Kinetics of food biopolymer film dehydration: Experimental studies and mathematical modeling

The dehydration process in thin biopolymer-based films, involving the two main macromolecules in food systems, namely, proteins and polysaccharides, has been investigated. Experimental measurements of the weight loss of polymer films were carried out in an enclosed Perspex chamber under controlled conditions of temperature, relative humidity, and air flow. We found that all of the experimental moisture ratio data for films of pure biopolymers, as well as their mixtures, can be superimposed on a single scaled dehydration curve. According to theoretical calculations, this curve cannot be described by a moisture diffusion-dominated drying process, irrespective of whether film shrinkage is included or ignored. We also derived an analytical expression for evaporation-controlled drying. When film shrinkage is fully taken into account in the calculations, very good agreement between the experiments and the theoretically derived curve is obtained. However, the theoretical results can be improved even further by considering a model that includes both diffusion and evaporation processes, with diffusion being fast but nonetheless finite, so as to only play a minor secondary role in the dehydration of such films. Theoretical models have also been extended to describe systems in which film shrinkage can also arise from changes in the partial molar volume and not just loss of moisture. Using a variable "effective" partial molar volume for water provides a possible method for modeling the entire drying process over different stages of dehydration in which the solid phase adopts significantly contrasting structures.