Effect of co-solute concentration on the diffusion of linoleic acid from whey protein matrices

Structural relaxation of a delivery vehicle has been identified in the literature as a limiting parameter for the controlled diffusion of a bioactive compound. To further examine the kinetics of controlled release, this study designed a system of homogeneously dispersed linoleic acid in a high-solid whey protein matrix. Amorphous glucose-syrup liquid was added systematically up to about 48% (w/w), with the total solids level in the formulation being kept constant at 80% (w/w), and experimental observations were obtained over a broad timeframe and temperature range. Structural and physicochemical properties of the blend were monitored using dynamic oscillation in-shear, WAXD, FTIR and confocal laser scanning microscopy. A specific chromogenic reaction with UV-vis spectroscopy was employed to follow the release of linoleic acid under perfect sink conditions. Variation in the amount of glucose syrup, as the co-solute, within the high-solid whey protein network affected profoundly the values of the mechanical glass transition temperature. That was reflected in the kinetics of fatty acid diffusion from the glass transition region of our composite materials. Transport phenomena were quantified via the concept of effective diffusion coefficient, which increases with higher levels of glucose syrup leading to structural relaxation of the delivery vehicle.