Structural and dynamic properties of milk proteins spread at the air-waterinterface

We have studied the effect of monolayer structure on dilatational character istics (surface dilatational modulus and its elastic and viscous components ) of protein monolayers ( beta -casein, caseinate, and whey protein isolate (WPI) spread on the air-water interface, at 20 degreesC, and at pH 5 and 7 . The stress response to compression-expansion sinusoidal deformation of th e interface in a modified Wilhelmy-type trough with two oscillating barrier s was measured as a function of deformation amplitude (within the range of 1-20% of the initial area), frequency (within the range of 1-300 mHz), and superficial density (within the range of 1-8 mg/m(2)). The same experimenta l device coupled with Brewster angle microscopy makes it possible to determ ine the structure, morphology, and relative reflectivity of the monolayer. The monolayer structure and, especially, the conditions at which the monola yer collapses determine the viscoelastic behavior of the monolayer and the linear response of the stress to area deformation. The nonlinear viscoelast ic behavior of the interface has been associated with the protein monolayer collapse. It was found not only that the dilatational modulus is determine d by the interactions between spread molecules (which depend on the surface density) but also that the structure of the proteins spread on the monolay er also plays an important role. (C) 2001 Academic Press.