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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.

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