Food Lipids: Chemistry, Nutrition, and Biotechnology

in food systems, transition metals often exist chelated to other compounds. Many
compounds will form complexes with metals, resulting in changes in catalytic activity.
Some metal chelators increase oxidative reactions by increasing metal solubility
and/or altering the redox potential [49]. Chelators also increase the prooxidant activity
of transition metal activity by making them more nonpolar, thereby increasing
their solubility in lipids [50]. Chelators that exhibit antioxidative properties inhibit
metal-catalyzed reactions by one or more of the following properties: prevention of
metal redox cycling, occupation of all metal coordination sites, formation of insoluble
metal complexes, and stearic hindrance of interactions between metals and lipids or
oxidation intermediates (e.g., peroxides) [51]. The prooxidative/antioxidative properties
can depend on both metal and chelator concentrations. For instance, EDTA is
prooxidative when ratios of EDTA to iron are 1 or less and antioxidative when
EDTA/iron � 1 [49].
The most common metal chelators used in foods contain multiple carboxylic
acids (e.g., EDTA and citric acid) or phosphate (e.g., polyphosphates and phytate)
groups. Chelators are typically water soluble but some will exhibit solubility in lipids
(e.g., citric acid), thus allowing the chelator to inactivate metals in the lipid phase
[52]. Chelator activity depends on pH, since the chelator must be ionized to be active.
Therefore, as pH approaches the pK a of the ionizable groups, chelator activity decreases.
Chelator activity is also decreased by the presence of other chelatable ions
(e.g., calcium), which will compete with the prooxidative metals for binding sites.
Although most food-grade chelators are unaffected by food processing operations
and subsequent storage, polyphosphates are an exception. Polyphosphates are
stronger chelators and antioxidants than mono- and diphosphates [53]. However,
some foods contain phosphatases, which hydrolyze polyphosphates, thus decreasing
their antioxidant effectiveness. This can be observed in muscle foods, where polyphosphates
are relatively ineffective in raw meats that contain high levels of phosphatase
activity [54] but are highly effective in cooked meats, where the phosphatases
have been inactivated [55]. Nutritional implications should also be considered when
chelators are used as food antioxidants, since chelators influence mineral bioavailability.
For instance, EDTA enhances iron bioavailability while phytate decreases iron,
calcium, and zinc absorption [56].
Prooxidant metal activity is also controlled in biological systems by proteins.
Proteins with strong binding sites include transferrin, ovotransferrin (conalbumin),
lactoferrin, and ferritin. Transferrin, ovotransferrin, and lactoferrin are structurally
similar proteins consisting of a single polypepetide chain with a molecular weight
ranging from 76,000 to 80,000. Transferrin and lactoferrin bind two ferric ions
apiece, while ovotransferrin has been reported to bind three [29,57,58]. Ferritin is a
multisubunit protein (molecular weight 450,000) with the capability of storing up to
4500 ferric ions [59]. Transferrin, ovotransferrin, lactoferrin, and ferritin inhibit ironcatalyzed
lipid oxidation by binding iron in its inactive ferric state and possibly by
sterically hindering metal–peroxide interactions [29,60]. Reducing agents (ascorbate,
cysteine, superoxide anion) and low pH can cause the release of iron from the proteins,
resulting in an acceleration of lipid oxidation reactions [29,61]. The activity
of copper can also be controlled by binding to proteins. Serum albumin binds one
cupric ion [62] and ceruloplasmin binds up to six cupric ions [63].
Amino acids and peptides can chelate metals in a manner that decreases their
reactivity. Both the chelating and the antioxidant activities of the skeletal muscle
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