Food Chemistry

200 3 Lipids

Table 3.29.Linoleic acid hydroperoxidesa: decomposition by heavy metal or heme compounds at 23◦C. Relative
reaction rates krelare given at two pH’sa

Heavy
metal
ionb

krel Heme

compoundb

krel

pH 7 pH 5.5pH7pH5. 5

Fe^3 + 1102 Hematin 4. 103 4. 104
Fe^2 + 14 103 Methemoglobin 5. 103 7. 6. 103
Cu^2 + 0. 21. 5 Cytochrome C 2. 6. 103 3. 9. 103
Co^3 + 6. 102 1 Oxyhemoglobin 1. 2. 103
Mn^2 + 0 0 Myoglobin 1. 1. 103
Catalase 1
Peroxidase 1

aLinoleic acid hydroperoxide is emulsified in a buffer.
bReaction rate constant is related to reaction rate in presence of Fe 3 +at pH 7 (krel=1).

ions with depleted hydration shells. In addition,
ESR spectroscopic studies show that food drying
promotes the formation of free radicals which
might initiate lipid peroxidation. As the water
content starts to increase, the rate of autoxidation
decreases. It is assumed that this decrease in rate
is due to hydration of ions and also of radicals.
Above an awof 0.3, free water is present in
food in addition to bound water. Free water
appears to enhance the mobility of prooxidants,
thus accounting for the renewed increase in
autoxidation rate that is invariably observed at
high moisture levels in food.

3.7.2.1.7 Heme(in)Catalysis......................................

Heme (Fe^2 +) and hemin (Fe^3 +) proteins are
widely distributed in food. Lipid peroxidation
in animal tissue is accelerated by hemoglobin,
myoglobin and cytochrome C. These reac-
tions are often responsible for rancidity or
aroma defects occurring during storage of
fish, poultry and cooked meat. In plant food
the most important heme(in) proteins are
peroxidase and catalase. Cytochrome P 450 is
a particularly powerful catalyst for lipid per-
oxidation, although it is not yet clear to what
extent the compound affects food shelf life “in
situ”.
During heme catalysis, a Fe^2 + protoporphyrin
complex (P–Fe^2 +), like in myoglobin, will
be oxidized by air to P–Fe^3 + as indicated in
Formula 3.66. The formed superoxide radical
anion O− 2 , whose properties are discussed

below, will further react yielding H 2 O 2 .Hy-

drogen peroxide will then oxidize P–Fe^3 +

to the oxene species P–Fe=O. The reac-

tion with H 2 O 2 is accelerated by acid/base

catalysis, facilitating the loss of the water

molecule; the hemin protein and one car-

boxylic group of the protoporphyrin system

acts as proton acceptor and proton donor

respectively.

Oxene is the active form of the hemin catalyst. It

oxidizes two fatty acid hydroperoxide molecules

to peroxy radicals that will then initiate lipid per-

oxidation.

In comparison with iron ions, some heme(in)

compounds degrade the hydroperoxides more

rapidly by several orders of magnitude (cf. Ta-

ble 3.29). Therefore they are more effective as

initiators of lipid peroxidation. Their activity is

also negligibly influenced by a decrease in the

pH-value.

However, the activity of a heme(in) protein to-

wards hydroperoxides is influenced by its steric

accessibility to fatty acid hydroperoxides. Hy-

droperoxide binding to the Fe-porphyrin moiety

of native catalase and peroxidase molecules is ob-

viously not without interferences. The prosthetic

group is free to promote hydroperoxide decom-

position only after heat denaturation of the en-

zymes. Indeed, a model experiment with peroxi-

dase showed that the peroxidation of linoleic acid

increased by a factor of 10 when the enzyme was

heated for 1 minute to 140◦C. As expected, the

enzymatic activity of peroxidase decreased and

was only 14%. Similar results were obtained in

reaction systems containing catalase.