Molecular mobility and oxygen permeability in amorphous β-lactoglobulin films

An understanding of the solid-state matrix mobility that underlies macromolecular behavior is important to exploit edible film technology effectively. This study used phosphorescence of erythrosin B to monitor molecular mobility in amorphous thin films (∼40 μm) of β-lactoglobulin as a function of temperature. Emission peak energy and bandwidth were determined by fitting emission spectra to a log-normal bandwidth function; peak emission energy gradually decreased with temperature indicating that the matrix dipolar relaxation rate increased with temperature. Phosphorescence lifetimes were determined from stretched exponential analyses of emission decays in air and in nitrogen; lifetimes were interpreted in terms of the rate constants associated with the various processes that contribute to de-excitation of the probe's excited triplet state. The lifetime in nitrogen decreased gradually at low and more steeply at high temperature indicating that matrix collisions increased with temperature. The lifetime was identical in air and in nitrogen at -20 °C but was lower in air at all higher temperatures indicating that oxygen quenching of the triplet state was activated at higher temperatures. The oxygen-quenching rate, which reflects oxygen permeability, was directly proportional to the matrix collision rate over the entire temperature interval from -20 to 120 °C.