Coffee bean particle motion in a rotating drum measured using Positron Emission Particle Tracking (PEPT)

Physicochemical transformation of coffee during roasting depends on the applied time-temperature profile (i.e., rate of heat transfer), with heat transfer phenomena governed by particle dynamics. Positron Emission Particle Tracking (PEPT), a non-invasive imaging technique, was used here to characterise the granular flow of coffee in a real, pilot-scale rotating drum roaster. The experimental study established the impact of drum speed, batch size and bean density (i.e., roast degree) on the system's particle dynamics. Particle motion data revealed two distinct regions: (i) a disperse (low occupancy, high velocity) region of in-flight particles and (ii) a dense (high occu-pancy, low velocity) bean bed. Implications of these results for heat transfer suggest that controlling drum speed for different density coffees will provide roaster operators with a tool to modulate conductive heat transfer from the heated drum to the bean bed. These comprehensive data thus inform roasting best practices and support the development of physics-driven models coupling heat and mass transfer to particle dynamics.

Automated summary

The physicochemical transformation of coffee during roasting is affected by the time-temperature profile, which is in turn influenced by heat transfer mechanisms governed by particle dynamics. This study employed Positron Emission Particle Tracking (PEPT) to analyze the granular flow of coffee in a pilot-scale rotating drum roaster. The results revealed that the drum speed, batch size, and bean density significantly impact the particle dynamics within the system. These findings suggest that controlling the drum speed can modulate conductive heat transfer from the heated drum to the bean bed, thereby influencing the roasting process.