Water Vapour Transport in Biopolymeric Materials: Effects of Thickness and Water Vapour Pressure Gradient on Yeast Biomass-Based Films

Biopolymer-based films are hydrophilic biodegradable matrices that exhibit poor water vapour barrier. Ideally, water vapour permeability does not depend on the water vapour pressure gradient and film thickness. However, hydrophilic films interact with water and this affects the solubility and diffusion of water in the matrix. This may produce anomalies such as the dependence of permeability on the thickness and the water pressure gradient. In this work, the water transport through yeast biomass-based films, as an example of a biopolymeric matrix, was studied against changes in film thickness and water pressure gradients. Water sorption isotherms and water vapour permeability measurements allowed the evaluation of water solubility and diffusion as separate phenomena that contribute to permeability. Solubility and diffusion turned out to be strongly dependent on the water pressure gradient. Permeability revealed a non-linear behaviour as a function of pressure gradient, which was mainly due to the contribution of the water solubility. At all pressure gradients, the thickness increase produced a linear growth of the permeability, which was a consequence of the increase in diffusion. The thickness effect observed even at low water vapour pressures advised that, in addition to the effect of the matrix affinity to the water, the formation of a different structure produced at different thicknesses after casting must also be considered to explain the phenomenon. Results revealed the importance of both the assessment of environmental conditions and film thickness during end-use applications to optimize the performance of biopolymeric matrices.