The role of vitrification on the molecular transport and oxidation of bioactive compounds in condensed systems of natural polymers

The utilisation of the glass transition concept to understand the delivery of bioactive compounds in functional foods has received considerable attention. The concept reveals a temperature prediction at which a reversible transition occurs between the glassy and rubbery state of the material. Prediction of bioactive molecular mobility within these two stages helps ensure the stability of the system. The properties of the selected food biopolymer determine the system's structural features, leading to different physicochemical properties. The phase transformation of matrix biopolymers in the food systems becomes important in controlling the kinetics of the entrapped bioactive compounds within the amorphous substances. Hence, estimating the glass transition temperature of polymeric matrices as delivery vehicles for entrapped bioactive compounds remains essential. This work encompasses research on the determination of glass transition temperature of various high solid systems comprising native food biopolymers as delivery vehicles for bioactive compounds, followed by an investigation of their role in affecting different molecular processes, i.e. diffusion and lipid oxidation. Characterisation of the biopolymers and bioactives' molecular properties during their phase transition utilised multiple techniques, such as Fourier transform infrared spectroscopy, scanning electron microscopy, wide-angle X-ray scattering, modulated differential scanning calorimetry and small-deformation rheology. In addition, a study on the kinetics of several bioactive compounds was conducted as a function of time and temperature using visible spectrophotometry protocols.  In the first experimental chapter, the work focused on the estimation of the glass transition temperature of condensed gelatine/glucose syrup mixtures and its relationship with the diffusion kinetics of nicotinic acids. The system used two types of gelatin, i.e. bovine and fish gelatin, with different molecular weights. Physicochemical analyses were carried out to allow the determination of the calorimetric (Tgc) and the mechanical (Tgm) glass transition temperatures, which were then related to the micro and macromolecular properties of these systems. An experiment using UV-vis spectroscopy was also performed at a temperature range of 5-40⁰C to investigate the effect of calorimetry and mechanical Tg values on the release of nicotinic acid within high-solid bovine and fish gelatin/glucose syrup matrices. This resulted in strong evidence to indicate that the molecular weight of biopolymer affects the structural characteristics and determines the glass transition temperatures of condensed systems. It was determined that mechanical Tg appears to be the main driver behind the release kinetics of the entrapped vitamin when compared to its calorimetric counterpart. In the second experimental chapter, a model of high solid systems consisting of κ-carrageenan/polydextrose was proposed to investigate the influence of counterion on the glass transition value of the mixtures and its effect in controlling the delivery of caffeine. A series of potassium chloride (KCl) concentrations were used to evaluate the structural properties’ dependency on potassium ion concentrations. A combination of calorimetric, mechanical and UV spectroscopy was used in the working protocol to correlate the relationship between the glass transition concept and the diffusion of the bioactive compound. Furthermore, the physicochemical properties of the polymeric matrix were also investigated to serve a fundamental insight into the molecular mechanisms involved during vitrification. The prominent effect of the mechanical over the calorimetric glass transition temperature in governing the release of the bioactive compound was confirmed in this work. In addition, the contribution of potassium counterion to stabilize the κ-carrageenan helices was shown to significantly impact the decoupling effect between matrices mobility and the release of the bioactive compound. The significance of the glass transition temperature concept on a different molecular process was introduced in the third experimental chapter, where a range of κ-carrageenan and glucose syrup matrices were prepared with high solid content to control the oxidation of linoleic acids. A series of measurements using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), modulated differential scanning calorimetry (MDSC), and small deformation dynamic oscillation in shear was performed to determine the polymeric matrix's characteristics. Monitoring the accumulation of hydroperoxide (ROOH) in the vicinity of the estimated Tg of the systems (-25 to 0°C) was performed using UV-vis spectroscopy. A sigmoidal kinetic model was proposed to fit the oxidation data, which represents the initiation and propagation phases of lipid oxidation. The role of Tgm in the rate of ROOH formation (k_f) and decomposition (k_d) in the polymeric systems was also correlated to Tgc. The important role of mechanical glass transition temperature was confirmed in this work, as evidenced by the drop of the oxidation rate constants at a temperature close to the estimated Tgm. Finally, in the fourth experimental chapter, gelatin mixed with polydextrose was prepared as a matrix to entrap linolenic acid.  The effect of glass transition temperature on fatty acid oxidation was observed by tracking the development of its primary products within the system. Transformation in the microstructural properties of the polymeric networks relied on the concentration of genipin as a crosslinker added to the systems. Evaluation of the lipid oxidation data was performed using mathematical modelling and showed that statistically, the sigmoidal model best described the kinetics of the fatty acid oxidation. The oxidation of linolenic acids from the gelatin/polydextrose network was shown to be entirely controlled by the Tgm as opposed to its Tgc counterpart, which agrees with the outcomes obtained in the preceding work.The contribution of this study was highlighted in the final chapter of the thesis, focusing on the determination of and comparison between mechanical and calorimetry glass transition temperatures in controlling the molecular mobility of biopolymer systems as a delivery vehicle for bioactive compounds. Comprehensive characterisation of the biopolymers' macro and microstructural properties as well as their physicochemical characteristics confirm the molecular interaction of the systems. Diffusion and lipid oxidation studies on the systems with various combinations of biopolymers/co-solute matrices with distinct glass transition temperature values from two different approaches (rheology and calorimetry), highlights the extent to which the molecular processes within the systems are governed by the Tgm as opposed to Tgc.