Antioxidants multiphase systems

Multiphase Systems Antioxidant activity depends very much on the lipid substrate used for evaluation and the hydrophilic/lipophihc nature of the anti-oxidative compound. Solubility and partition properties of the compound in the medium affect the activity of antioxidants in the bulk hpid systems. As most foods cannot be related to bulk oil systems (e.g., meat, fish, eggs, mayonnaise, salad dressings, etc.), evaluation of antioxidants in multiphase systems is more relevant to their physical and chemical nature. Because of the very same reasons, several studies have found that compounds exhibiting strong activity against oxidation of lipids in bulk systems are often inefficient in colloidal and emulsion systems. 

Three systems are generally used for such evaluations emulsions (water-in-oil, oil-in-water), liposomes (uni- or multUamellar vesicles formed with aqueous phase and phospholipids), and micelles (emulsions formed with free fatty acids and aqueous phase). A lengthy discussion about how to use these multiphase systems in evaluating antioxidant activity is provided by Frankel (19). 

The behavior of antioxidants in food and their antioxidative ability can vary markedly, depending on the lipid-containing systems. Food products are predominantly multiphase systems in which the activity of the antioxidants depends on the lipid composition, pH and ionic strength, non-lipid constituents, additives such as emulsifiers, and the presence of metal ions. These factors may affect several important physicochemical properties of the antioxidant, such as interphase transport,... 

Knowledge on the sites of prooxidant and antioxidant action in multi-component systems is essential to better predict the oxidative stability of complex foods and biological systems. Foods of improved quality may be developed if the association and driving forces of prooxidants and antioxidants can be controlled in multiphase systems. 

This book discusses these and other pitfalls inherent in basic studies that use artificial radical initiators with unrealistic models and homogeneous systems. The application of simple research models may lead to numerous problems in the ultimate interpretation of results, because lipid oxidation proceeds by a complex sequence of reactions influenced by many factors, all of which become extremely difficult to unravel in real food and biological systems. These systems are multi-phased and controlled by complex colloidal phenomena affecting different sites of oxidation and antioxidation. In interpreting the effects of prooxidant and antioxidant compounds their effective concentrations in different phases must be considered. A dimension of lipid oxidation that is important to better understand control methods deals with the relative partition of oxidants and antioxidants in multiphase systems. This topic has not received sufficient attention. 

Lipids exist in most foods as multiphased colloidal systems bound by surface-active phospholipids, proteins and emulsifiers. The oxidative stability of food lipids is greatly affected by the partitioning of the lipid substrates, metal initiators and antioxidants, which is complex and depends on the physical properties of the food. We may consider three types of food systems (see Chapter 10) ... 

Liposomes Phospholipids spontaneously form bilayers called liposomes when dispersed in water. Meat and biological membranes are examples of liposomes. Hydrophilic antioxidants and prooxidants would be oriented at the polar end of the phospholipid bilayers, while the lipophilic antioxidants and prooxidants would be located between the long chain of the lecithin and the water phase. Multiphase food systems will be discussed further in Chapter 10. 

The activity of natural antioxidants is greatly affected by complex interfacial phenomena in emulsions and multi-component foods. The methodology to evaluate natural antioxidants must be carefully interpreted depending on whether oxidation is carried out in bulk oils or in emulsions, and which analytical method is used to determine extent and end-point of oxidation (Chapter 10). To understand and better predict how natural antioxidants may protect foods against oxidation, the following complex questions need to be carefully considered for the judicious choice of antioxidant evaluation protocols a) What are the protective properties of antioxidants b) What substrates are oxidized and what products of oxidation are inhibited c) In a multiphase food system is the antioxidant located where oxidation takes place d) Are there any... 

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