Effect of film ageing on the surface properties of lactoglobulin and lactoglobulin plus sucrose stearate monolayers

The change in beta-lactoglobulin and beta-lactoglobulin + sucrose stearate spread monolayers over time (ageing effect) at room temperature was studied. Measurements of the surface pressure pi-mean area per molecule A isotherms of the monolayers were used to probe any time dependent changes. The increase of the protein film area with time was attributed to protein unfolding. At certain ratios of surfactant to protein, a higher film area increase was observed. This correlated well with the excess film areas of 45 h aged films. The excess film areas for newly spread films of the same composition seemed to follow the opposite trend indicating that the protein-surfactant interactions change as the conformational changes of the protein molecules progress. (C) 1999 Elsevier Science B.V. All rights reserved.