Antioxidants and Shelf Life of Foods - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 183

TABLE 6.4
Sources of Tocopherols and Tocotrienols (ppm)

Source α T β T γ T δ T P-8 α T3 β T3 γ T3 δ T3

Oils
Soybean 101 — 593 264 — — — — —

Canola 313 — 287 7 74 — — — —

Sunflower 1000 — 5 1 — — — — —

Flax 25 — 201 8 131 — — — —

Rice 124 40 50 5 — 184 21 570 —

Palm 279 — 61 — — 274 — 398 69

Coconut 5 — — 6 — 7 1 19 —

Cereals
Wheat 14 7 — — — 33 — — —

Wheat germ 239 90 — — — 30 100 — —

at 5 1 ——— 11 2 ——

Rye 16 4 — — — 15 8 — —

Rice (brown) 6 1 1 — — 4 — 10 —

Rice bran 3 15 4 2 — 1 14 22 29

Barley 2 4 — 1 — 11 3 2 —

Barley bran 11 16 36 4 — 36 25 19 11

Abbreviations: T = tocopherol isomer; T3 = tocotrienol isomer; P-8 = plastochromanol.

dl -tocopherol, ±-tocopherol or 2DL,4 D-tocopherol). Individual tocopherol ste-

reoisomers have been found to have different antioxidant activities in biological
systems ( Figure 6.7 ).
Tocotrienols have one chiral center in phytol chain at the second carbon atom
so that only two stereoisomers are possible, 2D and 2L. The presence of double
bonds in the phytyl chain at 3

″ D,8 ′

carbon atom, however, generate four cis/trans
isomers per tocotrienol molecule. The antioxidant activities of tocotrienol isomers
have not been evaluated. 45
Chromanols are probably the most efficient lipid antioxidants produced by
nature. The antioxidant activity of these components is related to the following:
(1) phytyl chain with phenolic ring make them lipid soluble; (2) lipid radicals react
with them several times faster than with other lipid radicals; 46 and (3) one tocopherol
molecule can protect about 10 3 to 10 8 molecules of polyunsaturated fatty acid
molecules at low peroxide values. 47 Tocopherols act as antioxidants by donating a
hydrogen atom from the hydroxyl on the ring system to a free radical. 48 Unsubstituted
phenols are not hydrogen donors, while the reactivity of substituted phernols is
mainly attributed to two factors: (1) inductive effects of electron-releasing substitutes
in the position ortho - and para- to the hydroxy group/function, and (2) stereoelec-
tronic effects related to the orientation of substituents to the aromatic ring. 49 Electron-
releasing substituents present in the ortho- and para- position increases the electron
density of the active center(s) promoting release of hydrogen from hydroxyl group
and improving reactivity with peroxy radicals. 50 From this mechanism,

′ and 7 ′

α -tocopherol,