Quantifying Microstructure to Better Control Bioactive Compound Delivery in Protein-Based Systems from Low- to High-Solid Preparations for Improved Human Health

Proteins have potential to form the next generation of delivery vehicles for functional food and nutraceutical applications. Improved water solubility, biocompatibility and non-toxicity make them an attractive prospect for a health-conscious society. Research unveils these biopolymers as efficient encapsulators of bioactive compounds for controlled release, however, much of the literature does not explore the microstructural properties and physical mechanisms governing release from such systems. Of particular interest is the role of the aqueous solvent in controlling small molecule diffusivity. At a low level of solids, the presence of solvent alters the physical landscape of the protein and defines critical parameters such as crosslink density, mesh size and intermolecular coupling constant as tuneable properties to control release. As the level of solids increases, the landscape again shifts. Here, protein molecules can be treated using the free volume theory to ascribe a link between the mechanical glass transition temperature and bioactive compound release. While the focus of this review is on proteins, the industrialist must also consider protein and polysaccharide mixtures, as they closely resemble industrial formulations. Here, we demonstrate how the use of fundamental rheology-based blending laws provides a mechanistic understanding of these composite gels in relation to bioactive compound diffusion.