Mechanical structure after roasting

Hardness of a roasted bean isn't a fixed property. During roasting, water is lost, density decreases, and tissue structure changes; as a result, beans become more brittle and easier to break than green beans. Research by Pittia and colleagues shows strength, toughness, and deformation ability change with roasting conditions, with moisture loss and structural change both contributing. So when saying a batch is “harder” or “softer,” you need to specify which mechanical indicator you're referring to and under what moisture state.

Hardness and brittleness

Hardness and brittleness are related but shouldn't be used as synonyms. Hardness usually describes the force needed to create deformation or breakage under a given test; brittleness relates to how a material fractures and how much deformation occurs before breaking. For roasted coffee, its porous, brittle structure makes grinding different from a dense material. Research on the effect of water activity shows that as a roasted bean absorbs moisture, mechanical properties change, and at a sufficiently high water activity, a plasticizing effect removes the characteristic brittleness. This explains why the same type of coffee can show different grinding behavior when its moisture state changes.

Grinding energy

Grinding energy also needs to be defined carefully. A machine's electrical power over a time period isn't entirely the energy used to break the bean. Part goes toward overcoming friction, rotating components, heating the machine, and other losses. So if you want to study the relationship between bean structure and grinding energy, you should record power or energy consumption together with coffee mass and operating time, then place that data alongside PSD results.

Roast level and breaking requirements

Roast level can change breaking requirements through porosity, density, and brittleness. But you shouldn't infer that lower energy always means better grounds quality. An easily broken bean can still produce a very different distribution if the grinding mechanism's geometry or settings aren't suitable. Conversely, a higher energy level can still come with a desired PSD in a different system. Output needs to be measured directly rather than using energy as a proxy for quality.

Designing the test

A useful test could use the same batch of coffee, different roast levels, the same mass, and the same grinder. Measure input mass, grinding time, energy or power, and PSD. If possible, also measure moisture and density to help explain the mechanism. With this design, you can see how specific energy per unit mass changes and whether that change accompanies PSD change. This is a way to separate the physical relationship from a subjective impression when operating the machine.

The biggest limitation is that grinding energy isn't a direct sensory indicator. A high or low energy number doesn't by itself say coffee will taste better, more aromatic, or extract better. It only has value when placed within a data chain including bean state, grinding conditions, and grounds characteristics. For quality control, the practical goal should be detecting unusual changes in the system and identifying the cause, rather than searching for a single “correct” energy level applicable to every type of coffee.

If you want to compare energy across roast levels, use the same measuring device and the same calculation method. Instantaneous power can fluctuate as beans enter the grinding zone; cumulative energy over time is usually easier to compare, but still needs normalizing by mass. An idle-running machine also consumes energy, so if the goal is estimating the portion related to grinding, you need to determine the machine's baseline consumption and consider how to subtract or clearly report it.

Moisture state matters especially because water can change mechanical properties. A roasted bean that absorbs moisture doesn't just get “heavier” — it can also lose brittleness, changing how it cracks. So if a batch shows higher grinding energy, moisture should be checked before attributing it to roast level. This is a clear example of a mechanical data point only carrying meaning within the context of the specific material.

A mechanical test also needs to distinguish a single bean from a bulk of beans. The energy to break one individual bean doesn't necessarily reflect the energy a machine consumes when processing a stream of material. In an actual grinder, the number of beans present simultaneously, friction, and the grounds exit flow all affect power draw. So for quality-control purposes, data at the operating scale usually has more value than a single mechanical measurement.

Moisture and water activity also need to be recorded according to the specific goal. Two samples with the same total moisture but different bound-water states may not have the same mechanical properties. So when studying water's effect, you should clearly define which quantity is being controlled and the measurement method. This matters especially when comparing roasted beans stored in different environments.

References

• Pittia, Dalla Rosa & Lerici (2001), LWT 34, 168–175, DOI 10.1006/fstl.2000.0749.
• Pittia, Nicoli & Sacchetti (2007), Journal of Texture Studies 38, 116–134, DOI 10.1111/j.1745-4603.2007.00089.x.
• Crushing of roasted Arabica coffee beans (2014), Acta Universitatis Agriculturae et Silviculturae Mendelianae Brunensis.
• Pignatone et al. (2026), Applied Sciences 16, 4886, DOI 10.3390/app16104886.