218 3 Lipids
includes the 2,3-double bond. The phenoxyl
radical arises from the trapping reaction of
peroxyl radicals with quercetin. Another factor is
the lipophilicity. Catechin is more hydrophobic
than caffeic acid, which is present as an anion at
pH 7.4 and, consequently, cannot penetrate the
linoleic acid micelle. For this reason, although
caffeic acid has OH-groups in ortho position, it is
not an antioxidant under the conditions given in
Table 3.40.
Polyphenols in wood, such as lignin, undergo
thermal cracking, resulting in volatile phenols,
during the generation of smoke by burning wood
or, even more so, sawdust. These phenols de-
posit on the food surface during smoking and then
penetrate into the food, thus acting as antioxi-
dants.
It was also demonstrated that vanillin, in food
items where its aroma is desired, plays an import-
ant role as an antioxidant. Finally, some of the
Maillardreaction products, such as reductones
(cf. 4.2.4.4), should be considered as naturally ac-
tive antioxidants.
3.7.3.2.2 SyntheticAntioxidants...................................
In order to be used as an antioxidant, a synthetic
compound has to meet the following require-
ments: it should not be toxic; it has to be highly
active at low concentrations (0.01–0.02%); it has
to concentrate on the surface of the fat or oil
phase. Therefore, strongly lipophilic antioxidants
are particularly suitable (with low HLB values,
e. g. BHA, BHT or tocopherols, dodecylgallate)
for o/w emulsions. On the other hand, the more
polar antioxidants, such as TBHQ and propyl
gallate, are very active in fats and oils since they
are enriched at the surface of fat and come in
contact with air. Antioxidants should be stable
under the usual food processing conditions.
This stability is denoted as the “carry through”
effect. Some of the synthetic antioxidants used
worldwide are:
(3.85)Propyl (n=2); octyl (n=7) and dodecyl (n=11)
gallate(3.86)2,6-Di-tert-butyl-p-hydroxytoluene (BHT)(3.87)tert-Butyl-4-hydroxyanisole (BHA)
Commercial BHA is a mixture of two isomers,
2- and 3-tert-butyl-4-hydroxyanisole(3.88)6-Ethoxy-1,2-dihydro-2,2,4-trimethylquinoline
(ethoxyquin)(3.89)Ascorbyl palmitate
ESR spectroscopy has demonstrated that a large
portion of ethoxyquin is present in oil as a free
radical(3.90)and stabilization by dimerization of the radical
occurs. The radical, and not the dimer, is the ac-
tive antioxidant.
tert-Butylhydroquinone (TBHQ) is a particularly
powerful antioxidant used, for example, for stabi-
lization of soya oil. The “carry through” proper-