Developing predictive models for determining physical properties of coffee beans during the roasting process

Abstract

This study aims to evaluate and model the variation in the physical properties of coffee beans in isothermal roasting conditions, providing mathematical expressions that can be used for heat and mass transfer models for coffee roasting. Arabica coffee beans were studied with an initial moisture content of 0.129 kgw kgdm−1 and roasted in a direct gas burning roaster. Five temperatures were set inside the cylinder (200, 220, 240, 260 and 280 °C). The beans were roasted uniformly by suspension in the center of the drum. A thermocouple recorded the temperature every 5 s. X-ray microtomography was used to analyze the evolution of the internal matrix during the roasting process. The moisture content and physical properties (volume, surface area, and density) of each coffee bean were evaluated every 20 s. Empirical models were fitted to represent the physical properties as a function of the moisture content. It was observed that the volumetric expansion is isotropic at roasting temperatures above 220 °C. The final bean volume can reach up to 1.8 times the initial volume. The bean density varied linearly with the moisture content, presenting a larger drop at a higher roasting temperature.