Morphological changes in adsorbed protein films at the oil-water interface subjected to compression, expansion, and heat processing

Adsorbed films of milk proteins at the oil-water (O-W) interface have been imaged using a Brewster angle microscope (BAM). Special adaptations were made to the BAM to allow imaging of the O-W interface and to enable in situ heating and cooling of the adsorbed films. The proteins beta-lactoglobulin (beta-L) and alpha(s1-), beta-, and kappa-casein were studied over a range of bulk protein concentrations (C-b) and surface ages at pH 7 and for beta-L at pH 5 also. The adsorbed films were subjected to incremental compression and expansion cycles, such that the film area was typically varied between 125% and 50% of the original film area, and the resulting film structure was recorded via the BAM at 25.0 C. Structuring of beta-L films (the formation of ridges and cracks) was more pronounced at pH 5 (closer to the protein's isoelectric point) than at pH 7 and for longer adsorption times and/or higher C-b. Structuring was also much more apparent at the O-W interface than at the A-W interface on compression/expansion/aging, especially at pH 7. After heating beta-L films adsorbed at low C-b (0.005 wt %) to 80 or 90 degrees C, an even greater degree of film structuring was evident, but beta-L films adsorbed at higher C-b (>= 0.05 wt %) showed fewer but larger fractures. The adsorbed caseins showed little evidence of such features, either before or after heating, apart from slight structuring for the heated films of alpha(s1-) and kappa-casein films after 1 day. Changes in the dilatational elastic modulus of the beta-L films (C-b = 0.005 wt %) were correlated with the variations in the structural integrity of the films as observed via the BAM technique. In particular, there was a marked increase in the elastic modulus on heating, while the cycle of compression and expansion appeared to result in a net film weakening overall. The beta-L films adsorbed at higher C-b (>= 0.05 wt %) behaved as if an even stronger elastic skin completely covered the interface. The overall conclusion is that interfacial protein films subjected to these types of thermal and mechanical perturbations, which are typical of those that occur in food colloid processing; can become highly inhomogeneous, depending on the type of protein and the bulk solution conditions. This undoubtedly has implications for the stability of the corresponding emulsions and foams.