Heat during the grinding process

Coffee beans don't enter a grinder in a thermally static state. The bean's starting temperature, rotation speed, grinder load, and how the ground stream exits the grinding chamber all jointly determine the thermal state during the breaking process. So when talking about “hot grinding,” at least two issues need separating: the heat generated during grinding, and the actual temperature the bean or grounds reach. These two quantities are related but not identical. Research by Uman and colleagues shows bean temperature before grinding can change grinding behavior and ground-particle characteristics; that doesn't mean a hot-feeling machine automatically means coffee quality has declined.

From bean temperature to PSD

The heat source in grinding mainly relates to friction, deformation, and mechanical work converted into heat. Part of that energy breaks down the bean's structure; the rest dissipates through the grinding mechanism, the machine body, the air, and the ground coffee itself. Static electricity is also a factor related to the state of the grounds after leaving the grinding mechanism. Recent research shows the electric charge generated during grinding depends on roast level and moisture; so the same machine can behave differently with different types of coffee. This matters when assessing heat: a feeling like “the machine is hotter” shouldn't be treated as representative of the entire process.

Heat and aroma change

A harder question is whether temperature changes particle-size distribution. The answer depends on experimental design, since bean temperature can affect mechanical properties and breaking behavior, while grinding speed and machine load also change at the same time. A 2026 study on pre-grinding thermal treatment of beans continues to show bean temperature is a notable variable for ground-coffee and beverage characteristics. However, this doesn't allow inferring that every temperature increase in a grinder worsens PSD. PSD needs to be measured directly using a suitable method.

Designing the temperature measurement

When considering the risk of aroma change, heat especially shouldn't be separated from time. A brief temperature spike during grinding doesn't carry the same meaning as grounds staying at high temperature for an extended period. The sharply increased surface area after grinding also opens a pathway for gas and volatile compounds to escape. So if you want to test heat's effect on aroma, the test needs to hold bean type, roast level, time since roasting, batch mass, and post-grind packaging constant. Only changing thermal conditions or grinding speed and then tracking change over time can separate the effect of grinding from the effect of storage.

Limits of the heat indicator

Measuring heat inside a grinder is also a measurement problem, not just a matter of placing a sensor on the machine body. Sensor position, sampling moment, bean mass, rotation speed, continuous run time, and machine condition all need to be recorded. If the goal is knowing grounds temperature, machine-body temperature can't automatically substitute for grounds temperature. A good procedure should include measurement before grinding, during or right after grinding, and, if needed, after an equilibration period. When repeating, the same position and same sampling conditions matter more than having one very precise but non-reproducible number.

The final point is the limit of using heat as a quality indicator. Higher temperature can be a signal worth investigating, but on its own it doesn't prove aroma loss, poor PSD, or poor extraction. Heat needs to be placed alongside output data: particle-size distribution, input-output mass, moisture, electrostatic state, or sensory indicators if the goal is assessing experience. This approach avoids two extremes: ignoring heat entirely, or turning temperature into a general “quality threshold” for every machine and every coffee.

A practical way to test is creating three grinding conditions with the same mass but different operating speeds. At each condition, record bean temperature before grinding, grounds temperature right after grinding, grinding time, and PSD. If temperature rises but PSD stays within normal variation, there isn't yet a basis to say heat has worsened the grounds' quality. If PSD shifts at the same time, you need to further check the grinding mechanism's condition and residual grounds before concluding. This design also allows separating the effect of speed from the effect of machine load.

For aroma, avoid measuring at just one point in time. Grounds can lose volatile compounds quickly at the start, then change more slowly afterward. So a curve over time is more meaningful than a single before-after measurement. If the goal is comparing two thermal conditions, both curves need to be sampled at the same time points and in the same packaging. When chemical data, temperature, and sensory evaluation all point in the same direction, the evidence for a real effect becomes stronger.

A heat measurement should also record the machine's condition before starting. A machine that just ran continuously and one that's been resting a while can have different baseline heat. If this isn't recorded, two measurements matching in speed and mass may still not be equivalent. For production, data should be collected under representative operating states, such as the start of a shift and after a run of continuous batches, to know whether heat accumulates.

When reading results, avoid confusing correlation with causation. If temperature and PSD both rise, there could be a third variable like machine load or moisture. A follow-up test needs to change the suspected variable in isolation. This step-by-step reasoning matters more than setting a neat-looking but unfounded temperature threshold. Heat only becomes a strong indicator when repeated measurements show it consistently precedes or accompanies a reproducible quality change.

References

• Uman et al. (2016), The effect of bean origin and temperature on grinding roasted coffee, Scientific Reports 6, 24483, DOI 10.1038/srep24483.
• Méndez Harper et al. (2024), Strategies to mitigate electrostatic charging during coffee grinding, iScience 27, 110639, DOI 10.1016/j.isci.2024.110639.
• Pignatone et al. (2026), Effects of Coffee Bean Thermal Treatments on Particle Size Distribution and Espresso Bioactive Compounds, Applied Sciences 16, 4886, DOI 10.3390/app16104886.