Diffusion and relaxation contributions in the release of bioactive compound from a moving boundary of genipin-crosslinked biopolymer matrices

The search for the efficient release of bioactive compounds from food materials has pushed the industry to develop strategies for encapsulation, preservation, and delivery of micronutrients to the gastrointestinal tract. Nevertheless, the kinetics of controlled release of bioactivity in simulated release media pose a challenging hurdle to overcome. Meaningful predictions of the release mechanism are required for successful formulation engineering. Recently, there has been an increasing accumulation of data on addressing aspects of controlled delivery under conditions of a stationary boundary, which provide an initial approach to the subject. This PhD study aims to extend the recent advance into model systems undergoing controlled swelling due to adsorption of water or other biological fluids into the carrier matrix hence creating moving boundary conditions on the diffusion kinetics of microconstituents. The relative contribution of the principal mechanisms governing micronutrient transport from the swellable carrier matrix is then explored. It also focuses on how the morphological characteristics of the carrier matrix play a critical role in facilitating carrier's network design by identifying key parameters and molecule-release mechanisms. Therefore, a prediction of the release behaviour of micronutrients based on theories of network formation and mathematical modelling would facilitate the development of tailored delivery systems for specific applications in added value food and nutraceutical industries. <br><br> In the first experimental chapter, gelatin molecules were crosslinked with genipin to design a novel delivery system. Successful crosslinking created excipients of a variable degree of swelling thus allowing a reliable estimation of morphological parameters including the molecular weight between adjacent crosslinks and the network mesh size via the Flory-Rehner theory. The swellable genipin-crosslinked gelatin matrices control the delivery of vitamin B6 and the morphological characteristics of the matrix impact on release kinetics of vitamin B6. In addition, the moving boundaries associated with swelling of the gelatin matrix resulted in anomalous vitamin transport that was described with the interplay of diffusional and relaxational kinetics via the Peppas-Sahlin equation.<br><br> In the second experimental chapter, research was advanced to understand the effect of the network morphology of genipin-crosslinked bovine serum albumin (BSA) on the controlled release of vitamin B6. Up to ninety percent crosslinking was achieved and electron microscopy images showed a reduction in the size of the matrix pores with a higher degree of crosslinking. The hydrophilic nature of the BSA-genipin matrix allowed swelling with water adsorption, which was monitored with the modified Flory-Rehner theory to predict the molecular weight between adjacent crosslinks, network mesh size and crosslinking density as a function of crosslinker addition. Results obtained theoretically and experimentally confirm that mass transport of the vitamin is governed by steric constraints of the changing network morphology with the degree of crosslinking. The work also showed the importance of relaxational contributions for vitamin release in this delivery vehicle of moving boundaries.<br><br> The previous results led us to develop the next phase of the work to provide insights into the effect of pH on the changing microstructural characteristics of swollen BSA networks and the concomitant release profiles of an entrapped bioactive compound. Transportation of entrapped vitamin B6 from crosslinked BSA networks was followed over long periods of observation in release media of acidic, neutral and alkaline conditions. Utilisation of diffusion theory predicted the apparent diffusion coefficient and diffusion exponent for the release of the bioactive compound, which was correlated to the mesh size of the BSA network in the crosslinked protein network via the change in pH.<br><br> The last experimental phase of the work presented in this thesis (chapter 6) explored the diffusion patterns of vitamin B6 from a high solid genipin-crosslinked whey protein isolate matrix. The morphology of WPI matrices at various degrees of crosslinking was examined, and values of the diffusion exponent emphasise the critical role of swelling in the anomalous diffusion of the bioactive compound. Combination of diffusion and swelling equilibrium theories revealed a measurable effect of the WPI network characteristics on vitamin B6 release, consistent with the results obtained from chapters 3-4.<br><br> The final chapter of the thesis concludes by highlighting the contribution of this work that focuses on the morphological characteristics of the swellable biopolymer networks affecting significantly the diffusion kinetics of bioactive compounds. A clear relationship was developed between network mesh size and rapid diffusion kinetics via the apparent diffusion coefficient, which was controlled by the degree of crosslinking. This finding constitutes an original attempt to develop a mathematical expression for characterising the mobility of natural bioactive compounds in relation to molecular structure of the swellable biopolymeric matrices for food use. Moreover, this new mathematical equation would be useful to identify the extent of coupling between a tracer molecule and a swellable crosslinked biopolymer matrix.