Food Chemistry

3.7 Changes in Acyl Lipids of Food 199

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 metal-
  bound 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 dur-
  ing 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.

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 composi-

tion 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 un-

saturated acyl lipids only when they contain hy-

droperoxides. That is, the presence of a hydroper-

oxide group is a prerequisite for metal ion ac-

tivity, which leads to decomposition of the hy-

droperoxide group into a free radical:

Men⊕+ROOH→Me(n+1)⊕+RO·+OH

(3.63)

Re(n+1)⊕+ROOH→RO· 2 +H⊕+Men⊕ (3.64)

Reaction rate constants for the decomposition

of linoleic acid hydroperoxide are given in

Table 3.29. As seen with iron, the lower oxi-

dation 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 decomposi-

tion. 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⊕+Men⊕ (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 (aw)

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