Effects of food structure and ingredient interactions on antioxidant capacity - Oxidation in Foods and Beverages and Antioxidant Applications - page 324

multilamellar and unilamellar liposomes have been used to investigate anti-
oxidant effects and synergy between antioxidants. Iron and copper have been
used to initiate peroxidation in some studies of antioxidant- containing lipo-
somes, but the majority of studies have used 2,2 0 -azobis(2-amidinopropane)
dihydrochloride (AAPH) and 2,2 0 -azobis(4-methoxy-2,4-dimethylvaleronitrile)
(AMVN), which are water-soluble and lipid-soluble thermo labile azo initiators
respectively. These azo compounds form peroxyl radicals after initially
decomposing to give carbon radicals and a nitrogen molecule. The peroxyl
radical can attack the lipids and induce lipid peroxidation (13.2).
R±N=N±R ÿ! R · N 2 R · 13.1

R · O 2 ÿ! RO 2 · ÿ! 13.2

Liposomes are very useful structures for investigating synergy in the activity
of antioxidants. Lipophilic antioxidants may be incorporated into the liposome
membrane whereas polar antioxidants are in the external aqueous phase.
Synergy in the antioxidant effects of mixtures of carotenoids has been
effectively demonstrated using both unilamellar (Liang et al., 2009) and
multilamellar vesicles (Stahl et al., 1998). Synergy between vitamin C and -
tocopherol (Roberts and Gordon, 2003), and between vitamin A and -
tocopherol (Tesoriere et al., 1996) has been effectively demonstrated using
unilamellar liposomes.
Synergy between antioxidants has been reported in a range of different
media, including oils, emulsions, liposomes, microemulsions, fish and meat
muscles. In some reports, the antioxidants used in a combination only have
additive effects, but the term synergy should be restricted to situations where the
combination of antioxidants has a greater effect than the sum of the effects
contributed by each antioxidant when studied separately. Synergy between
antioxidants may vary both with the medium and the nature of the lipids. Caffeic
acid was effective in protecting -tocopherol in fish muscle but not in an oil-in-
water emulsion, and this was important in the synergistic effect of a mixture of
caffeic acid and -tocopherol in retarding lipid oxidation in the fish muscle
(Iglesias et al., 2009). -Tocopherol showed a strong synergistic effect with
quercetin in a methyl oleate in water emulsion, but the effect was reduced in
phospholipid liposomes and the combination of -tocopherol and quercetin had
a shorter induction time than quercetin alone, when the oxidative stability was
assessed in an oil by the Rancimat test (Becker et al., 2007).

13.4 Interactions of antioxidants with other components
13.4.1 Interactions with metals
Iron and copper ions induce peroxidation of lipids, but many antioxidants have
the ability to chelate metal ions and to thereby reduce the pro-oxidant effect of
these metals. Some plant polyphenols possess a remarkable transition metal
chelating activity and form redox stable complexes with transition metal ions