Keywords: encapsulation, microcapsule, protein, fibril, protein-polysaccharide complex, controlled release, interfacial rheology, lysozyme, ovalbumin
This thesis describes the design of encapsulation systems using mesostructures from proteins and polysaccharides. The approach was to first investigate the physical properties of the encapsulating materials (protein fibrils and protein – polysaccharide complexes). Subsequently, microcapsules with tunable release rate and mechanical strength were developed.
Firstly, the effect of steady shear and turbulent flow on the formation of protein fibrils from lysozyme was studied. We determined the conversion and size distribution of fibrils obtained by heating lysozyme solutions at pH 2. The formation of fibrils was quantified using flow-induced birefringence. The size distribution was fitted using decay of birefringence measurements and Transmission Electron Microscopy. The morphology of Lys fibrils and kinetics of their formation varied considerably depending on the flow applied. With increasing shear or stirring rate, more rod-like and shorter fibrils were obtained, and the conversion into fibrils was increased.
Secondly, we have investigated the surface rheological properties of oil – water interfaces stabilized by fibrils from lysozyme (long and semi-flexible, and short and rigid ones), fibrils from ovalbumin (short and semi-flexible), lysozyme – pectin complexes, or ovalbumin – pectin complexes. We have compared these properties with those of interfaces stabilized by the native proteins. The surface dilatational and surface shear moduli were determined using an automated drop tensiometer, and a stress controlled rheometer with biconical disk geometry. Results show that interfaces stabilized by protein – pectin complexes have higher surface shear and dilatational moduli than interfaces stabilized by the native proteins only. At most of the experimental conditions, interfaces stabilized by protein fibrils have the highest surface rheological moduli. The difference between long semi-flexible lysozyme fibrils or short rigid lysozyme fibrils is not pronounced in interfacial dilation rheology but significant in interfacial shear rheology. The complex surface shear moduli of interfaces stabilized by long semi-flexible fibrils are about ten times higher than those of interfaces stabilized by short rigid fibrils, over a range of bulk concentrations. Interfaces stabilized by short and more flexible ovalbumin fibrils have a significantly higher surface shear modulus than those stabilized by the somewhat longer and more rigid short lysozyme fibrils.
Finally, encapsulation systems are developed using layer-by-layer adsorption of food-grade polyelectrolytes on an emulsion droplet template. The first encapsulation system was built with alternating layers of ovalbumin fibrils and high methoxyl pectin. By varying the number of layers, the release of active ingredients can be controlled: increasing the number of layers of the shell from four to eight, decreases the release rate by a factor six.
The other encapsulation systems were built with alternating layers of protein – pectin complexes and protein fibrils. Two types of proteins (ovalbumin and lysozyme) and three types of fibrils were used: short and semi-flexible from ovalbumin, short and rod-like, and long and semi-flexible from lysozyme. At low number of layers (less than five), microcapsules from ovalbumin complexes and fibrils were stronger than microcapsules prepared from lysozyme complexes and fibrils. Increasing the number of layers, the mechanical stability of microcapsules from lysozyme complexes and fibrils increased significantly, and capsules were stronger than those prepared from ovalbumin complexes and fibrils with the same number of layers. The contour length of the Lys fibrils did not have a significant effect on mechanical stability of the lysozyme complexes and fibrils capsules. These results show that mechanical properties of this type of capsule can be tuned by varying the flexibility of the protein fibrils.
|Qualification||Doctor of Philosophy|
|Award date||23 Oct 2012|
|Place of Publication||S.l.|
|Publication status||Published - 2012|