Unraveling molecular interactions in whey protein-kefiran composite films to understand their physicochemical and mechanical properties

To enhance health promoting properties of edible whey protein films, the inclusion of kefiran to develop composite films was studied. Physicochemical and mechanical properties of films prepared with different wpi:kefiran ratios, different percentage of plasticizer and subjected or not to a heat treatment were analyzed. The inter- and intra-molecular interactions produced by the incorporation of kefiran into whey-protein films were studied to understand the effect exerted by kefiran on the protein network that may affect the properties of the films. Differential-scanning-calorimetry and Fourier-transform–infrared-spectroscopy studies revealed that kefiran does not affect proteins denaturation, but rather modifies protein-protein interactions in the films, exhibiting a change in wpi secondary structure and a drop in films' melting temperature. Composite wpi:kefiran films had higher opacity, lower water content and lower water activity compared to single wpi films. Incorporation of kefiran into the films decreases tensile strength and elastic modulus and produces an increment in the elongation at break value. It was confirmed that the addition of a whey-protein denaturation step produces an improvement in the protein network that is crucial for the mechanical behavior of the films. Understanding the molecular interactions within composite films greatly aids in obtaining novel formulations with tailor-made physicochemical properties.