Experimental and mathematical investigation of mass transfer in food andhydrogel systems using magnetic resonance imaging and NMR relaxometry

Çıkrıkcı, Sevil
Nuclear magnetic resonance (NMR) and Magnetic Resonance Imaging (MRI) are well-known non-invasive characterization methods used in a wide range of areas; from medical to food applications. NMR experiments are conducted either through spectroscopy with high resolution systems or with relaxometery (Time Domain NMR) through mid or low field systems. Time domain NMR is primarily based on relaxation times and diffusion measurements from the signal coming from the whole sample while MRI enables to visualize the inside of the materials on a macroscopic scale without disturbing the sample based on the differences on relaxation and diffusion. For biological imaging, proton NMR ( ıH) is used mostly. Since the signal comes from protons, this technique could easily be employed to monitor different transport processes in food systems that include moisture or oil transport. In this dissertation, model food and gel systems were selected and mass transport was analyzed through both NMR and MRI in which mathematical models of the transport were developed and validated. Monitoring oil migration from hazelnut paste layer to chocolate layer (sweetened with sucrose, stevia, splenda or with their combinations) and developing the mathematical vi models were the first section of the study. Oil migration is a common problem in chocolate confectionery products leading to quality defects, particularly fat bloom. So, knowing the migration model for the product could have enabled us to predict the shelf life of the chocolate. In the second section, a hydrogel system was selected as the system of interest and alginate-gum tragacanth (ALG-GT) hydrogels at different ALG replacement ratios were designed for controlled release of insulin in simulated gastrointestinal (GIT) conditions and characterized by NMR/MRI to analyze mass transfer and waterpolymer, polymer-polymer interactions. Since insulin is a therapeutic protein, it could not have given such a high signal to be observed in MRI but insulin was confirmed by NMR spectroscopy and its interactions were studied by NMR relaxometry. When hydrogels are placed in a solution, they usually respond to the environment by swelling and thus modelling mass transport becomes challenging due to moving boundaries. However, the studied hydrogels did not show significant swelling which were also validated by MR images. On the other hand, insulin release was quantified by High Performance Liquid Chromatography (HPLC). In both sections of this dissertation, effective diffusion coefficients (D) of the systems were predicted by fitting experimental data to the assumed mathematical model by MATLAB. In the first part of the study, for five chocolate formulations stored at 30 °C over a time frame of 22 days, experimental data acquired through MRI were modeled using a Fickian based mathematical model to calculate D values. Using two different equations for boundary condition at upper chocolate surface, two models were evaluated and logistic type boundary model was shown to exhibit a better fit. In addition, associated constants (C0, β, t0) for time dependent upper boundary conditions were determined. Average diffusivities of all samples varied in the order of 10−11 m2 /s. This study addressed the potential use of MRI for visualization and quantification of migration for different chocolate formulations. vii In the second part of the study, insulin entrapped alginate-gum tragacanth (ALG-GT) hydrogels at different ALG replacement ratios (100, 75, 50, 25) were prepared through an ionotropic gelation method, followed by polyelectrolyte complexation (PEC) with chitosan (CH). Retention of almost the full amount of entrapped insulin in a simulated gastric environment and sustained insulin release in simulated intestinal buffer indicated the pH sensitivity of the gels. Insulin release from hydrogels with different formulations showed significant differences (p < 0.05). D values of the gel samples were predicted in the order of 10−10 m2 /s. Time domain (TD) NMR relaxometry experiments showed the differences for different formulations, and the presence of CH revealed that ALG-GT gel formulation could be used as an oral insulin carrier at optimum concentrations. Texture, FTIR and SEM analyses supported less firm structure, interactions between polymers and more heterogenous structure with the increase of GT ratio in the formulations. The hydrogels formulated from biodegradable, biocompatible, and nontoxic natural polymers were seen as promising devices for potential oral insulin delivery. In summary, this dissertation referred the potential of magnetic resonance for validation of transport processes, identification of molecular interactions and characterization of conformational changes occurred in a food and gel matrix. This study could give an insight for further studies undergoing in food and biomedical applications