2009_Book_FoodChemistry

202 3 Lipids
Table 3.30. Rate constants of reactions of reactive oxygen species with food constituents
Constituent 1 O 2 HO • O ⊖ 2
HOO •
Lipid k (l × mol −1 × s −1 )
Oleic acid 5.3 ×10 4
Linoleic acid 7.3 ×10 4 No reaction
Linolenic acid 1.0 ×10 5 1.2 ×10 3
Arachidonic acid 1.7 ×10 3
Cholestererol 2.5 ×10 8 3.1 ×10 3
Amino acids
Histidine 4.6 ×10 7 4.8 ×10 9 <1.0 <95
Tryptophane 1.3 ×10 7 1.3 ×10 10 <24
Cysteine 5.0 ×10 7 1.9 ×10 10 <15 <600
Cystine 2.1 ×10 9 <4.0 ×10 −1
Methionine 1.3 ×10 7 7.4 ×10 9 <3.3 ×10 −1 <49
Sugar
Glucose 1.4 ×10 4 1.5 ×10 9
Fructose 1.6 ×10 9
Sucrose 2.5 ×10 4 2.3 ×10 9
Maltose 2.3 ×10 9
Vitamins
β-Carotene 5.0 ×10 9
Riboflavin 6.0 ×10 7 1.2 ×10 10
Ascorbic acid 1.1 ×10 7 8.2 ×10 9 1.6 ×10 4
Vitamin D 2.3 ×10 7
α-Tocopherol 13.2 ×10 7 No reaction 2.0 ×10 5
other substances. It easily decomposes with the
formation of radicals, which can start lipid peroxidation.
Hydrogen peroxide, H 2 O 2 , is the second intermediate
of oxygen reduction. In the absence of
heavy metal ions, energy-rich radiation including
UV light and elevated temperatures, H 2 O 2 is
a rather indolent and sluggish reaction agent. On
the other hand, the hydroxy radical (HO • ) derived
from it is exceptionally active. During the abstraction
of an H-atom,
a complex system such as food, the following
question is always pertinent: “Has the HO •
radical actually reached the unsaturated acyl
lipid, or was it trapped prior to lipid oxidation by
some other food ingredient?”.
The reaction of the superoxide radical anion with
hydrogen peroxide should be emphasized in relation
to initiation of autoxidation. This is the
so-called Fenton reaction in particular of an Fecomplex:
R—H + HO • −→ R • + H 2 O (3.70)
the energy input in the HO-bond formed is
497 kJ/mol, thus exceeding the dissociation
energy for abstraction of hydrogen from each
C–H bond by at least 75 kJ/mol (cf. Table 3.27).
Therefore, the HO • radical reacts non-selectively
with all organic constituents of food at an almost
diffusion-controlled rate. Consequently, it can
directly initiate lipid peroxidation. However, in
(3.71)
The Fe-complex (e. g. with ADP) occurs in food
of plant and animal origin. The Fe 2+ obtained
by reduction with O − 2 can then reduce the H 2O 2
present and generate free HO • radicals.