Milk Membrane Lipids - Dispersions, Emulsions and Interactions with Proteins
Sammanfattning: Milk is a dilute aqueous dispersion with a complex mixture of proteins, fat, carbohydrates, vitamins and minerals. Milk membrane lipids are a part of the milk fat globule membrane (MFGM), which covers the fat globules. During dairy processing membrane material is released and surface structure features are altered. The presence of free membrane material as well as various surfaces might influence various technical properties of relevance to the dairy industry. This thesis focuses therefore on the polar milk membrane lipids self-assembly structures in dispersions in oil-based emulsions, and their interactions with the major serum protein, beta-lactoglobulin and the major casein, beta-casein. The main polar membrane lipids of the MFGM are the zwitterionic lipids: phosphatidylcholine (PC), phosphatidylethanolamine (PE) and sphingomyelin (SM), and the anionic lipids: phosphatidylserine (PS) and phosphatidylinositol (PI). A higher PE content led to reversed hexagonal structures. At compositions resembling the MFGM a lamellar phase was determined. At this composition mainly unilamellar spherical vesicular structures were visualized using cryogenic transmission electron microscopy (Cryo-TEM). Also bilamellar and multilamellar vesicles were present in addition of salt. Vesicles in cream showed facetted structures, in buttermilk and vitamized skim milk combined vesicle-emulsion aggregates were revealed. Deformed and tubular structures were observed at high PE content, close to the phase boundary of the lamellar to the reversed hexagonal phase. Combining a serum depletion method with radiolabelled proteins and Cryo-TEM interactions between membrane lipid vesicles and beta-casein or beta-lactoglobulin were investigated. At neutral pH and with a mild heat treatment low adsorbed amount of the proteins were observed, however an increase in adsorbed amount of beta-lactoglobulin and aggregation of the vesicles at lower pH was observed. Competitive adsorption between milk proteins and membrane lipids at the oil-water interface and how it depends on the formation of emulsions were investigated. Surface structures were described with different models. Adding vesicles to the protein emulsified emulsion led to adsorption / deposition of the vesicles at the oil-water interface. Furthermore, adding protein to a membrane lipid emulsified emulsion only a small amount of protein was adsorbed at the oil-water interface. Competitive adsorption at the oil-water interface was observed when beta-casein or beta-lactoglobulin was emulsified together with the membrane lipids. This study shows that the kinetics aspects can control the structures rather than the thermodynamic equilibrium, and that this may result in structures more complex than an adsorbed monolayer. Hence, it can be expected that procedures, such as the mixing order, during emulsion preparation is of crucial importance to the emulsification performance.
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