton, Five Senses Coffee

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