Antioxidants and Shelf Life of Foods - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 181

OH OH

CH 3

HO OH C(CH 33 OC 25

H 3
H 3

N

H

COOC 37 OH

Propyl Gallate (PG) TBHQ Ethoxyquin

FIGURE 6.5 Structure of propyl gallate, tertiary -butylhydroquinone (TBHQ) and ethoxyquin.

NATURAL ANTIOXIDANTS

Most natural antioxidants, with the exception of tocopherols, are phenolic com-
pounds, containing ortho -substituted active groups, whereas synthetic antioxidants,
with the exception of gallates, are para -substituted. 30
The term “phenolic compound” includes a large number of secondary plant
products which differ in chemical structure and reactivity, ranging from simple
compounds to highly polymerized compounds. Many properties of plant products
are associated with the presence of differentpolyphenolic compounds. 31 Phenolic
compounds possess an aromatic ring bearing one or more hydroxyl groups together
with a number of other constituents.At least 5000 phenolic compounds have been
identified with an early reference classifying plant phenolics into 15 groupings. 32
Only some of the major groups will be considered including phenolic acids, cou-
marins, and flavonoid compounds such as anthocyanidins. Commercial formulations
of natural antioxidants available are listed in Table 6.3 .

T OCOPHEROLSAND T OCOTRIENOLS

Tocopherols and tocotrienols, known as chromanols, are well recognized for their
efficient protection against lipid oxidation in food and biological systems. These
components are synthesized by plants and provide essential nutrients for humans
and animals. Tocopherols are present in green parts of higher plants, leaves, and oil
seeds 33 ( Table 6.4 ). There are eight structurally different compounds in the toco-
pherol family ( Figure 6.6 ); four known as tocopherols and four known as toco-
trienols. 34,35 -Tocopherol is present mainly in the plant cell chloroplasts, while the

other three isomers are found outside of these organelles. 33 Tocopherols are present
in refined vegetable oils at levels ranging from 60 to 110 mg/100 g. 36 Tocotrienols
are absent or present in only very small amounts in most vegetable oils with the
exception of palm. 37,38 They are found in the bran and germ parts of cereals and
some seeds 39 ( Table 6.4 ) . Frega and co-workers 40 recently found tocotrienols were
the only fat-soluble antioxidants in the lipid fraction of Annatto ( Bixaorellana L.)
seeds, mainly

α

-tocotrienol. The basic structure of all eight of these compounds is
similar ( Figure 6.6 ) , consisting of a 6-chromanol aromatic ring system containing a
hydroxyl group and a 16-carbon phytol side chain. 35,41 Tocotrienols differ from

δ