| Pickering emulsions have attracted considerable research interest for the long term stabilization due to their effective hinder the coalescence of droplets and application as precursors to fabricate the new materials with complex hierarchical structure. To stabilize emulsion, various kinds of particles have been widely used, such as hydrophobic silica, clays, latex, and microgels. Recently, biological origin particle emulsifiers are receiving reasonable attention. Compared to synthesized polymer and inorganic particle emulsifiers, biological origin particle emulsifiers are more superior for application in food, biomedicine and cosmetics. Among the numerous of biological origin particle, starch based particle emulsifiers have important potential to be used in food, cosmetics and pharmaceuticals due to their inexpensiveness, abundance, and edible. In this dissertation, the preparation of starch based particle emulsifiers was studied and then theirs emulsifying propert ies, emulsifying mechanisms and application in lipid oxidation stability were also investigated. The objective of this research is to obtain a novel edible particle emulsifier with excellent emulsifying property.The main research works of this thesis are as follows :(1) The surface properties of native starches, namely rice, waxy maize, wheat and potato starch were characterized by contact angle measurement, zeta potential, surface tension and other test methods, then the emulsifying ability of these native starch granules was investigated by using liquid paraffin as oil phase and starch granules as sole emulsifier. The results showed that emulsions can be prepared by using rice, waxy maize and wheat starch granules as emulsifier except potato starch granules which has no emulsifying ability even when the concentration of starch granules as high as 15 wt% relative to water phase. Rice starch granules which had the smallest particle size was the best emulsifier among these native starches. The emulsion stabilized by rice granules proved to be stable when the starch granules concentration was above 3 wt% relative to water phase. With the increasing of the rice starch concentration, the sizes of droplets decreased and the stability of emulsions enhanced. At the low starch granules concentration, the droplet surface coverage also was low and particles formed monolayer close-packed ―clumps‖. When the starch concentration progressed, the droplet surface coverage increased and became monolayer dense packing. The pH value of aqueous and ionic concentration had no significant influence on the packing structure of native starch granules at interface. After heating the emulsion at gelatinization temperature of starch, the emulsion did not break, but turned into gel- like emulsion. This could be used to prepare the novel starch microencapsulation.(2) The composite granules with different hydrophobicity were prepared by using physical method to coat the surface of rice starch granules with edible beeswax and chitosan, then they were characterized by means of SEM,OCA and other instruments and their emulsifying properties were also investigated. The results showed that beeswax and chitosan indeed coat on the surface of rice starch granules and the hydrophobicity is increased, the emulsifying property of the composite granules can be significantly improved only by adding tiny amounts of these hydrophobic materials. The emulsifying property of beeswax/rice starch composite granules increased with the increasing of hydrophobicity while the emulsifying property of chitosan/rice starch composite granules was the best in a certain hydrophobicity and it declined when the hydrophobicity is getting higher or lower.(3) Taking waxy maize starch as a raw material, the starch nanocrystals(SNC) were successfully prepared by sulfuric acid hydrolysis method and the influence factors of SNC-stabilized emulsion were still investigated. The results showed that the surface tension of SNC dispersions decreases from 71.7 mN/m to 45.8 mN/m when the SNC concentration grows from 0.05 wt% to 3.0 wt%. Further tests on the emulsifying ability of supernatant of centrifuged SNC dispersions and the emulsion stability at the melting temperature of SNC confirmed that the emulsions were indeed stabilized by SNC instead of small surface active molecules. The size of the droplets decreased with the raising of the concentration of SNC. These emulsions were very stable to coalescence over months and the creaming of emulsions decreased with the increasing of SNC concentration. The rheological results indicated that the inhibition of creaming at high SNC concentration mainly due to the formation of inter-particle network in the emulsions. Because of the amount of sulfuric groups at the surface, SNC was negatively charged. Therefore, the electrostatic repulsion of SNC was reduced at low p H value of aqueous or high ionic concentration which further caused the SNC aggregation and size increase of the emulsions stabilized by SNC. The emulsion cannot be formed when pH≤4.4 and the droplets size of emulsion increased to about 1000 μm when the concentration of sodium chloride was 500 mmol/L.(4) At the low pH value of aqueous(pH≈2.2), emulsions could not be formed by using amino acid or SNC as sole emulsifier. Pickering emulsions were prepared by using a mixed dispersion of amino acid and SNC, the effect of amino acid species on the stability of emulsion and the interaction between amino acids and SNC were studied. The results showed that valine, leucine, isoleucine, methionine, phenylalanine, tyrosine and tryptophan can be cooperate with SNC to stable emulsion because of their high hydrophobicity. Amino acids attach to the surface of SNC mainly by electrostatic force, hydrophobic interaction and esterification, the stable emulsion can be prepared due to the reduced aggregation of the particles because of the formation of steric hindrance between the carbon chain of amino acids. The stability of emulsion increased with the increasing ability to form steric hindrance of amino acids that adsorbed on the surface of SNC which was determined by adsorption quantity and the side chain properties of amino acids.(5) The alkenyl succinic anhydride was grafted to the surface of SNC by esterification to reduce the flocculation of particles in aqueous, the aim is to prepare the stable emulsion at low pH value of aqueous or high ionic concentration. The different degrees of substitution of octenyl succinic anhydride modified starch nanocrystals(OSA-SNC) and a similar degree of substitution of different anhydride modified starch nanocrystals were prepared by adjusting the reaction conditions of esterification, while their mechanisms to stabilize emulsions were also discussed. The results showed that the dispersion of OSA-SNC in aqueous increases with the increasing degree of substitution and it can be stably dispersed in aqueous for a month without flocculation when the degree of substitution is 0.019 while it can be completely flocculated within one hour when the degree of substitution is 0.006. For the SNC modified by anhydride with different alkyl chain length(similar degree of substitution), the dispersibility of particle in aqueous increased with increasing chain length of anhydride. The stable Pickering emulsion cannot be prepared when the OSA-SNC particle had an excellent dispersion or strong flocculation in aqueous, whereas, the OSA-SNC with slight flocculation ability in aqueous had excellent emulsifying ability. Similarly, the SNC modified by different anhydride to stable emulsion followed the same rule. According to this rule, the stable emulsion can be prepared at low p H value of aqueous or high ionic concentration by adjusting the dispersibility of particles in aqueous.(6) Taking olive oil as oil phase, the Pickering emulsions stabilized by OSA-SNC with different degrees of substitution were prepared. The effects of OSA-SNC concentration and pH value of aqueous phase on the properties of emulsions were investigated. The peroxide value(POV) of emulsions separately stabilized by Tween-20 and OSA-SNC were determined and the effects of OSA-SNC concentration and pH value of aqueous phase on the lipid oxidation of emulsions were also investigated. The results showed that the lipid oxidation stability of emulsions stabilized by OSA-SNC is significantly better than that of Tween-20. The POV of emulsions stabilized by Tween-20 increases while that of OSA-SNC decreases with the increasing of concentration; The POV of emulsions stabilized by Tween-20 first decreases and then increases with increasing pH value of aqueous, but the effect of pH value of aqueous on POV of emulsions stab ilized by OSA-SNC related to the degree of substitution of OSA-SNC. Generally speaking, the POV of emulsions stabilized by Tween-20 was positively correlated to its interfacial surface area, while the POV of emulsions stabilized by OSA-SNC was combined affected by the behavior of particles on the oil/water interface, the interfacial surface area, the system viscosity(or G′) and other factors. |
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