Food Lipids: Chemistry, Nutrition, and Biotechnology

droxycoumarin (scopoletin), and hydroxybenzoic acids (4-hydroxybenzoic, ellagic,
gallic, gentisic, protocatechuic, salicylic, and vanillic acids) are phenolic compounds
that can form metal complexes. Antioxidant activity of these compounds varies
greatly and is also dependent on the food system. Hydroxycinnamic acid esters were
found to be more active than the free acids in model systems involving linoleic
acid [53].
Flavones (apigenin, chrysoeriol, diosmetin, isovitexin, luteolin, and nobiletin)
and flavonols (gossypetin, isorhamnetin, kaempferol, myricetin, robinetin, and quercetin)
occur in fruits as glycosides. These compounds are also prevalent in vegetables,
tea, and wine [54]. Quercitin has gained attention as a very potent antioxidant
[55]. It has been shown to be very effective in linoleic model systems [56]. Phenolic
acids, flavonoids, and other phenolics have potential as food antioxidants. Contents
of specific phenolic compounds in plants can be very low, requiring large amounts
of raw material to obtain sufficient amounts of these antioxidants. Usage can be
limited because the compounds are often present in the form of glycosides and are
not soluble in oil. In addition, some flavonoids are toxic.
b. Other Natural Sources of Antioxidants. Numerous plants have been identified
as sources of phenolic compounds with antioxidant activity. The list of natural
antioxidants is growing as a result of the amount of research that is being conducted
to isolate and identify these compounds in plants. A comprehensive listing of all the
sources of antioxidant compounds identified in plant materials is beyond the scope
of this chapter. Recent reviews by Pratt [56] and Pratt and Hudson [57] contain
detailed information on numerous phenolic compounds. Some recent research findings
are given below. The diversity of plant sources and compounds considered to
be antioxidants is evident even from this brief description.
For example, flavonoid derivatives have been recently identified as potent antioxidants
found in apples [58,59] and chrysanthemum [60]. Antioxidant activity has
been correlated to flavonoids found in rice, buckwheat, barley, and malt [61–64].
Other examples of antioxidants include tannins from bark, lignans from papua mace,
and capsaicin from peppers. Compounds with antioxidant activity isolated from high
alpine plant species [65], marine sources [66], and wood smoke [2] have also been
characterized.
Tea. Tea extracts are a source of natural antioxidants. Tea catechins (Fig. 6)
have potent antioxidant activity [67–71]. Extracts of green and black tea contain
epicatechin, epicatechin gallate, epigallocatechin, epigallocatechin gallate, and gallocatechins
[72]. During the fermentation process required to produce black tea,
catechins are oxidized to produce flavins and flavin gallates that have antioxidant
activity [73]. Tea antioxidants have been patented for use in several food products
[74–76].
Sesame seed compounds. Several antioxidative compounds have been isolated
and identified from sesame seed oil (Fig. 7). The compounds are lignanphenols such
as pinoresinol sesaminol, sesamol (and its dimer), and sesamolinol [77–79]. Sesame
seed oil has a much longer shelf life than many edible oils because of the presence
of these phenolic compounds [80].
Soybean. Soybeans have several antioxidative constituents that include soy
proteins [81]. Soy and other vegetable hydrolysates contain phenolic compounds with
antioxidant activity. Fermented soy products contain isoflavones and genistein, which
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