The role of structural relaxation in governing the mobility of linoleic acid in condensed whey protein matrices

The classical limiting case of simple diffusion as described by Fick's second law was examined in the transport of a small molecule, linoleic acid, through a condensed polymer matrix, whey protein. Experimental protocol was based on small-deformation dynamic oscillation in-shear, wide angle X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, FTIR micro-spectroscopy imaging, ANS fluorescence spectroscopy, and the sulfo-phospho-vanillin assay. This mass transfer problem for the omega-6 fatty acid was examined in relation to whey protein forming a glassy system with a glass transition temperature, T-g, of -16 degrees C. Diffusion followed a more complicated pattern than Fick's equation that could be described at temperatures above T-g with the so-called "anomalous transport". The diffusion coefficient of linoleic acid was estimated within the glass transition region and glassy state of the whey protein network delineated with changing environmental temperature. The free-volume theory of transport was then considered to provide a useful vehicle for rationalising molecular motion and, in doing so, we established a generalised relationship between diffusion coefficient of bioactive compound and fractional free volume of polymeric matrix. (C) 2016 Published by Elsevier Ltd.