By Jonathan Parent

Myths and legends:Does specialty coffee break the grinders?

Turning to specialty coffee generally means grinding harder and less crumbly beans, since roasted lighter. The question arises: can these grains break your grinder? All grinding wheels wear out over time and eventually need to be replaced. Consuming specialty coffee means slightly faster wear, not to mention breakage..


To grind, grinding wheels introduce fractures into the grain structure by applying four types of force: compression, traction, shear and torsion.

  • The compression force occurs when the grains are pushed down and thus compressed.
  • The tensile force occurs when the grains are stretched.
  • Shear force occurs when a surface is pulled in two opposite directions.
  • Twisting force occurs when an object is simply twisted.

These different forces exerted simultaneously by the mill cause the grains to stretch, shrink and twist until they can no longer withstand such pressure and break. When we remove the pressure, we notice that the grains return to their initial shape as the rubber bands do. These are therefore both malleable and fragile. They go from elasticity to rupture very quickly.

What are the effects of roasting on the fragility of beans?

It is known that green beans have a compact cell structure, that the vacuole (organelle containing the nutrients and maintaining the water balance in the plant cell) is intact and rigid before roasting. In contact with heat, the cells begin to swell, openings form and the water contained in the grain evaporates little by little. These changes in the cellular structure of the grain thus make it more fragile and brittle, mainly due to its dehydration.

Heat and air inlets are used during roasting to reduce moisture content and dry the beans. At the first crack, the steam, CO2 and volatile compounds will create pressure inside the cell, causing the cell walls to compress against each other until they rupture. Breaking the membrane is essential, since it opens the cells. Water can then penetrate there to dissolve the nutrients which, in turn, will be transformed into aromatic compounds.

By continuing the roasting further, we arrive at a second crack (second crack). The cell wall is then very fractured. We see the appearance of small tunnels from the inside of the grain to the outside, which allow the oils to migrate to its surface. This explains why dark kernels are usually oily.

The study conducted by Monika Fekete, founder of the Coffee Science Lab, illustrates how dark roast beans are more fragile than light roast ones. When ground, beans roasted just after the first crack yield a median particle size of 305 micrometers, while beans roasted after the second crack have a median particle size of 120 micrometers. The observation is obvious: the longer the roasting, the more easily the beans break and produce fine particles.

Trends in the specialty market

Rare are specialty coffees roasted after the second crack. We have, more often than not, apparently dry grains with residues of silver film lodged in their furrow. Their color is more brown than black. We find, despite everything, a nice range of roasting levels: from dark brown to pale brown. By opting for a specialty coffee, expect the beans to be stronger, tougher and rougher on the wheels of your grinder.

What you need to know about grinding

A dark roast coffee versus a light roast coffee does not have the same solubility and extracts differently. The darker one is less dense and contains less moisture, we will opt for a slightly coarser grind. The paler one will require, for the same extraction, a finer grind. This fineness will increase the contact surface between the water and the ground beans in order to facilitate the extraction of the aromatic compounds.

Due to their density and stiffness, light roast beans will take more time and effort to grind. This is clearly seen when switching from one type of roast to another or when grinding for espresso. It is normal to have to wait longer to obtain a dose, since the burrs will have to exert more pressure to break up the beans.

However, Alexander Choppin from Baratza warns us: avoid grinding green beans and under-roasted beans, which really risk breaking the components of your mill.

What you need to know for maintenance

It's not wrong to say that light roast coffees wear out your grinder's burrs faster. On average, we grind 226.8 kilograms (500 lbs) of coffee before having to replace the burrs. Drinking only light roasts therefore requires replacing them a little earlier.

On the other hand, using dark, oily beans requires more frequent maintenance of your grinder. Oily streaks left in the hopper and on the grinding wheels can reduce the grinding speed and get mixed up with your new shots. In addition, they increase the chances of the coffee backing up and clogging the chamber by narrowing the passage, which is called clogging. Bi-weekly cleaning is recommended to avoid any hassle with such grains, whereas with dry grains one can wait up to a month.

It should also be noted that, to be designated as a specialty, batches of coffee must meet certain standards, including that of having no foreign bodies (rock, wood, screws, etc.) or having few of them. With lower grade coffees, often darker roasts, you are more likely to come across one of these intruders which can be damaging to your grinder too. Since the production of low-grade coffee is more focused on volume than on quality, sorting on the farm is less rigorous than that carried out in the specialty industry.


After cleaning your grinder and making sure the burrs are in good condition, if some beans are still not going through, you have two possible explanations: the beans chosen may be under-roasted or your grinder motor may not be roasted. isn't strong enough to crush them. It is necessary to opt for more friable grains, therefore darker, if you wish to keep the same equipment.


Research and writing: Chloé Pouliot



Alexander Choppin, Dark Roast Versus Light: What It Means For Your Grinder, Baratza.

Why Are Some Coffee Beans Harder To Grind Than Others, Perfect Daily Grind.

Monika Fekete, Inside A Coffee Roast, Beanscene Magazine.