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theses

Selecting the best antioxidant, AO, for a particular food application is still a major problem in food science because AO e ciencies are determined by a variety of factors. One major di culty in establishing a scale of AO e ciencies has been the lack of reliable methods for determining AO distributions between the di erent regions of aggregated systems, which arises from the physical impossibility of separating the interfacial region from the aqueous or oil regions. We have developed a on-invasive" approach to estimate AO distributions in various aggregated systems. The observed rate constant, kobs, for the reaction of a chemical probe, 4-n-hexadecylbenzenediazonium ion (16-ArN+2), that is located in the interfacial region of an aggregation system with an AO is measured by a chemical derivatization method or directly by UV-Vis spectroscopy depending upon the system turbidity. The kinetic data is interpreted by using a well established pseudophase kinetic model that was originally developed for treating chemical reactivity in micelles and microemulsions, and in this thesis we demonstrate that the model can be applied to nonionic emulsions, ionic emulsions, and vesicles. Chapter 1 is a general introduction on pertinent background information including dynamic equilibrium in surfactant aggregates, basic assumptions of the pseusophase kinetic model and logic of the chemical trapping method, and e ect of AO distributions on AO e ciency. Chapter 2 describes the application of the pseudophase kinetic model to cationic and anionic emulsions in the absence and presence of added salt to obtain estimates of the partition constants of t-butylhydroquinone, TBHQ, between the oil and interfacial region, PIO, and the aqueous and interfacial region, PIW, and the second-order interfacial rate constant, kI, that is independent of AO distributions. Chapter 3 reports measurements of kobs for 16-ArN+2 reacting with TBHQ in C12E6 nonionic emulsions of constant composition but di erent droplet size distributions together with hydration number estimates for C12E6. The results support the pseudophase model assumptiont that rate constants for reactions in emulsions are insensitive to changes in droplet size and that the medium properties of the interfacial region are virtually constant. Chapter 4 demonstrates that the AO reactivity of a homologous series of gallate esters as characterized by the observed rate constant, kobs, for their reactions with 16-ArN+2 plateaus in vesicular solutions once the AOs are fully associated with the vesicles. This plateau di ers from the cuto e ect" observed in oil-inwater emulsions, the AO activity increases with the alkyl chain length of AO and reaches a maximum at an intermediate chain length, after which further increase in AO chain length results in a decrease in activity.

ETD_6631

Ph.D.

Includes bibliographical references

by Qing Gu

doi:10.7282/T3RN39T3

ETD doctoral

The author owns the copyright to this work.