Structural effects on the permeability of whey protein films in an aqueous environment

Permeability of whey protein isolate films for salts in an aqueous environment is clearly dependent on the concentration of plasticizer. With increase in glycerol concentration from 20% to 40% the diffusion of salt (CaCl2) through the film increases from 2.5×10−10 to 3.6×10−10 m2/s. To describe diffusion experiments with a dye (fluorescein free acid (FFA)), an effective diffusion coefficient which increases in time has to be used: for a whey protein film with 30% glycerol an effective diffusion coefficient, which increases from 8.0×10−12 to 3.2×10−11 m2/s results in an accurate fit with experimental results. The observed increase in the diffusion coefficient is likely due to a microstructural change on the length scale of the diffusing molecule, caused by the washing out of the glycerol.The glycerol content plays an important role in the structure development in the film when the films are produced. Based on SAXS results it is observed that films are formed in a phase-separating process, where a sponge-like structure is formed around glycerol domains. The results imply that the diffusion coefficient for salt in a film is a function of the initial glycerol content while preparing the film rather than the actual glycerol content during the permeation experiment. Furthermore, this sponge-like picture helps to understand the observed linear relation between the elastic modulus (EM) and the tensile strength (TS) of the films.The observations in this paper will contribute to control barriers, based on whey protein isolate, with respect to delayed or prolonged release of ingredients, brought into contact with an aqueous environment.