Numerical Evaluation of the Temperature Field inside Specialty Coffee Beans during Microwave Drying

The drying process that is responsible for ensuring the coffee quality, reduces the moisture of the coffee bean thus avoiding unwanted microorganisms. Traditional drying methods take between 16 and 45 hours to dry coffee beans, which results in high operational costs. Therefore, the application of the microwave drying technique is a possible alternative, as it can reduce drying periods by four times. In this study, the main goal was to evaluate the heating kinetics and distribution of a single coffee bean due to microwaves to verify its heterogeneity. Initially, a single coffee bean was designed and inserted into a monomode microwave cavity. The geometry of the coffee bean was simplified as a semi-ellipsoid with its diameters measured experimentally and with four different curvatures at the edges. As a result of the numerical simulations, the temperature distribution on the coffee bean over time was obtained as well as the average temperature and temperature at its center. The heating kinetics plot described a linear curve for every geometry, having a higher inclination in the sharp edge and decreasing as the curvature increases. Also, when comparing the average and center temperatures, the temperature in the center is 107.58 °C and the average temperature of the coffee bean is 96.09 °C, which corroborates the fact that the microwave heating occurs from inside out. In conclusion, the microwave heating in coffee beans is heterogeneous, starting in its center, favoring the mass transfer phenomenon during drying and can be influenced by the sharpness of the coffee beans edges.