Abstract
A recently proposed moving-boundary model for food isothermal dehydration was applied to analyze the dehydration kinetics of ellipsoidal cocoa beans, characterized by a moderate shrinkage and a non-uniform initial distribution of water content between the core and the shell of the bean. The aim is to predict the influence of air velocity and non-uniformity of the initial water distribution on the dehydration rates, as well as the temporal evolution of the water content in the core and in the shell and of the characteristic lengths of the ellipsoidal bean. The model proved capable of accurately describing the two-phases dehydration process: an initial fast dehydration of the shell, characterized by higher dehydration rates, followed by a slower dehydration of the core, characterized by a linear relationship j d = δ ( T ) X r between the dehydration rate j d and the moisture ratio X r . A shortcut method to estimate the effective water diffusivity D is also proposed, deriving from the basic observation that the asymptotic exponential behaviour of the dehydration curve X r ( t ) for an ellipsoidal bean coincides with that of an equivalent sphere, with the same surface-to-volume ratio.
Highlights
The cocoa beans extracted from the fruit of the cocoa tree (Theobroma cacao, see Figure 1) are the raw material for chocolate production [1]
The drying operation can be provided by two methods: natural drying [7], when cocoa beans are spread on a support and exposed directly to sun, or artificial convective drying [3], in which cocoa beans are placed in a dryer and exposed to an air flow with controlled velocity, temperature and humidity
The water content in the core and the shell can be very different and this may have a strong influence on the dehydration kinetics that is accompanied by a moderate volume reduction, about 30% of the initial volume
Summary
The cocoa beans extracted from the fruit of the cocoa tree (Theobroma cacao, see Figure 1) are the raw material for chocolate production [1]. In [9,10], Herman et al investigate the temporal evolution, during convective drying, of the water content in the core and the shell of fermented Amazonian cocoa beans, in order to better characterize the drying kinetics and to investigate the influence of operating parameters, namely the air velocity and the drying temperature. The moving-boundary model is applied to 3-d ellipsoidal beans, accounting for the specific internal structure of the bean (core and shell), the non-uniformity of the initial water content and the volume shrinkage in order to better understand and to accurately describe the two-phases dehydration process of cocoa beans, as observed by Herman et al [9,10]: an initial fast dehydration of the shell, characterized by higher dehydration rates, followed by a slower dehydration of the core, characterized by a linear relationship between the dehydration rate jd and the moisture ratio Xr. The article is organized as follows. (ii) an accurate prediction of the influence of air velocity and non-uniformity of the initial water distribution on dehydration kinetics; (iii) a correct description of the temporal evolution of the moisture content and thickness of core and shell
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