Food Lipids: Chemistry, Nutrition, and Biotechnology

dipeptide carnosine depend on metal ion type [64–66]. Carnosine more effectively
inhibits the oxidation of phosphatidylcholine liposomes catalyzed by copper than by
iron. Decker et al. [66] found that the carnosine can chelate and inhibit the prooxidant
activity of copper but more effectively than its constituent amino acid histidine.
Ceruloplasmin is a copper-containing enzyme that catalyzes the oxidation of
ferrous ions:
2� 3�
4Fe � 4H� �O2 → 4Fe � 2H2O This ferroxidase activity inhibits lipid oxidation by maintaining iron in its oxidized,
inactive state [63]. Since ceruloplasmin is primarily a constituent of blood, one would
not expect to find it in most foods, other than muscle foods. Addition of ceruloplasmin
to muscle foods in a pure form or as part of blood plasma has been found to
effectively inhibit lipid oxidation [67,68].
B. Control of Singlet Oxygen
Singlet oxygen is an excited state of oxygen in which two electrons in the outer
orbitals have opposite spin directions. Initiation of lipid oxidation by singlet oxygen
is due to its electrophilic nature, which leads to the formation of lipid peroxides
from unsaturated fatty acids [69].
Singlet oxygen can be inactivated by both chemical and physical quenching.
Chemical quenching of singlet oxygen by �-carotene will lead to the formation of
carotenoid breakdown products containing aldehyde and ketone groups as well as
�-carotene-5,8-endoperoxide. �-Carotene-5,8-endoperoxide, which occurs mainly
upon the oxidation of �-carotene by singlet oxygen, therefore may provide a unique
marker that could be used to monitor singlet oxygen/carotenoid interactions in foods
and biological systems [70]. Tocopherols can chemically quench singlet oxygen in
reactions that lead to the formation of tocopherol peroxides and epoxides [69]. Other
compounds, including amino acids, peptides, proteins, phenolics, urate, and ascorbate,
can chemically quench singlet oxygen, but little is known about the resulting
oxidation products [69,71,72].
While carotenoids are capable of chemically inactivating singlet oxygen, these
reactions cause carotenoid autoxidation, leading to loss of antioxidant activity. Therefore,
the major mechanism of singlet oxygen inactivation by carotenoids is physical
quenching. The most common energy states of singlet oxygen are 22.4 and 37.5 kcal
above the ground state [69]. Carotenoids physically quench singlet oxygen by a
transfer of energy to the carotenoid to produce an excited state of the carotenoid and
ground state, triplet oxygen. Energy is dissipated from the excited carotenoid by
vibrational and rotational interactions with the surrounding solvent to return the
carotenoid to the ground state [73]. Nine or more conjugated double bonds are necessary
for physical quenching [74]. The presence of six carbon-oxygenated ring
structures at the end of the polyenes increases the ability of carotenoids to physically
quench singlet oxygen [74]. While it is generally believed that the physical quenching
of singlet oxygen by carotenoids does not cause destruction of the carotenoid,
these reactions may result in trans or cis isomer conversions [73].
Tocopherols and amines can physically quench singlet oxygen by a charge
transfer mechanism. In this reaction, the electron donor (tocopherol or amine) forms
a charge transfer complex with the electron-deficient singlet oxygen molecule [70].
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