Convective drying of fermented Amazonian cocoa beans (Theobroma cacao var. Forasteiro). Experiments and mathematical modeling

Abstract

The general objective of this work is to contribute to the characterization and the mathematical modeling of the convective drying of fermented Amazonian cocoa beans (Theobroma cacao var. Forasteiro). A combined approach is used, including the analysis of the bean physical characteristics, the determination of the bean sorption isotherms, the realization of drying experiments in a convective dryer (for different operating conditions) and the development of a mathematical model, based on the different transport (heat and mass) phenomena involved inside and outside the beans during their drying. The results show that the drying of an Amazonian cocoa bean can be divided in two phases. It is demonstrated that this is due to the combination of two elements. First, the initial volumetric concentration of water is higher in the shell than in the core of the bean. Second, at the beginning of the drying, the drying rate is controlled by both internal and external transport phenomena while, in the second part of the drying (for moisture content of the bean below 0.5kg of water/kg of dry matter), it is totally controlled by the water diffusion inside the bean. The model is compared to the experimental results in order to characterize the effective diffusion coefficient of water inside a bean. It is shown that this diffusion coefficient approximately doubles when the temperature is increased from 30 to 60°C. It is close to 1.5×10−10m2/s at 60°C. Finally, measuring the polyphenol content of the dry beans allows assessing that, regarding chocolate quality, the fermented Amazonian cocoa beans should be dried at 30–40°C.