Understanding oxidation processes in foods - Oxidation in Foods and Beverages and Antioxidant Applications - page 27

Table 1.6 Electron donor/acceptor classification of potential radical scavengers a

R a Low electron acceptor index High electron acceptor index

R d (bad acceptors) (good acceptors)

Low electron donation index Good radical scavengers Best radical scavengers

(good donors) Good antioxidants Example: -carotene

Example: Vitamin E

High electron donation index Poor radical scavengers Good radical scavengers

(bad donors) Example: Vitamin C Good antireductants

Example: Astaxanthin

a

R a and R d are relative to fluor and sodium, respectively, as defined in Table 1.5. Based on Martinez
et al. (2008).

flavonoids like quercetin and the tea polyphenols (Wolfe and Liu, 2007) in
agreement with the two-dimensional classification. Still ascorbic acid may
prevent oxidation reaction but rather through oxygen depletion or through
regeneration of phenolic antioxidants. The highly coloured anthocyanins have
been classified as relatively good electron donors (R d moderately high) and bad
electron acceptors (R a low) resulting in antioxidant properties between vitamin
C and vitamin E (Table 1.6). However, solvents effect seems significant due to
their positive charge (MartÂnez, 2009). It should further be noted (Table 1.5) that
the most efficient singlet-quenchers are found among the good antireductants,
while the best antioxidants are the best phenoxyl scavengers and best reductors.
Antioxidant synergism between /-carotene and tocopherols/tocotrienols
has been observed in red palm oil (Schroeder et al., 2006). Since -tocopherol
and -tocotrienol were shown to regenerate the carotenes from their radical
cations, rather than the opposite, and in agreement with their respective standard
reduction potentials as seen in Table 1.5, it was concluded that the carotenoids
were oxidized sacrificially, in effect protecting the tocopherols/tocotrienols as
the better (chain-breaking) antioxidants. The two-dimensional classification of
the actual compounds now available (Table 1.5) seems to confirm this previous
conclusion with the carotenes being the best radical scavengers and the
tocopherols/tocotrienols the best antioxidants. This important type of antioxidant
synergism in model systems depends on differences in reaction rate and may be
classified as a kinetic effect. For systems where the more efficient chain-
breaking antioxidants are regenerated by the antioxidant less efficient as chain
breakers but being more reducing, the synergism is rather the result of
thermodynamic control. Such examples may be found in the interaction between
-tocopherol and plant phenols. Although the BDE predicts the tocopherols to
be more reducing than the plant phenols, as seen for quercetin and the tea
polyphenols in Table 1.3, solvent effects including pH are very important for the
redox potential of plant phenols, which in solution may become the more
reducing as seen for quercetin as compared to -tocopherol (Pedrielli and
Skibsted, 2002).