Oxygen permeation through an oil-encapsulating glassy food matrix studied by ESR line broadening using a nitroxyl spin probe

A non-invasive method based on the broadening of electron spin resonance (ESR) lines in the presence of oxygen (oximetry) has been developed to determine the rate of permeation of oxygen from head space into oil, encapsulated in a glassy matrix (a food model made from sucrose, maltodextrin and gelatine by freeze-drying). The lipophilic nitroxide 16-doxylstearic acid, 16-DSA, was used as a spin-probe, and it was found to be concentrated mainly in the oil phase in the glassy matrix. The concentrations of oxygen in the freshly made glasses were found to be similar to the concentration in atmospheric air, and the process of freeze-drying is apparently not able to remove oxygen before the glassy system solidifies. Storing the oil-encapsulating glasses under oxygen increased the oxygen concentration inside the matrices, and the rate of permeation was found to increase with temperature. A kinetic model for the oxygen permeation was established, based on the rate data obtained up to full saturation of the oil with oxygen below the glass transition temperature (Tg=65°C ), and on data for partial oxygen saturation above the glass transition temperature. The kinetic model includes a temperature independent master curve and allows for structural heterogeneity. The energy of activation for oxygen permeation was found to be 74±6 kJ/mol for the glassy matrix, and the large value is in favour of the molecular model for oxygen diffusion rather than the free volume model, and accords with the zeroth-order kinetics for oxidation of lipids encapsulated in a glassy matrix, which has previously been observed to be associated with oxygen permeation as the rate-determining step.