Food Lipids: Chemistry, Nutrition, and Biotechnology

gallate. The antioxidant mechanism of the synthetic phenolics involves the formation
of a resonance-stabilized phenolic radical that neither rapidly catalyzes the oxidation
of other molecules nor reacts with oxygen to form antioxidant peroxides that autoxidize
[5].
Synthetic phenolic radicals can potentially react with each other by means of
mechanisms similar to that of �-tocopherol. These include reactions of two phenolic
radicals to form a hydroquinone and a regenerated phenolic, as well as the formation
of phenolic dimers. The phenolic radicals can also react with other peroxyl radicals
in termination reactions resulting in formation of phenolic–peroxyl species adducts.
In addition, oxidized synthetic phenolic undergo numerous degradation reactions (for
review, see Ref. 5). Since many of these degradation products still contain active
hydroxyl groups, the products may retain antioxidant activity. Therefore, the net
antioxidant activity of synthetic phenolics in food actually represents the activity of
the original phenolic plus some of its degradation products. Synthetic phenolics are
effective in numerous food systems; however, their use in the food industry has
recently declined, reflecting safety concerns and consumer demand for all-natural
products.
C. Ubiquinone
Ubiquinone, or coenzyme Q, is a phenolic conjugated to an isoprenoid chain. Ubiquinone
is primarily associated with the mitochondrial membrane [12]. Reduced
ubiquinone is capable of inactivating peroxyl radicals, but its radical scavenging
activity is less than that of �-tocopherol [13]. The lower free radical scavenging
activity of reduced ubiquinone has been attributed to internal hydrogen bonding,
which makes hydrogen abstraction more difficult [13]. Despite its lower radical scavenging
activity, reduced ubiquinone has been found to inhibit lipid oxidation in
liposomes [14] and low density lipoprotein [15]. Presumably, it could be an important
endogenous antioxidant in many foods.
D. Plant Phenolics
Plants contain a diverse group of phenolic compounds including simple phenolics,
phenolic acids, anthocyanins, hydrocinnamic acid derivatives, and flavonoids. Widely
distributed in plant foods such as fruits, spices, tea, coffee, seeds, and
grains, these phenolics have been estimated to be consumed in amounts greater than
1 g/day.
All the phenolic classes have the structural requirements of free radical scavengers
(see Fig. 7 for several examples). However, the antioxidant activity of these
compounds varies greatly, and some even exhibit prooxidant activity. Factors influencing
the antioxidant activity of plant phenolics include position and degree of
hydroxylation, polarity, solubility, reducing potential, stability of the phenolic to food
processing operations, and stability of the phenolic radical. In addition, many phenolics
contain acid or ring groups that may participate in metal chelation. These
metal chelation properties, in addition to high reducing potentials, can accelerate
metal-catalyzed oxidative reactions, leading to the prooxidative activity of plant
phenolics under certain conditions [16,17].
Herbs and spices are sources of phenolic antioxidants used in foods. Rosemary
extracts are the most commercially important source of an antioxidant ingredient
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