Understanding oxidation processes in foods - Oxidation in Foods and Beverages and Antioxidant Applications - page 12

but is an effective radical scavenger. The concentration of urate, which is a
breakdown product of nucleotides (cf. Fig. 1.3), may be increased in milk
through feeding the dairy cows a low-fat feed with high rumen fermentability
using wheat rather than maize, in effect increasing the oxidative stability of milk
especially when exposed to light (éstdal et al., 2008). Cystein containing
peptides including glutathione became through evolution important as thiol/
disulfide redox switches under conditions of oxidative stress in the cytosol or in
relation to protein functionality adjustment (Wouters et al., 2010).
1.4 Lipid phase oxidations
Lipids are more energy-dense than carbohydrates, which initially are formed as
electron-rich compounds in photosynthesis, and especially the apolar lipids such
as the triglycerides are important for energy storage in many plants and in
animals. The more polar lipids including the phospholipids are building blocks
for membranes. Besides classifications according to polarity, lipids may also be
classified as saponifiable including triglycerides, phospholipids and waxes, and
non-saponifiable including terpenes, carotenoids, tocopherols and steroids. Ease
of oxidation for lipids depends on unsaturation, however, all lipids are subject to
oxidation.
Oxidation of lipids is initiated by irradiation including exposure to visible
light, by enzymes and metal catalysis. Heat and pressure accelerate lipid
oxidation when initiated. From Fig. 1.5 the three major reaction paths involved
in initiation of lipid oxidation may be identified:
(I) Oxygen activation by metal catalysis including activation by oxido-
reductases to yield the hydroxyl radical subsequently abstracting an allylic
or bis-allylic hydrogen from an unsaturated lipid (LH). Other radicals like
the chlorine radical from disinfectants or generated by myeloperoxidases
may also initiate the chain reaction through formation of a carbon-centered
lipid radical (L · ), which freely reacts with ground state oxygen.
(II) Lipoxygenases incorporating oxygen in the unsaturated lipid to yield lipid
hydroperoxides like in chicken meat and vegetables.
(III) Photosensitized oxidation as described in eq. 1.3±1.5 for riboflavin, to yield
singlet oxygen which as an electrophile may add to the double bond of
unsaturated lipids to yield lipid hydroperoxides directly as for lipoxy-
genases. In addition to the singlet oxygen mediated (Type II, physical
quenching) photosensitized lipid oxidation shown in Fig. 1.5, triplet excited
photosensitzers may as strong oxidants form lipid radicals by hydrogen
atom abstraction or by electron abstraction followed by deprotonation from
the lipid substrate (Type I, chemical quenching) to form radicals as will
initiate chain reaction as described for metal catalysis (I).
The lipid hydroperoxides formed in the lipoxygenase initiated lipid oxidation or
in the Type II photosensitized lipid oxidation may be cleaved reductively by