Evolution of shrinkage, real density, porosity, heat and mass transfer coefficients during indirect solar drying of cocoa beans

Abstract

Drying is a main process of conservation used for cocoa beans and it is so essential to control its drying parameters. In this study, shrinkage, density, porosity, and heat and mass transfer coefficients of cocoa beans during indirect solar drying were investigated. The results showed that shrinkage and porosity increased with decrease in reduced moisture content. The real density varied during drying process. Its value decreased from 825.10 kg/m3 at the beginning of the drying to about 696.25 kg/m3 at the end of the drying process. The cocoa beans had a final porosity approaching 25% and most of its removed water during drying was replaced by gas. Also, the values of heat and mass transfer coefficients increased from 1.92 × 10−4 to 8.08 × 10−2 W/m2 K and from 1.88 × 10−7 to 7.88 × 10−5 m/s, respectively, for an indirect solar drying of cocoa beans. The effective moisture diffusivity was influenced by shrinkage. Effective moisture diffusivities were calculated by Fick’s diffusion law and their values varied from 5.49 × 10−10 to 4.26 × 10−10 m2/s. The cocoa beans thermophysical properties were obtained and fitted to nonlinear correlations, describing their behaviour as a function of moisture content. Keywords: Shrinkage, Porosity, Density, Heat and mass transfer, Cocoa beans