Emulsion microgel particles: from design to oral to gastrointestinal digestion

Lipophilic active molecules, such as fat soluble vitamins, flavourings and fatty acids pose challenges for their application in food matrices due their water-insolubility, rapid oxidation and degradation during physiological transit. Based on the perceived knowledge gap from the literature review, a novel encapsulation strategy, targeting the delivery of lipophilic molecules, was developed in this thesis: emulsion microgel particles. These engineered smart particles are soft gel like particles encapsulating several oil droplets. These particles can alter their network structure, swelling behaviour, permeability or mechanical strength in response to internal physiological stimuli (e.g., change in pH, ionic strength, temperature, mechanical shear, enzymatic action, etc.). By investigating the nature of two specific biopolymer networks (i.e., starch gel or whey protein gel) and the interactions between the filler (i.e., emulsion droplets) and the matrix (i.e., the gel), two different approaches (top-down or bottom-up) were used to design these emulsion microgel particles. The unique design of the emulsion microgel particles developed in this PhD was that both the emulsifying agent and the matrix material were composed of the same biopolymer without the need of additional hydrocolloids. The characterization techniques used in this thesis included particle sizing, microscopy at various length scales, zeta-potential, rheology, tribology, sodium dodecyl sulphate polyacrylamide gel electrophoresis, pH-stat titrimetric measurements supported by theoretical calculations. In view of the above, the rheological properties and microstructural breakdown of the model emulsion gels were first analysed. The oil content, particle size, filler to matrix ratio and matrix polymer properties were also examined. Emphasis was then placed on the different response mechanisms of the starch- or whey protein-based emulsion microgel particles when subjected to different environmental stresses, such as acidic pH, enzyme activity (α-amylase for oral relevance, pepsin or trypsin for gastrointestinal relevance) and mechanical shear. Successfully, the emulsion microgel particles contributed to the improvement of food based delivery systems for lipophilic materials. Starch-based emulsion microgel particles were found to be suitable for the protection of lipophilic molecules against oral shear and coalescence whilst providing appropriate lubrication via both enzyme and shear-response mechanism. Whey protein-based emulsion microgel particles were established to protect the lipophilic molecules from harsh gastric environment whilst enabling complete release of free fatty acid during the intestinal phase due to both enzyme and pH-assisted mechanisms. The findings from this PhD thesis thus provide guidelines to develop new bio-responsive materials where these emulsion microgel particles can be used to offer excellent site-dependent controlled release properties for lipophilic materials