Effect of thermally induced denaturation on molecular interaction-response relationships of whey protein isolate based films and coatings

The functional properties of whey protein isolate (WPI) based films and coatings have been the subject of several studies. However, quantitative information on the molecular interactions is rare. The solubility studies presented here allow qualitative statements to be made about the molecular interactions in WPI-based films. The other objective of this study was to determine the cross-linking density (CLD). Swelling studies were performed on WPI-based films containing different amounts of denatured WPI. The results of the swelling studies showed that the degree of denaturation has a significant influence on the CLD, which is directly proportional to the number of disulfide bonds in the WPI-based network. As a result, the structural stability of the polymer network was improved. The swelling studies showed that there was a significant linear increase in CLD of 1.17·10−4 mol cm−3 on going from 50% to 100% denatured WPI-based films (0.22·10−4 mol cm−3 → 1.39·10−4 mol cm−3). The solubility study showed that WPI film solubility is mainly influence by non-covalent bonds up to a degree of denaturation of 75%. In general, there was a correlation between CLD and the mechanical properties of the films. The Younǵs modulus increased by 320% on going from 25% to 100% denatured WPI-based films (26.70 MPa → 85.30 MPa). The CLD had only a minor effect on the barrier properties of the WPI-based films. Thus, this scientific paper provides new knowledge for researchers and material developers because qualitative information about the covalent intermolecular interactions in whey protein isolate based films and coatings has been obtained for the first time.