Diffusion coefficient modelling of natural bioactives from crosslinked hydrocolloid matrices under moving boundary conditions
Creators: name: Whitehead, Felicity
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 300602
subject: Food chemistry and food sensory science

description: Increased bioactivity and bioavailability of bioactive compounds encapsulated/entrapped within food-based biopolymer systems is a recent focus of food science research. This can be achieved in crosslinked biopolymer systems which retain and protect the bioactive molecule during processing, transport and storage and can deliver the active agent to the target location within the body. Controlling bioactive compound release kinetics is important in developing effective delivery devices for functional foods and nutraceuticals. This work extends research in this field to model the diffusion kinetics of bioactive compounds in swelling biopolymer systems at distinct pH conditions of industrial relevance in relation to product development.

In the first experimental chapter, swellable genipin-crosslinked gelatin matrices with intermediate solids content were used to control the delivery of ascorbic acid. A range of degrees of crosslinking within the gelatin networks were induced by altering crosslinker concentration, which resulted in variations in microstructural parameters of the crosslinked networks, including crosslink density, network mesh size and average molecular weight between crosslinks. Physicochemical characteristics of the network were observed during polymer gelation and were quantified based on crosslinker concentration. Swelling and diffusion studies revealed initial Case I Fickian transport kinetics, which experienced a considerable decrease as the system approached thermodynamic equilibrium. Relative rates of water adsorption into the system and ascorbic acid diffusion revealed the apparent infusion coefficient of water as the rate-limiting process in the delivery vehicle. Strong evidence that modulation of the extent of biopolymer crosslinking impacts hydrogel morphology and structural characteristics proves promising in controlling the release kinetics of entrapped vitamins.

In the second experimental chapter, genipin-crosslinked gelatin networks were analysed over a wide range of concentrations. Swelling of the initially high-solid system was followed and the modified Flory-Rehner theory was used to predict changes in average molecular weight between crosslinks and network mesh size. Genipin-crosslinked gelatin was prepared with a wide range of solids contents, mimicking changes in the network throughout the swelling process. Physicochemical and thermomechanical analyses of these systems revealed a single-polymer composite system with two thermodynamically distinct structural knots. The amorphous nature of the system allowed estimation of thermal glass transition temperatures from modulated differential scanning calorimetry results. These were compared with mechanical glass transition temperatures obtained from rheological measurements treated with the combined framework of free volume and reaction rate theories.

An alternative biopolymer system was introduced in the third experimental chapter, where genipin-crosslinked chitosan matrices were prepared encompassing a range of degrees of crosslinking and biopolymer concentrations. Chemical crosslinking within the biopolymer networks was confirmed by molecular miscibility in Fourier transform infrared spectroscopy measurements and by amorphicity of the systems as determined by wide angle X-ray diffraction tests. The modified Flory-Rehner theory was used to predict swelling-induced changes in network mesh size and average molecular weight between crosslinks in the genipin-crosslinked chitosan hydrogels. Network behaviour during biopolymer swelling was modelled using genipin-crosslinked chitosan dried to a range of total solids. From these samples, complementary mechanical glass transition temperature at intermediate solids and thermal glass transition temperature at high solids were measured. Manipulation of hydrogel properties by network crosslinking highlighted the suitability of this system as a delivery vehicle of bioactivity.

Finally, genipin-crosslinked gelatin and chitosan hydrogels were prepared and network swelling and bioactive compound release were observed at distinct pH conditions of commercial interest for the diffusion of bioactive compounds in the human GI tract. Changes in microstructural properties of the networks were dependent on the pH of the swelling environment in relation to the isoelectric point of gelatin in the genipin-crosslinked gelatin matrices and on the pKa of chitosan in the chitosan-based systems. Mathematical modelling was applied to ascorbic acid diffusion data and revealed the Peppas-Sahlin model and zero-order equation as models that statistically best described the kinetics of bioactive compound release. Diffusion from the gelatin-genipin network was entirely diffusion controlled at low pH, whereas at neutral pH there was an increased contribution of structural relaxation to the overall vitamin release mechanism. Ascorbic acid release from genipin-crosslinked chitosan was characterised by bioactive compound diffusion at both pH conditions.

The final chapter of the thesis highlights the contribution of this work, focusing on the network parameters that affect kinetics of biopolymer swelling and bioactive compound diffusion. Comprehensive characterisation of physicochemical, thermomechanical and microstructural properties of the genipin-crosslinked gelatin and chitosan networks reveal their suitability as bioactive compound delivery devices. Diffusion studies on these systems at distinct pH highlight the extent to which vitamin release may be controlled by manipulation of network parameters and conditions of the release environment. These results are encouraging for functional food and nutraceutical applications in the manipulation of bioactive compound release kinetics from crosslinked biopolymer networks controlled by swelling and environmental pH conditions.