Mark Al-Shemmeri, Kit Windows-Yule, Estefania Lopez-Quiroga, and Peter Fryer
University of Birmingham & Jacobs Douwe Egberts, UK
Coffee’s in-cup flavour & aroma is determined by its physicochemical development during roasting. The coffee’s development depends on the applied time-temperature profile (i.e., rate of heat transfer), which in turn depends on the system’s particle dynamics. Understanding coffee bean particle motion within a roaster is key to the optimization of the applied time-temperature profile and control of the coffee’s development during roasting. Coffee bean particle motion in a rotating drum roaster was tracked using Positron Emission Particle Tracking (PEPT) - a non-invasive technique that can characterise flow behaviour in granular systems. The drum’s rotation speed, coffee’s batch size and bean density (i.e., roast degree) influence particle dynamics and were thus varied to understand their impact on flow patterns. The explored range of product properties and process parameters reflects realistic variations that a roaster might employ. Experimental design followed a 2-factor (batch size and drum rotation speed) full-factorial design carried out for 3 coffees of different density (green, part-roasted and full-roasted). Occupancy profiles revealed a dense bean bed of high occupancy, as well as a dilute region corresponding to beans in-flight. Both Bean Bed Mass Fraction (BBMF) and median particle velocity (vp) were affected by bean density, rotation speed and batch size such that: (i) coffee of greater roast degree (with greater bean volume) decreases vp, yielding a greater BBMF (ii) a larger batch size decreases vp, inducing greater BBMF (iii) a higher rotation speed increases vp, reducing BBMF. PEPT provides real particle motion data for granular systems that can be used for physics-driven models to relate heat transfer and particle dynamics and thus optimise time-temperature roasting profiles.