Trace amounts of metals, e.g. Fe, Cu, act as promoters of
lipid oxidation in the presence of hydroperoxides.
Transition metal ions are involved in one-electron redox
reaction, which leads to hydroperoxide decomposition
[Bondet et al., 2000]:
Men+ + ROOH ® RO• + Me(n+1)+ + OH–
Me(n+1)+ + ROOH ® ROO• + Men+ + H+
The alkoxyl and peroxyl radicals formed initiate the chain
reaction of the autoxidation. A low concentration of ascorbic
acid accelerates the decomposition of hydroperoxides as
ascorbic acid reduces ferric ions. The lower oxidation state
provides over ten-fold faster decomposition rate than the
higher state [Belitz & Grosch, 1999]. Metals are thought to
play a key role in the initiation of autoxidation process, as it
is difficult to eliminate traces of metals during food processing.
Metal ions may originate from plant and animal tissues.
Influence of processing and storage conditions is also not
negligible. Optimal pH for Fe and Cu activity is 5.5–6