Fundamental studies on the diffusional mobility of unsaturated fatty acids in high-solid hydrocolloid matrices
Creators: name: Paramita, Vilia
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 300602
subject: Food chemistry and food sensory science

description: Fundamental aspects of the controlled delivery of bioactive compounds have been focusing on methods of administration to achieve a certain rate of release. Incorporating the bioactive agents in high solid biopolymers is a viable means to sustain bioavailability and biofunctionality of the natural ingredients in processed foods and nutraceuticals. An issue arising from this approach is the complexity of entrapping materials and their association with the active ingredients. Biophysical approaches were thus elaborated in this thesis to understand the mechanism behind a sustainable release of bioactive compounds. Diffusion of unsaturated fatty acids was modelled to study the diffusion patterns of these oleophilic materials in high solid matrices in the vicinity of their glass transition temperature. The concepts of Fick’s second law of diffusion, the reaction rate and free volume theories were utilised to rationalise the transport mechanisms of these active ingredients.<br><br>Such processes were examined by preparing glassy hydrocolloid slabs loaded with fatty acid and submerging them in a solvent to allow unidirectional diffusion of the lipid fraction to the liquid phase. The excipients were slabs of kappa-carrageenan, high-methoxy pectin and whey protein isolate, whereas the entrapped bioactive constituents were in the form of unsaturated fatty acids, i.e. oleic, linoleic and alpha-linolenic acids. Once a range of condensed hydrocolloid-fatty acid systems was prepared, the release kinetics of the bioactive compound as a function of experimental temperature was monitored. Characterisation techniques included small-deformation dynamic oscillation analysis, modulated differential scanning calorimetry, confocal and scanning electron microscopy, Fourier transform infrared spectroscopy fitted with an imaging system, wide angle x-ray diffraction and UV-vis spectroscopy in order to measure fatty acid diffusion.<br>In the first experimental chapter, oleic acid mobility was examined in a high-methoxy pectin/glucose syrup system. Structural relaxation was investigated under dynamic oscillation by monitoring the cooling and frequency sweep profiles of this composite system over a wide range of temperatures from -30 to 22°C. Results were modelled using the Williams, Landel and Ferry equation in conjunction with the modified Arrhenius equation to determine the effect of glass transition on the diffusion rate of oleic acid in a matrix composed of gelling polysaccharide and low molecular weight co-solute. Findings from this thesis show, for the first time, that the concept of the Less Fickian diffusion was capable of explaining the transport behaviour of fatty acid in a biopolymer composite. The diffusion patterns were identified based on the diffusion exponent (n) obtained from the power law equation suggesting a low rate of fatty acid migration in the vicinity of the glass transition temperature (Tg).<br>In the second experimental chapter, research was advanced to understand the influence of two carbohydrate materials, i.e. k-carrageenan and polydextrose that acted as co-solute, in regulating the transport behaviour of a-linolenic acid. Diffusion of fatty acid was modelled based on the half time diffusion of Fick’s second law. The relationship of the diffusion coefficient determined in this equation and the fractional free volume determined from WLF equation of the polymeric matrix suggested constant increment of diffusion as a function of temperature and free volume within the system.<br><br>The third experimental chapter studied the effect of the structural relaxation of single whey protein systems on the diffusion patterns of linoleic acid. Transport of linoleic acid, above and below the glass transition temperature, was quantified and modelled using Fick’s second law of diffusion. The results indicated a limiting molecular mobility below the glass transition temperature, which increased following an anomalous diffusion mechanism at the Tg. Data suggested an agreement between diffusion pattern and availability of voids within the polymeric matrix, which were fed into a mathematical expression proposed by Panyoyai and Kasapis yielding a linear relationship between the logarithm of the diffusion coefficient and the inverse value of fractional free volume. The equation produces a coupling parameter that correlates the macromolecule displacement relative to the entrapped fatty acid mobility within the composite system. Quantification of the concerted movement of fatty acid and protein assists in the manipulation of the release profile of small-molecules in condensed phases.<br>The last experimental chapter dealt with the effect of various concentrations of low molecular weight co-solute (i.e. glucose syrup) in altering the diffusion patterns of linoleic acid in condensed whey protein matrices. Results suggested a continuous increase in the rate of fatty acid release from the matrix with increasing amounts of glucose syrup. Such event displayed the plasticising effect of co-solute on the polymeric structure of whey protein system. This work highlighted the decrease in the Tg value with increasing substitution of protein by glucose syrup and as such resulted in an increase in the fatty acid diffusional mobility. Anomalous diffusion, or non-Fickian diffusion behaviour, was observed for a matrix at the glass transition and rubbery states. Values of the coupling parameter suggest an increasing freedom of transportation of the fatty aid within the supporting matrix with higher levels of glucose syrup in the system. Increasing mobility of polymeric molecules as well as fatty acid molecules was confirmed by a reduction in the value of the activation energy to overcome configurational rearrangements from one state to another.<br><br>The final chapter of the thesis concludes by highlighting the contribution of this work that focuses on the metastable properties of biopolymer networks affecting significantly the diffusion kinetics of bioactive compounds. That was shown to be the case in high solid samples of protein and polysaccharide supporting a homogeneous distribution of polyunsaturated fatty acids. By considering a free volume theory of diffusion, it was found that at T &gt; Tg the effective diffusion coefficient of microconstituent transport would increase in accordance with the free volume of the polymer matrix. Fitting experimental diffusivity data in glassy polymers to a free volume based theory generates a two-parameter equation that calculates the extent of molecular interaction between macromolecule and microconstituent as a function of the effective diffusion coefficient and the inverse of fractional free volume. Results can be manipulated by plasticising the polymeric matrix to profoundly affect the level of interaction and diffusion of fatty acids in condensed biomaterials.