Jmr. Patino et al., Structural and morphological characteristics of beta-casein monolayers at the air-water interface, FOOD HYDROC, 13(5), 1999, pp. 401-408
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.