Emulsion formation and stability - Impact of the dispersed state of the emulsifier on emulsification
Abstract: Emulsions find a variety of applications in for example food, cosmetics and pharmaceutical preparations. In order to obtain emulsions with desired properties it is important to understand how different factors, such as the properties of the emulsifier and the emulsification procedure, impact the final emulsion properties. The studies performed in relation to this thesis have mainly been focused on the impact of the dispersed state of phospholipids on emulsification. Previous studies have indicated that larger structures are transported more rapidly to the interface during high-pressure homogenization and, thus, dominate the interfacial layer. Phospholipids are chosen as emulsifiers since they can be dispersed in different ways but would provide the same stabilization of the interface once they have adsorbed and spread. Three different phospholipid products from soy are used; phosphatidylcholine (PC), PC-enriched lecithin and deoiled lecithin. The emulsifier is either dispersed in the oil or in the aqueous phase as aggregates of different size. The aggregates are large lamellar structures, large vesicles or small vesicles. The vesicle size is studied by cryo TEM and asymmetrical flow-field flow fractionation. For PC-enriched soy lecithin and deoiled soy lecithin also the conformation as a function of vesicle size is studied. Emulsions are prepared with different phospholipid compositions and dispersed states. For soy-PC the smallest emulsion droplets are obtained with the emulsifier dispersed as small vesicles. Monolayer adsorption is observed for the vesicles whereas multilayer formation is indicated for the larger aggregates as well as when the emulsifier is dispersed in the oil. Hence, a larger area can be covered with the same amount of emulsifier when vesicles are used and this may explain the higher efficiency. For emulsions prepared with PC-enriched soy lecithin and deoiled soy lecithin the droplets are smallest with the emulsifier dispersed in the oil. These products contain some charged material and the observed differences are likely explained by electrostatic repulsion. When the ionic strength is increased the charged systems behave similar as soy-PC. The results show that the dispersion of the emulsifier may have a large impact on emulsification. Emulsions are often complex systems containing a mixture of emulsifiers and additives. Interactions between emulsifiers may affect emulsion properties. The interactions between hydrophobically modified starch and egg proteins, in solution and at the interface, are studied. Moreover, we often want to heat or freeze emulsions products. Freeze-thaw stability of mayonnaise-type oil-in-water emulsions is another topic that is treated in this thesis.
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