2009_Book_FoodChemistry

3.7 Changes in Acyl Lipids of Food 199
The concentration of heavy metal ions that results
in fat (oil) shelf-life instability is dependent on the
nature of the metal ion and the fatty acid composition
of the fat (oil). Edible oils of the linoleic acid
type, such as sunflower and corn germ oil, should
contain less than 0.03 ppm Fe and 0.01 ppm Cu to
maintain their stability. The concentration limit is
0.2 ppm for Cu and 2 ppm for Fe in fat with a high
content of oleic and/or stearic acids, e. g. butter.
Heavy metal ions trigger the autoxidation of unsaturated
acyl lipids only when they contain hydroperoxides.
That is, the presence of a hydroperoxide
group is a prerequisite for metal ion activity,
which leads to decomposition of the hydroperoxide
group into a free radical:
Me n⊕ + ROOH → Me (n+1)⊕ + RO· + OH ⊖ (3.63)
Re (n+1)⊕ + ROOH → RO·2 + H ⊕ + Me n⊕ (3.64)
Fig. 3.25. Side reaction of a branched furan fatty acid
with singlet oxygen (R 1 : (CH 2 ) 7 COOH)
which then propel the radical chain reaction
of the autooxidation process. Fats, oils and
foods always contain traces of heavy metals,
the complete removal of which in a refining
step would be uneconomical. The metal ions,
primarily Fe, Cu and Co, may originate from:
• Raw food. Traces of heavy metal ions are
present in many enzymes and other metalbound
proteins. For example, during the
crushing and solvent extraction of oilseeds,
metal bonds dissociate and the free ions bind
to fatty acids.
• From processing and handling equipment.
Traces of heavy metals are solubilized during
the processing of fat (oil). Such traces
are inactive physiologically but active as
prooxidants.
• From packaging material. Traces of heavy
metals from metal foils or cans or from
wrapping paper can contaminate food and
diffuse into the fat or oil phase.
Reaction rate constants for the decomposition
of linoleic acid hydroperoxide are given in
Table 3.29. As seen with iron, the lower oxidation
state (Fe 2+ ) provides a ten-fold faster
decomposition rate than the higher state (Fe 3+ ).
Correspondingly, Reaction 3.63 proceeds much
faster than Reaction 3.64 in which the reduced
state of the metal ion is regenerated. The start of
autoxidation then is triggered by radicals from
generated hydroperoxides.
The decomposition rates for hydroperoxides
emulsified in water depend on pH (Table 3.29).
The optimal activity for Fe and Cu ions is in the
pH range of 5.5–6.0. The presence of ascorbic
acid, even in traces, accelerates the decomposition.
Apparently, it sustains the reduced state of
the metal ions.
The direct oxidation of an unsaturated fatty acid
to an acyl radical by a heavy metal ion
RH+ Me (n+1)⊕ → R· + H ⊕ + Me n⊕ (3.65)
proceeds, but at an exceptionally slow rate. It
seems to be without significance for the initiation
of autoxidation.
The autoxidation of acyl lipids is also influenced
by the moisture content of food. The reaction rate
is high for both dehydrated and water-containing
food, but is minimal at a water activity (a w )
of 0.3 (Fig. 0.4). The following hypotheses are
discussed to explain these differences: The high
reaction rate in dehydrated food is due to metal