Water vapor transmission rate prediction: The case-study of a biopolymer-based capsule for food applications

The effect of environmental humidity on the moisture barrier properties of biobased packaging remains a critical challenge for shelf life prediction. Indeed, biobased materials often exhibit variable moisture permeability, which typically increases with rising environmental humidity, leading to undesired moisture uptake during storage. In this context, the present study aims to predict the influence of environmental humidity on steady-state water vapor mass flux using a case study of a commercial biopolymer-based capsule designed for packaging dry food powders. To achieve this goal, water vapor sorption tests were conducted across increasing water activity (a_w) levels to estimate the water sorption isotherm and the diffusion coefficient. A sorption apparatus equipped with a climatic chamber that automatically measured the samples’ weight was employed, with equilibrium defined as a mass variation not exceeding 0.01 % over 7 h. Subsequently, the obtained water vapor sorption kinetics were modelled using a non-Fickian approach that simultaneously accounts Fickian diffusion and polymer matrix relaxation. These results were then used to predict the dependence of the water vapor transmission rate (WVTR) on upstream and downstream a_w. To validate the model, WVTR tests were performed at various upstream a_w levels while maintaining downstream a_w at zero. The relative percent difference between predicted and measured WVTR values was 32.2, which is an acceptable value, considering the complexity of the phenomena involved, and the simplicity of the model used.