Food drying

In the context of increased global competition in the food drying business, staying at the state of the art with respect to research and innovation is proving crucial.[3,4,8] Lately, following consumer trends, focus has been set on developing mathematical models allowing to model shrinkage of fruits and vegetables during drying.[1,6,7,9-17]

In this research project, the modelling of potato shrinkage during convective drying was considered using COMSOL Multiphysics software to implement the equations. Furthermore, experiments were carried out in a drying tunnel at different temperatures and air velocities of 1.5m/s with relative humidity of 25% in order to verify the model output experimentally. During the experiments, weight and colour were measured at regular intervals. Colour was measured with a Hunterlab Miniscan EZ colorimeter that uses the CIELAB colour scale. The geometry used for the potato slabs was of 4.5cm diameter and 1.5cm height cylinders. This geometry was chosen in order to enable colour measurements throughout shrinkage as the cylinder’s top surface –taken as reference for colour measurements– diminishes in size.

The mathematical model was developed in 2D, using axial symmetry to extend it to 3D. The GAB isotherm was used to calculate equilibrium moisture content, while volumetric shrinkage was assumed to be linearly proportional to moisture content decrease, following results from research conducted by other authors.[1,7,16] The velocity profile was approximated as constant on top of the cylinder, with a decrease along the lateral side to zero value at the bottom edge. Mass transfer coefficients were calculated based on a proportionality relation with the heat transfer coefficients from Rovedo et al., initially proposed by Blackwell et al., while the heat transfer coefficients were obtained based on adimensional numbers of Nusselt, Reynolds and Prandtl.[17]

Future work should include an extension of the model to 3D and a more accurate description of the volumetric shrinkage, taking into account modification of porosity inside the solid as a function of time, coupled with moisture changes. Additionally, volume measurements should be carried out with the help of imaging techniques, as has been done by other authors previously.[2,16] This would help to fine-tune the model leading to a more accurate description of reality.

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