Food Lipids: Chemistry, Nutrition, and Biotechnology

e cyclical, with regeneration of the lower oxidation state of the metal. Nevertheless,
reaction (15) occurs much more slowly than reaction (14) [7]. Although the metal–
hydroperoxide mechanisms are generally accepted as the most important for metal
catalysis of autoxidation, it is unclear whether redox-active transition metals promote
lipid peroxidation directly through the formation of metal–lipid complexes or by
forming peroxy and oxy radicals.
Chelation of metals by certain compounds decreases their prooxidant effect by
reducing their redox potentials and stabilizing the oxidized form of the metal. Chelating
compounds may also sterically hinder formation of the metal hydroperoxide
complex. Citric acid (and its lipophilic, monoglyceride ester), phosphoric acid (and
its polyphosphate derivatives), and ethylenediaminetetraacetic acid (EDTA) can chelate
metals. EDTA forms a thermodynamically stable complex with metal ions. The
metal-chelating ability of oligophosphate increases with phosphate group number up
to six residues. Carboxyl groups of citric acid are thought to be responsible for
binding with metals and forming complexes. Malic, tartaric, oxalic, and succinic
acids bind metals in the same manner.
In addition to their antioxidant activity, many of these compounds have other
unique functions as food additives. Citric acid, malic acid, and tartaric acid are
important food acidulants. Phosphates are added as buffers, emulsifiers, and acidulants
and water binders. Chelating antioxidants are also referred to as synergists
because they enhance the activity of phenolic antioxidants. This synergism is sometimes
referred to as acid synergism when the chelator is citric or other acids.
b. Oxygen Scavengers and Reducing Agents. Ascorbic acid, ascorbyl palmitate,
erythorbic acid, sodium erythorbate, and sulfites prevent oxidation by scavenging
oxygen and acting as reductants. Oxygen scavenging is useful in products with head
space or dissolved oxygen. Reducing agents function by donating hydrogen atoms.
Ascorbic acid and sulfites react directly with oxygen and eliminate it from the food
product [11]. L-Ascorbic acid has strong reducing properties, and its most significant
chemical property is its ability to oxidize through one- or two-electron transfers
[12,13]. One-electron reactions involve an L-ascorbic acid radical (semidehydroascorbic
acid). A proton is lost, whereupon a bicyclic radical, which is the intermediate
leading to dehydroascorbic acid, is formed. A two-electron transfer occurs when
transition metals catalyze ascorbate autoxidation. In this process, L-ascorbate and
oxygen form a ternary complex with the metal catalyst. Two � electrons from Lascorbate
shift to oxygen through the transition metal. Oxidation of ascorbic acid
has been reviewed by Liao and Seib [14].
Ascorbic acid represents a truly multifunctional antioxidant. Schuler [15] identified
the following classes of reactions as significant to the antioxidant action of
ascorbic acid in food systems: quenching of singlet oxygen, reductions of free radicals
and primary antioxidant radicals, and removal of molecular oxygen in the presence
of metal ions.
c. Singlet Oxygen Quenchers. Singlet oxygen is a high energy molecule that is
responsible for photooxidation of unsaturated fats and the subsequent generation of
hydroperoxides. Singlet oxygen quenchers deplete singlet oxygen of its excess energy
and dissipate the energy in the form of heat. Carotenoids, including �-carotene,
lycopene, and lutein, are active singlet oxygen quenchers at low oxygen partial pressure.
Figure 1 gives an overview of lipid oxidation and the interaction of antioxidants.
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