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Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Food Science

Year Degree Awarded

2014

First Advisor

Eric Andrew Decker

Subject Categories

Food Chemistry

Abstract

Lipid oxidation is a great concern for food manufacturers and consumers as it negatively impacts not only food quality and nutritive values of food lipids, but also consumer health. Lipid oxidation in bulk oil is impacted by chemical factors, such as, prooxidants and antioxidants, and is also related to the existence of physical structures. Bulk oils contain a variety of surface active minor components which are able to form physical structures known as association colloids. These physical structures create oil-water interfaces which seem to be an important site where lipid oxidation occurs in bulk oil. Thus, this research focused on how the surface active minor components in bulk oil impact physical structure formation and oxidative stability in bulk oil. In the first study, the influence of polar lipid oxidation products isolated from used frying oil on the oxidative stability of bulk oils and oil-in-water (O/W) emulsions was investigated. Polar compounds were added to bulk stripped corn oil (with and without reverse micelles formed by dioleoylphosphatidylcholine, DOPC) and O/W emulsion to evaluate their prooxidative activity by following the formation of lipid hydroperoxides and hexanal. Polar compounds increased lipid oxidation in bulk oil with and without DOPC. The presence of DOPC reverse micelles decreased the prooxidant activity of the polar oxidation products. On the other hand, there was no significant effect of the polar compounds on oxidation of O/W emulsions. ix Besides phospholipids, other surface active minor components in commercial oils such as free fatty acids may impact lipid oxidation rates and the physical properties of association colloids. Thus, in the second study, the effects of free fatty acids on changes in the critical micelle concentration (CMC) of DOPC in stripped corn oil were determined by using the 7,7,8,8-tetracyanoquinodimethane (TCNQ) solubilization technique. Different free fatty acids including myristoleic, oleic, elaidic, linoleic and eicosenoic were added at 0.5% by wt along with the DOPC (1-2000 μmol/kg oil) into the bulk oils. There was no significant effect of free fatty acids with different chain length, configuration and number of double bonds on the CMC value for DOPC in bulk oil. However, increasing concentrations of oleic acid (0.5, 1, 3 and 5 % by wt) caused the CMC value for DOPC in bulk oils to increase from 400 to 1000 μmol/kg oil. Physical properties of DOPC reverse micelles in the presence of free fatty acids in bulk oils were also investigated by the small angle X-ray scattering technique. Results showed that free fatty acid could impact on the reverse micelle structure of DOPC in bulk oils. Moreover, free fatty acid decreased pH inside reverse micelle as confirmed by the NMR studies. The oxidation studies revealed that free fatty acids exhibited prooxidative activity in the presence and absence of DOPC. Different types of free fatty acids had similar prooxidative activity in bulk oil. In the last experiment, multiple surface active minor components including DOPC, dioleoylphosphatidylethanolamine (DOPE), oleic acid, diacylglycerols (DAG) and stigmasterol were incorporated to form nanostructures in stripped corn oil. Individual component significantly decreased the oil-water interfacial tension on which the DOPC and DOPE exhibited the strongest impact. However, the CMC study shows that only DOPC and DOPE could form aggregates at the CMC of 40 and 200 μmol/kg oil. The CMC of the mixed components was as low as 20 μmol/kg oil. The absence of a component did not significantly change the CMC value. x However, in the absence of DOPC, we were not able to observe the CMC of the mixed components in bulk oil. The NBD-PE probe was used to study the interfacial activity of minor components. The addition of mixed components caused the emission fluorescence intensity increase, suggesting that these components were at the oil-water interface. Again, the absence of a component from the mixture did not significantly change the fluorescence intensity, except when lacking of the DOPC. This indicates that the DOPC plays an important role on association colloid formation. The oxidation study showed that the association colloids formed by adding 100 μmol/kg oil of mixed components decreased the oxidative stability of bulk oil. In addition, the impact of mixed minor components at below (10 μmol/kg oil) and above their CMC (100 μmol/kg oil) on antioxidant activity of α-tocopherol and Trolox (water soluble derivative of tocopherols) at 10 and 50 μmol/kg oil was investigated. The addition of α-tocopherol and Trolox at 10 μmol/kg oil already compensated the prooxidant activity of association colloids. Trolox exhibited stronger antioxidant activity than α-tocopherol. However, the association colloids did not influence the antioxidative effectiveness of either α-tocopherol or Trolox in this study. In conclusion, the surface active minor components formed complex association colloids that decreased the oxidative stability of bulk oil. The presence of reverse micelle impacted the physical location of components such as polar lipid substrates, thus influenced their prooxidant activity. The physicochemical properties of association colloids could change according to the composition of minor components presenting at the oil-water interface. For example, the addition of free fatty acids extended the CMC and altered the pH of the water core of DOPC reverse micelles. The combination of multiple surface active components physically and chemically impacted the oxidative stability and activity of antioxidants in bulk oil. This research xi demonstrates what happens in real commercial oils which are complicated and could provide an idea of how to protect the oil from lipid oxidation.

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