Structural and morphological characteristics of beta-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 o f beta-casein monolayers at the air-water interface as a function of subpha se 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 wa s 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 thicknes s and film isotropy. A method was applied to measure the relative reflectiv ity and relative film thickness. The relative reflectivity versus surface p ressure plots reflects the surface equation of state for the spread beta-ca sein monolayer, and is particularly sensitive of the existence of transitio ns 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 con sequence of the more compact packing of beta-casein residues on acidic subp hases. (C) 1999 Elsevier Science Ltd. All rights reserved.