Structural and morphological characteristics of β-casein monolayers at the air-water interface

Brewster angle microscopy (BAM) coupled with surface pressure (pi)-area (A) isotherms was used to visualize and determine structural characteristics of beta-casein monolayers at the air-water interface as a function of subphase pH. The measurements were performed at 20 degrees C. From the results of pi-A isotherms, it can be concluded that beta-casein monolayers at the air-water interface adopt two different structures. The monolayer structure was more condensed on acidic aqueous solutions. From BAM images, the domains that the residues of beta-casein molecules adopt at the air-water interface appeared to be of uniform reflectivity, suggesting homogeneity in thickness and film isotropy. A method was applied to measure the relative reflectivity and relative film thickness. The relative reflectivity versus surface pressure plots reflects the surface equation of state for the spread beta-casein monolayer, and is particularly sensitive of the existence of transitions in beta-casein monolayers with compression. The relative film thickness increased with the film compression and was a maximum at the collapse point. At the same surface pressure, the relative film thickness is independent of the pH, but the surface density is higher at pH 5 than at pH 7, as a consequence of the more compact packing of beta-casein residues on acidic subphases. (C) 1999 Elsevier Science Ltd. All rights reserved.