Food Chemistry

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 × 104

Linoleic acid 7. 3 × 104 No reaction

Linolenic acid 1. 0 × 105 1. 2 × 103

Arachidonic acid 1. 7 × 103

Cholestererol 2. 5 × 108 3. 1 × 103

Amino acids

Histidine 4. 6 × 107 4. 8 × 109 < 1. 0 < 95

Tryptophane 1. 3 × 107 1. 3 × 1010 < 24

Cysteine 5. 0 × 107 1. 9 × 1010 < 15 < 600

Cystine 2. 1 × 109 < 4. 0 × 10 −^1

Methionine 1. 3 × 107 7. 4 × 109 < 3. 3 × 10 −^1 < 49

Sugar

Glucose 1. 4 × 104 1. 5 × 109

Fructose 1. 6 × 109

Sucrose 2. 5 × 104 2. 3 × 109

Maltose 2. 3 × 109

Vitamins

β-Carotene 5. 0 × 109

Riboflavin 6. 0 × 107 1. 2 × 1010

Ascorbic acid 1. 1 × 107 8. 2 × 109 1. 6 × 104

Vitamin D 2. 3 × 107

α-Tocopherol 13. 2 × 107 No reaction 2. 0 × 105

other substances. It easily decomposes with the
formation of radicals, which can start lipid perox-
idation.
Hydrogen peroxide, H 2 O 2 , is the second inter-
mediate of oxygen reduction. In the absence of
heavy metal ions, energy-rich radiation includ-
ing 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 abstrac-
tion of an H-atom,

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

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 re-

lation to initiation of autoxidation. This is the

so-calledFentonreaction in particular of an Fe-

complex:

(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 2 O 2

present and generate free HO•radicals.