Chemistry Reference
In-Depth Information
potential reason for this is the susceptibility of polyphosphates to hydrolysis
producing less effective mono- and diphosphates. This can be observed in
muscle foods, where polyphosphates are relatively ineffective in raw meats
which contain high levels of phosphatase activity (Li, 1993) but are highly
effective in cooked meats where the phosphatases have been inactivated (Trout,
1990). Phytate is another form of phosphate that can chelate metals and inhibit
lipid oxidation. However, phytate is not allowed as a food additive due to its
ability to decrease iron, calcium and zinc absorption (Miller, 1996).
10.4 Control of reactive oxygen species
10.4.1 Singlet oxygen
Singlet oxygen is an important prooxidant in foods because it is able to add
directly across the double bonds of unsaturated fatty acids, resulting in the
formation of lipid hydroperoxides which can then be decomposed into free
radicals by light, metals and high temperatures. Singlet oxygen differs from
triplet oxygen in that it has two electrons in the outer orbitals that have opposite
spin directions. This excited state of oxygen is highly electrophilic allowing it to
interact with double bonds (Bradley, 1992). In foods, singlet oxygen is typically
formed by photosensitizers (e.g., riboflavin and chlorophyll) in the presence of
light.
Both chemical and physical quenching pathways exist for the inactivation of
singlet oxygen. Chemical quenching of singlet oxygen typically occurs by
compounds with double bonds such as tocopherols and carotenoids. Singlet
oxygen interactions with -carotene will lead to the formation of -carotene-
5,8-endoperoxide and carotenoid breakdown products containing aldehyde and
ketone groups. -Carotene-5,8-endoperoxide mainly forms upon the oxidation
of -carotene by singlet oxygen and therefore may provide a unique marker
which may be used to monitor singlet oxygen/carotenoid interactions in foods
and biological systems (Stratton, 1993). Interactions between tocopherols and
singlet oxygen lead to the formation of tocopherol hydroperoxides and epoxides
(Bradley, 1992). Other compounds including amino acids, peptides, proteins,
phenolics, urate and ascorbate can chemically quench singlet oxygen; however,
much less is known about the resulting oxidation products (Bradley, 1992; Dahl,
1988; Kanofsky, 1990). Chemical quenching of singlet oxygen leads to
destruction of the antioxidant and loss of activity.
A more effective pathway for singlet oxygen inactivation is by physical
quenching, which primarily occurs through interactions between singlet oxygen
and carotenoids. As mentioned previously, singlet oxygen is in an excited state
with the energy of its two main forms being 22.4 and 37.5 kcal above the ground
state (Bradley, 1992). Carotenoids can physically quench singlet oxygen through
the transfer of this energy to the carotenoid to produce an excited state of the
carotenoid and ground state, triplet oxygen. The excited carotenoids can then
dissipate their energy to the surrounding environment through vibrational and
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