Understanding oxidation processes in foods - Oxidation in Foods and Beverages and Antioxidant Applications - page 14

Table 1.3 Bond dissociation energies (BDE) for hydrogen-oxygen bonds in phenolic
antioxidants in lipophilic environment a and standard reduction potentials in aqueous
solution for their one-electron oxidized radicals b

BDE (kJ mol ÿ1 ) -OH Eë / V vs NHE

-tocopherol 330.0 0.31

-tocopherol 335.3

-tocopherol 334.9

-tocopherol 341.5

Quercetin 343.0 0.29

(±) Epicatecin 343.2 0.33

Catecin 348.1 0.36

(±) Epigallocatecin 344.6

(±) Epigallocatecholgallate 339.0

a

From Denisova and Denisov (2008). Solvent for tocopherols is styrene, for the plant polyphenols
methyl linoleate micelles in water were used.
b

From Jùrgensen and Skibsted (1998). Solvent is aqueous solution with pH = 6.4 and 25 ëC. Value
reported for -tocopherol is based on analogy with the water-soluble Trolox.

bond strength shows large variation among individual plant phenols providing
the rich diversity in antioxidant properties known for this group of secondary
plant metabolites. The bond dissociation energy (BHE) may be based on
quantum mechanical calculations and be valid for the gas phase only (Ami and
LuciÂ, 2010), or be based on the rate for reaction with peroxyl radicals
corresponding to the reaction of eq. 1.17 in lipophilic solvents or in a lipophilic
environment (Denisova and Denisov, 2008). For a few phenolic antioxidants
values for BHE in lipophilic solvents or in a lipophilic environment for
discussion of antioxidant synergism in Section 1.6 are found in Table 1.3. -
tocopherol has the lowest BHE among the lipophilic antioxidants, which further
is smaller than the BHE for plant phenols. For comparison, the BHE for the
hydrogen-oxygen bond in lipid hydroperoxides is approximately 380 kJ mol ÿ1 ,
establishing the sequence for reactivity of radicals shown in Fig. 1.6.
Oxidation of neat lipids is relatively well understood, and the rate of lipid
oxidation depends mainly on the degree of unsaturation with fish oil oxidizing
faster than plant oils, and with animal fat especially of ruminants being most
stable. Lipid oxidation is characterized by a lag phase, which for comparable
unsaturation depends mainly on the nature and concentration of antioxidants
present in the oil or fat. In disperse systems initiation of oxidation is linked to
the increased surface of the lipids and lipids become more vulnerable to
oxidation especially in the presence of iron or copper ions which may bind to
negatively charged surfactants in the lipid/water interface. For oil-in-water
emulsions spatial location of antioxidants has been shown to be of outmost
importance (Laguerre et al., 2009). Traditionally the so-called polar paradox,
i.e. the fact that hydrophilic antioxidants are efficient in bulk oil while

lipophilic antioxidants are more efficient in emulsions, has been used for