2009_Book_FoodChemistry

200 3 Lipids
Table 3.29. Linoleic acid hydroperoxides a : decomposition by heavy metal or heme compounds at 23 ◦ C. Relative
reaction rates k rel are given at two pH’s a
Heavy k rel Heme k rel
ion b 7 pH 5.5
metal
pH
compound b
pH7 pH 5.5
Fe 3+ 1 10 2 Hematin 4.10 3 4.10 4
Fe 2+ 14 10 3 Methemoglobin 5.10 3 7.6.10 3
Cu 2+ 0.2 1.5 Cytochrome C 2.6.10 3 3.9.10 3
Co 3+ 6.10 2 1 Oxyhemoglobin 1.2.10 3
Mn 2+ 0 0 Myoglobin 1.1.10 3
Catalase 1
Peroxidase 1
a Linoleic acid hydroperoxide is emulsified in a buffer.
b Reaction rate constant is related to reaction rate in presence of Fe 3+ at pH 7 (k rel = 1).
below, will further react yielding H O . Hydrogen
O2 ,
ions with depleted hydration shells. In addition,
anion whose properties are discussed reaction systems containing catalase.
ESR spectroscopic studies show that food drying
peroxide will then
2
oxidize
2
P–Fe 3+
promotes the formation of free radicals which to the oxene species P–Fe=O. The reaction
might initiate lipid peroxidation. As the water
with H 2 O 2 is accelerated by acid/base
content starts to increase, the rate of autoxidation catalysis, facilitating the loss of the water
decreases. It is assumed that this decrease in rate molecule; the hemin protein and one carboxylic
is due to hydration of ions and also of radicals.
group of the protoporphyrin system
Above an a w of 0.3, free water is present in acts as proton acceptor and proton donor
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.
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 peroxidation.
In comparison with iron ions, some heme(in)
compounds degrade the hydroperoxides more
3.7.2.1.7 Heme(in) Catalysis
rapidly by several orders of magnitude (cf. Table
3.29). Therefore they are more effective as
Heme (Fe 2+ ) and hemin (Fe 3+ ) proteins are
widely distributed in food. Lipid peroxidation
in animal tissue is accelerated by hemoglobin,
initiators of lipid peroxidation. Their activity is
also negligibly influenced by a decrease in the
pH-value.
myoglobin and cytochrome C. These reactions
However, the activity of a heme(in) protein to-
are often responsible for rancidity or wards hydroperoxides is influenced by its steric
aroma defects occurring during storage of accessibility to fatty acid hydroperoxides. Hydroperoxide
fish, poultry and cooked meat. In plant food
binding to the Fe-porphyrin moiety
the most important heme(in) proteins are of native catalase and peroxidase molecules is obviously
peroxidase and catalase. Cytochrome P 450 is
not without interferences. The prosthetic
a particularly powerful catalyst for lipid peroxidation,
although it is not yet clear to what
extent the compound affects food shelf life “in
situ”.
During heme catalysis, a Fe 2+ protoporphyrin
group is free to promote hydroperoxide decomposition
only after heat denaturation of the enzymes.
Indeed, a model experiment with peroxidase
showed that the peroxidation of linoleic acid
increased by a factor of 10 when the enzyme was
complex (P–Fe 2+ ), like in myoglobin, will heated for 1 minute to 140 ◦ C. As expected, the
be oxidized by air to P–Fe 3+ as indicated in enzymatic activity of peroxidase decreased and
Formula 3.66. The formed superoxide radical
was only 14%. Similar results were obtained in