How does it change the performance?

In the last few years, a small number of grinders have hit the market that boast an adjustable grinding speed. The best known of these are Lynn Weber’s EG-1, Victoria Arduino’s Mythos 2, and most recently Ceado’s Hero E37Z. The marketing material for each of these grinders claims that being able to change the RPM allows you to somehow optimise the grind for different coffees, or produce different types of extraction — but none of them make any specific claims about how slower or faster grind speeds might achieve this. So is the adjustable RPM just a gimmick, or can it really improve our extractions?

Grind Size Distribution

The holy grail of coffee grinding is to produce higher, more even extractions, by producing particles that are more even in size. Unfortunately, the published evidence for the effect of RPM on the particle size distribution is limited, and the effect may well be different for different grinders.

What we can say is that faster RPMs typically deliver finer grinds for a given grind setting, producing more fines and a less modal grind size distribution. However, it’s not yet clear if this effect is any different to simply grinding finer.

Particle size distributions of coffee ground at different RPMs. Source: Blackwater Issue

This chart, adapted from an article in Korea’s Blackwater Issue, shows the grind size distribution changing as the burr speed changes, with the higher RPM resulting in a smaller median grind size and more fines. However, this effect appears to be very similar to the effects of simply changing the grind setting as shown in the graph below.

Particle size distributions of coffee ground at different grind settings. Source: Blackwater Issue

Selecting a finer grind setting also results in a smaller median grind size and more fines. This difference appears to be fairly similar to the effect of increasing burr RPM.

These results are backed up by Jonathan Gagne’s experiments with his EG-1: “‘With my burrs, more speed makes the distribution finer and that’s it,” he explained in Matt’s Telegram Channel. “If I compensate with the spacing I get the exact same results in [extraction] and taste.”

Extraction

We do however have one piece of evidence that changing the RPM changes extraction, beyond just making the grind finer. Gwilym Davies sent us some data from a series of shots he ran using a Mythos 2 at two different RPM settings. He also found that a higher RPM resulted in a finer grind and hence slower shot times — but if you take the shot time into account, then the lower RPM setting produced consistently higher extractions.

The effect of grinder RPM on the TDS of espresso shots. Grouping the data by shot time rather than grind setting shows a distinct difference in extraction between the two RPM settings. Data courtesy of Gwilym Davies

This shows that, for the Mythos 2 at least, changing the RPM is altering the grind quality beyond just making it finer or coarser. However, we can’t yet say if that’s because of changes to the particle size distribution, or some other factor — for example particle shape.

Dosing

A higher RPM allows coffee to be ground more quickly, which might make the high RPM settings desirable in a busy cafe. However, grinding faster means the dose becomes less accurate. The Mythos 2 compensates for this by grinding the last portion of each dose more slowly, to increase the accuracy of the dosing.

This can have unexpected results in other grinders. AC motors use the alternating frequency of the current to turn the motor, which means that a different mains frequency will produce a different RPM. Most countries use a mains frequency of 50 Hz, but in the Americas, and some parts of Asia, 60 Hz is more common. This means a grinder with an AC motor operates at a higher rpm in the USA than it does in Australia, for example. This can make the dosing for this type of grinder slightly less accurate in the USA, and might have a small effect on grind quality, too.

The effect of mains frequency (and hence RPM) on dosing. Source: Graeme Burton (@papamurgy)

Heat

Finally, changing the RPM also changes the amount of heat produced in the grinder — but not in the way you might expect. In his talk at the 2015 Barista Camp, Christian Klatt explained that increasing the RPM of the grinder may actually reduce the effect of temperature on grinding. At higher rpm, the coffee passes through the grinder more quickly, so it has less time to absorb heat from the chamber, he explains. The motor will also generate less heat, as it is operating for less time per grind.