Changes after roasting

After roasting and cooling, coffee beans keep changing. Part of the gas generated during roasting is held within the porous structure and released gradually. This process is usually called degassing, but the entire rest period shouldn't be equated with a single phenomenon. During this stage, gas, temperature, moisture state, and storage conditions all change at once. So to know whether rest time affects grinding behavior, you need to design a test capable of separating these variables.

CO2 and bean state

CO2 is an important component of the post-roast process. Anderson and colleagues showed part of the CO2 generated during roasting can be held and released gradually; once the bean is ground, the gas-escape pathway changes drastically. Smrke and colleagues also developed a time-resolved gravimetric method for tracking mass loss due to degassing. These results show post-roast time can be described with data instead of a common “enough days” convention for every coffee.

The possibility of PSD changing

The PSD question is more complex. If the bean's mechanical state changes along with the degassing process, there could be an effect on breaking, but the extent needs to be measured. You shouldn't take the rate of CO2 release as direct evidence that PSD has changed. Bean temperature also decreases with post-roast time and can affect grinding behavior. These two variables need to be tracked or controlled when comparing rest-time points.

Designing the measurement

A suitable design could use the same roast batch divided into identical storage samples. Measure PSD at multiple time points, while recording bean temperature before grinding. If a deeper mechanistic understanding is needed, mass loss or a degassing indicator can be tracked in parallel. The grinder, settings, sample mass, and PSD measurement method need to stay the same. This way, differences between time points have a stronger basis for being linked to rest time.

Limits of the rest-time milestone

It's important not to turn a result into an “optimal day” applicable to every coffee. Roast level, bean structure, cooling conditions, packaging, and ambient temperature can all change post-roast kinetics. A milestone can be useful for one specific system but isn't sufficient to be a general rule. Even if PSD changes over time, the impact on the cup still depends on the brewing method and other extraction variables.

Rest time should therefore be seen as a testable input variable. A good approach is building a data curve from after roasting to time of use, rather than relying on a word-of-mouth number. If PSD is stable but extraction changes, the cause needs to be sought elsewhere; if PSD shifts together with the post-roast state, that's a stronger basis for further study. The value of the measurement lies in distinguishing phenomena that are often lumped into a single concept.

If you want to test the CO2-and-grinding-behavior hypothesis, you can measure mass loss or degassing rate alongside PSD. That way, a PSD change can be placed alongside gas state rather than relying only on the number of rest days. However, if two variables change together over time, additional experiments are still needed to determine causation. This is why kinetic data has more value than a fixed date milestone.

Post-roast packaging can also change the test conditions. A sample in valved packaging, one in a sealed bag, and one exposed to air will have different gas-escape pathways. If the goal is studying rest time specifically, storage conditions must be the same across groups. If the goal is studying the interaction between rest and packaging, that's a different design requiring enough samples to separate the two factors.

Rest time can also interact with ambient temperature. A batch cooled quickly and one cooled slowly can reach the same number of hours since roasting but different physical states. So if studying rest time, the sample's thermal history should be controlled or recorded. This matters especially when comparing data collected on different days.

If results show PSD doesn't change significantly but sensory perception does, that's still a valuable result. It shows the effect of rest time may lie in gas, volatile compounds, or the brewing process rather than necessarily in particle size. Conversely, if PSD changes but sensory perception doesn't within the tested range, the significance of that physical change shouldn't be exaggerated.

When comparing rest times, the same roast batch should be split into samples right after cooling to reduce initial differences. Samples are then stored under the same conditions and taken out at defined milestones. If further study of packaging's effect is needed, that should be a separate variable in the design. Separating each variable helps identify whether an observed change comes from rest time or from the storage environment.

A post-roast log should record the end-of-roast time, cooling time, storage conditions, and grinding time. These milestones help distinguish real rest time from samples experiencing different conditions. Without this history, two samples both called “day three” could actually have gone through very different conditions. Time-based data only has value when the time markers are clearly defined.

References

• Smrke et al. (2018), Time-Resolved Gravimetric Method To Assess Degassing of Roasted Coffee, J. Agric. Food Chem. 66, 5293–5300, DOI 10.1021/acs.jafc.7b03310.
• Anderson et al. (2003), Journal of Food Engineering 59, 71–78, DOI 10.1016/S0260-8774(02)00432-6.
• Uman et al. (2016), Scientific Reports 6, 24483, DOI 10.1038/srep24483.
• The Craft and Science of Coffee (2017), The Grind—Particles and Particularities.