Grounds and moisture exchange

Ground coffee has a much larger surface area than whole beans, so moisture exchange with the environment is a variable worth considering when studying grounds state. When relative humidity changes, grounds can absorb water and change physical properties. That doesn't mean every change is due to water activity, since temperature, equilibration time, packaging, and sample handling also have an effect. A good measurement needs to separate these concepts.

Effect on physical properties

Water can affect the mechanical properties of coffee material. Research by Pittia and colleagues shows roasted-bean mechanical properties depend strongly on water activity; at a sufficiently high level, a plasticizing effect removes the characteristic brittleness. For grounds, moisture absorption can also cause particles to clump, changing friction and flow ability. So a more humid grounds sample can differ not just in water content but also in how it disperses and passes through a measuring system.

Moisture and measured PSD

Moisture can also change the measured PSD without necessarily changing individual particle size. If particles clump together, a measurement method might register a cluster as a single larger particle. Conversely, if the sample-dispersion process breaks apart clumps, the result may differ from the material's stored state. This is why dispersion method and measurement conditions need to be clearly recorded when comparing PSD across samples with different moisture levels.

Designing the test

A moisture-PSD test should control temperature, relative humidity, equilibration time, sample mass, and measurement method. Samples can be placed at defined humidity conditions then measured at time milestones to know the rate of change. If the goal is simulating storage, packaging and headspace also need to be accounted for. You shouldn't take a sample exposed to the environment for a few hours and treat it as representative of an entire packaging system.

Applying to storage

Coffee-storage research shows temperature and packaging conditions can affect quality indicators over long periods. For roasted coffee, sensory and volatile-compound measurements can change with storage conditions. This shows moisture is part of a system, not a standalone variable. If the ultimate goal is beverage quality, physical grounds data needs to be related to brewing or sensory results rather than stopping at “grounds absorb moisture.”

The limit when applying results is clear: moisture-exchange kinetics depend on packaging, contact area, coffee amount, temperature, and time. A relative-humidity value producing a lab result can't automatically translate into a real-world storage time. A safer approach is describing the specific system, measuring the change within that system, and keeping conclusions within the scope of the data. Done this way, moisture becomes a controllable variable rather than a cause invoked for every grounds change.

In a practical test, you should distinguish “ambient humidity” from “the grounds' final moisture.” Two samples placed at the same relative humidity but with different handling histories may not have reached the same equilibrium state. So contact time and starting conditions need to be recorded. If needed, you can directly measure the sample's moisture or water activity rather than just inferring it from room humidity.

Packaging is especially important when transferring results to a commercial product. A small bag opened many times has a very different moisture-exchange history from a large bag opened once and resealed. Headspace, number of openings, material, and temperature all have an effect. So lab data should be used to understand mechanism and build hypotheses, while storage conclusions must be confirmed on the actual packaging system and usage pattern.

For ground coffee, clumping is a particular issue when reading PSD. A measurement can reflect cluster size rather than original particle size if the dispersion method isn't adequate. So when moisture increases, you need to determine whether the result changed because the material genuinely changed or because the measurement is seeing more clusters. This is why the sample-preparation method must be described alongside PSD results.

In real storage, the number of times a bag is opened can matter just as much as average ambient humidity. Each opening creates a new gas-and-moisture exchange. A model using only ambient temperature and humidity without factoring in opening history may not accurately describe user behavior. Conclusions should stay within tested conditions and clearly state which variables haven't been simulated.

As moisture increases, you need to track clumping as well, since clumping can distort particle-size measurement. If the measurement method has a dispersion step, dispersion conditions must be the same across groups. Otherwise, the difference between two results could be due to how clumps were broken apart rather than the material itself. This is why sample-preparation method must be recorded alongside results.

When PSD results change with moisture, it's worth checking both flowability and clumping tendency of the grounds. Two samples with the same measured PSD but different flow characteristics can still behave differently in a hopper or brewing machine. This shows one indicator shouldn't be treated as representative of the entire material behavior. Measuring an additional related property can help explain why the same particle size gives a different operating experience.

References

• Pittia, Nicoli & Sacchetti (2007), Journal of Texture Studies 38, 116–134, DOI 10.1111/j.1745-4603.2007.00089.x.
• Singh et al. (1997), Thermophysical Properties of Fresh and Roasted Coffee Powders, Journal of Food Process Engineering 20, 31–50, DOI 10.1111/j.1745-4530.1997.tb00409.x.
• Błaszkiewicz et al. (2023), Scientific Reports 13, 6447, DOI 10.1038/s41598-023-33609-x.
• The Craft and Science of Coffee (2017), The Grind—Particles and Particularities.