Effects of processing and storage on antioxidant efficacy in foods - Oxidation in Foods and Beverages and Antioxidant Applications - page 368

stabilization: tert. butylhydroxytoluene (BHT), tert.butylhydroxyanisole (BHA),
polar, very efficient antioxidant propyl gallate (PG), less polar, but efficient
dodecyl gallate (DG) and tert. butylhydroquinone (TBHQ). They belong to the
pyrocatechol and hydroquinol series, which react with free radicals in nearly the
same way.
The first step of the antioxidant action is the reaction with lipid-derived free
radical, which is transformed into less reactive lipid hydroxylated derivative.
The antioxidant is converted into a free radical, which is less reactive than lipid
free radicals. An example of a pyrocatechol derivative is shown in Fig. 15.1.
Two antioxidant radicals are in equilibrium. The free radical antioxidant shown
in Fig. 15.1 can be transformed in a semiquinoid free radical, and again, the two
isomers are in equilibrium (Fig. 15.2).
A hydroquinol derivative behaves in a similar way (Fig. 15.3(a)), and it is
slightly less reactive. It may react with another lipid radical to form a biradical,
which is isomerized into a para-quinone (Fig. 15.3(b)). A pyrocatechol
derivative can form an ortho-quinone in a similar way.
Another possibility is the reaction with a lipidic free radical. A copolymer
with a peroxide structure is produced (Fig. 15.4). Analogous reaction can take
place with pyrocatechol free radicals, which are more frequent among natural
antioxidants. The co-polymerization may be produced with a lipid peroxy
radical, accompanied with a loss of oxygen (Fig. 15.5).
The last possibility is the polymerization by interaction of two antioxidant
free radicals (Fig. 15.6). The dimer again have antioxidant activity so that they
Fig. 15.1 Formation of antioxidant free radical: free radical activity located on an
oxygen atom: (a) substituted pyrocatecol, (b) and (c) isomeric free radicals.
Fig. 15.2 Formation of antioxidant free radical: free radical activity located in benzene
ring: (a) substituted pyrocatecol, (b) and (c) isomeric free radicals.