ABSTRACT
In oil-in-water emulsions, the lipids are not directly in contact with air, but they are surrounded by an interfacial film and dispersed in the aqueous phase. Thus, oxygen, water-soluble antioxidants and prooxidants must diffuse through the aqueous phase and go through the interface before reaching the lipid substrate. As a consequence, even if the mechanism of autoxidation in emulsions is not fundamentally different from that identified in bulk oils and fats (see Chapter 1), a number of additional factors intervene in the process of lipid oxidation (Table 7.1). These factors include the structure of the emulsion (i.e., size distribution of the oil droplets), the physicochemical properties of the aqueous phase, the structure and organization of the oil phase, and the properties of the droplet’s membrane. The partitioning of antioxidant and prooxidant molecules among the oil, the aqueous phase, the interfacial region, and anti-and prooxidant interactions with the emulsifiers and other constituents at the interface or in the aqueous phase may also play an important role in the development of oxidation (Jacobsen et al. 1999a, Mancuso et al. 1999a, Schwarz et al. 2000). The polar paradox described by Porter (1980 and 1993) and Frankel et al. (1994) is a good illustration of the effect of antioxidant partitioning on antioxidant activities. It corresponds to the observation that the efficiency of a lipophilic antioxidant is generally greater in oil-in-water emulsions than in bulk oils, whereas the opposite is often observed for hydrophilic antioxidants. Finally, lipid reaction products and free radical intermediates can react with other components, such as proteins, present either at the interface or in the aqueous
TABLE 7.1 Additional Factors That Influence the Rate and Course of Oxidation in Oil-in-Water Emulsions
phase of the emulsion. This leads to deviations in lipid oxidation pathways and kinetics, changing the proportions of the reaction products or even giving rise to other products and apparent or true antioxidant activities (see Chapter 9).
