Chemistry Reference
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water interface of an emulsion may also retard lipid oxidation (Lethuaut et al.,
2002; Kiokias et al., 2006). Hydroperoxide contents were lower but residual
oxygen content was also lower for oil-in-water emulsions of smaller droplet size
in the study of Kiokias et al. (2006). The pH of an emulsion influences the rate
of lipid oxidation by the repulsion of metal ions by positively charged oil
droplets at low pH (Hu et al., 2003) as well as affecting the degree of ionization
and solubility of antioxidants in the emulsion and the charge on the proteins,
which may interact with antioxidants. Studies of polyphenols in aqueous
solution at pH 7.4 showed that a catechol or galloyl structure is needed for
antioxidant activity at this pH. The order of antioxidant activity was proto-
catechuic acid < hydroxytyrosol < gallic acid < caffeic acid < chlorogenic acid
(Andjelkovic et al., 2006). Vanillic acid, syringic acid and ferulic acid, which do
not contain a catechol or galloyl structure did not show any iron complex
formation at this pH.
The relative rates of lipid oxidation reactions which convert lipids to
hydroperoxides and those that cause hydroperoxide decomposition may vary
depending on the lipid medium, and the effects of antioxidants on hydroperoxide
formation and decomposition may also vary depending on the medium. Methyl
carnosate was more active than Trolox in retarding the rate of formation of
hexanal in oil or in water-in-oil emulsions but the order of activity was reversed
in oil-in-water emulsions (Schwarz et al., 2000). Methyl carnosate is an ortho-
dihydroxy aromatic ester, which has metal chelating ability. The effect of the
medium on metal chelation by polyphenols is likely to be an important
mechanism by which the medium affects the relative ability of antioxidants to
reduce the rate of hydroperoxide decomposition relative to the rate of
hydroperoxide formation. The stability of metal-polyphenol complexes varies
depending on the medium, and polar solvents also provide ionic pathways by
which the complexes can degrade.
In many fatty foods, e.g. chocolate and margarine, the fat phase is partly
solidified. According to Porter (1980), amphiphilic antioxidants may be incor-
porated into lipid droplets which are then solidified, but if solid fats are cut to
reveal new surfaces, it would not be possible for antioxidant molecules to diffuse
to the new surface. Only non-polar antioxidants, which are homogeneously
distributed in the oil before solidification, can be effective in preventing oxidation
at these surfaces. When oils solidify on cooling, the antioxidant capacity can
change significantly. Cooling olive oil from the liquid state at 25 ëC to the solid
state at 3 ëC did not improve the oxidative stability of the oil, and this represented a
discontinuity in the effect of temperature on oil stability from the behavior
predicted from data in the range 25 ëC to 60 ëC. This effect was attributed to the
increase of unsaturated triacylglycerols and decrease of polyphenols in the liquid
phase as solid fat crystals were formed (Calligaris et al., 2006). Rapid cooling
which generates lipid crystals in less ordered polymorphs may allow impurities
such as antioxidant molecules to be incorporated into the crystal structure.
The incorporation of air into lipid containing foods such as aerated creams
clearly increases the surface area of the lipid system exposed to the air and is
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