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Coffee bean drying shrinkage comparison by finite element simulations and real image processing

Czech University of Life Sciences Prague, Fac. of Tropical AgriSciences - Dept. of Economics and Development, Czech Republic

Abstract

One of the most important physical properties of the coffee grain to consider when drying is the surface area, seeing that the moisture diffusion is a function of the size of the grain. Also, the coffee bed represents a porous volume throughout which the drying air circulates, such porosity is also given by the seed’s dimensions and void spaces within the grain layer. When the coffee bean is dried, a volumetric shrinkage occurs, and even though its change can be easily measured in terms of length, width and depth, the surface area variations must be approximated since the bean’s shape is irregular. Contemplating this, three wet coffee grains (Coffea arabica) were 3D scanned and digitised, upon which a transient mass diffusion finite element method (FEM) simulation was performed from an initial moisture content of 55% (w.b) until reaching the required moisture content of 10% (w.b). The superficial area of the three grains was evaluated at different moisture content stages during the drying simulation process, so the ongoing shrinkage could be recorded. Afterwards, real coffee grains at 55% (w.b) were laboratory-dried, keeping the same parameters as the simulation (air temperature, relative humidity, airflow, and direction); samples were removed at different moisture content levels and their parchment was withdrawn and spread on a cover glass; their image was taken and subsequently processed to obtain the actual surface area. Both the FEM simulation and the image processing results were compared, finding a level of similarity of 96.5%, where an average 7.3% volumetric shrinkage was observed. At the same time, the drying time was compared, displaying that both results shared an accurate and high similarity.

Keywords: Coffea arabica, coffee drying, image processing,mMass diffusion, seed shrinkage

Contact Address: Eduardo Duque-Dussan, Czech University of Life Sciences Prague, Fac. of Tropical AgriSciences - Dept. of Economics and Development, Kamýcká 129, 16500 Prague, Czech Republic, e-mail: duque_dussanftz.czu.cz