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Design and Fabrication of Functional Lipid Nanoparticles Based on Control of Interfacial Properties Using Biopolymers

The main objective of this research was to better understand the formation, stability and properties of emulsions having lipid nanoparticles with tunable functional properties by controlling the composition and structure of the biopolymer interface, in order to develop better food-grade delivery systems. Initially, the influence of environmental stresses (pH, heating and salts) on the physicochemical properties of cationic lactoferrin (LF)-stabilized oil-in-water emulsions was investigated. At ambient temperature, the emulsions were found to be stable at all times except when pH  pI. When LF-coated droplets were heated in distilled water, and then their pH was adjusted in the range 2 to 9, they were highly unstable to aggregation at pH 7 and 8. On the other hand, when the pH was altered in the range 2 to 9 first, and then they were heated, the LF-coated droplets were highly unstable to aggregation at pH ≥ 5 when heated above 50 ºC. This showed that the thermal stability of the emulsions to salt addition depended on pH and salt type, which was attributed to counter-ion binding and electrostatic screening effects. For NaCl, emulsions were stable from 0 to 200 mM at pH 3 and 9, but aggregated at ≥ 100 mM at pH 6. For CaCl2, emulsions were stable from 0 to 200 mM at pH 3, but aggregated with ≥ 150 mM CaCl2 at pH 6 and 9. These results have important implications for the formulation and production of emulsion-based products using lactoferrin as an emulsifier. Next, we studied the properties and stability of multilayer emulsions formed using the primary emulsifier lactoferrin and secondary polysaccharides like low methoxyl pectin (LMP), high methoxyl pectin (HMP) and alginate. At neutral pH, electrostatic attractions occurred between the anionic groups on the polysaccharide molecules and the cationic patches on the protein surfaces. In the absence of polysaccharide, the LF-coated droplets were highly unstable to aggregation when heated above about 60 ºC at pH 7, presumably because thermal denaturation of the adsorbed proteins increased droplet attraction. However, the addition of either LMP or HMP prior to heating greatly improved the thermal stability of the emulsions, with no aggregation being observed from 30 to 90 ºC. On the other hand, the presence of anionic polysaccharides had little effect on emulsion stability or even promoted emulsion instability when 0 to 200 mM NaCl or CaCl2 were added. If such emulsions are to be used to as functional ingredients to encapsulate bioactive lipids, it is important to study their fate in the gastrointestinal tract. Therefore, changes in the physicochemical properties and digestibility of both the primary LF and the secondary LF-polysaccharide emulsions, under simulated gastrointestinal conditions were monitored. LF, LF-LMP and LF-HMP emulsions were stable to droplet aggregation in the stomach but aggregated in the small intestine, whereas LF-alginate emulsions aggregated in both the stomach and small intestine. The presence of a dietary fiber coating around the initial lipid droplets had little influence on the total extent of lipid digestion in simulated intestinal fluid (SIF), but LF-alginate emulsions had a slower initial digestion rate than the other emulsions. These results suggest that the dietary fiber coatings may become detached in the small intestine, or that they were permeable to digestive enzymes. Pepsin was found to have little influence on the physical stability or digestibility of the emulsions. Next, we fabricated emulsions with oil droplets coated by sequential electrostatic deposition of cationic LF and anionic β-lactoglobulin (BLG) at pH 6.5: LF, LF-BLG, LF-BLG-LF, and LF-BLG-LF-BLG. Changes in the physicochemical properties of these systems were characterized when they were exposed to environmental stresses and simulated small intestine conditions. LF-coated droplets were stable throughout the entire pH range which was attributed to strong steric repulsion. All the nanolaminated droplets were unstable to aggregation at pH 5, which is between the isoelectric points of BLG and LF. LF-coated droplets were unstable to low levels of salt addition (≥ 10 mM NaCl, pH 6.5), whereas nanolaminated droplets were stable up to high salt levels (200 mM NaCl, pH 6.5). Under simulated small intestine conditions, there was a reduction in the initial rate of lipid digestion as the number of protein layers coating the lipid droplets increased (LF-BLG-LG-BLG < LF-BLG-LG < LF-BLG < LF), but eventually all of the systems were fully digested. Finally, a “premix” approach was utilized to fabricate interfacial coatings around the lipid droplets, instead of the LbL approach. This method involved mixing BLG and LF prior to emulsion formation and the influence of environmental stresses on the properties of these emulsions was examined. Droplets coated by BLG were unstable to aggregation near their isoelectric point (pH  5), whereas those coated by LF were stable across the whole pH range. The stability of emulsions to pH induced aggregation improved as the ratio of LF-to-BLG in the mixed systems was increased. Lipid droplets coated by either LF or BLG were unstable to aggregation at high salt concentrations (500 mM NaCl, pH 6.5), but those stabilized by mixed protein coatings (LF and BLG) were stable, which was attributed to an increase in interfacial thickness and steric repulsion. Droplets coated by BLG were stable to droplet aggregation after thermal treatment (30 to 90 oC, 0 mM, NaCl pH 7), whereas those coated by LF were highly unstable when heated above their thermal denaturation temperature. The thermal stability of the droplets decreased as the amount of LF in the mixed systems increased. Overall, this study shows that mixtures of LF and BLG are able to modulate the physicochemical properties and stability of protein-coated lipid droplets in oil-in-water emulsions. The knowledge obtained from this research is important for the design of emulsion-based delivery systems that are resistant to environmental stresses, to encapsulate lipophilic bioactive ingredients and control lipid digestibility. This may, in turn, be useful for developing functional foods to induce satiety or control the release of bioactive components within the gastrointestinal tract. These delivery systems could also be used to encapsulate, protect and release functional components in other industrial products such as pharmaceuticals, cosmetics, and personal care products.
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