Antioxidants and Shelf Life of Foods - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 187

COOH CH CHCOOH

R R R R

3 1 3 1

R R

2 2

Benzoic Acid Cinnamic Acid

Acid Functional

R

Group
R

Position
R

Acid Functional

R

Group
R

Position
R

1 2 3 1 2 3

p -Hydroxybenzoic
Protocatechuic
Gallic
Vanillic
Syringic

H
OH
OH
OCH
OCH

OH
OH
OH
OH
OH

H
H
OH
H
OCH

Coumaric
Caffeic
Ferulic
Sinapsic

H
OH
OCH
OCH

OH
OH
OH
OH

H
H
H
OCH

3

3

3 3

3 3

FIGURE 6.10 Structure of selected phenolic acids.

P HENOLIC A CIDSAND C OUMARINS

A range of substituted benzoic acid and cinnamic acid derivatives comprise two
families of phenolic acids commonly found in plants ( Figure 6.10 ). Both types occur
in conjugated and esterified forms. 72
Vanillic, p- hydroxybenzoic and syringic acids are found in lignin. Gallic acid
has been found less frequently than its dimeric condensation product, ellaigic acid. 32
Caffeic and p -coumaric acids are the most common cinnamic acids and usually
occur as chlorogenic acid, esters of quinic and shikimic acids, or as sugar esters.
The double bond in the side chain causes these acids and their derivatives to exist
as cis and trans isomers. 72
These compounds exist as a variety of sugar esters or glycosides, and are
restricted to a few plant families. 72

F LAVONOID C OMPOUNDS

These compounds are based on a C6-C3-C6 skeleton structure and include by far
the largest and most diverse group of plant phenolics. They are classified according
to substitution patterns, and the position of ring B 30 ( Figure 6.11 ). The major sub-
groups are flavonols, flavones, isoflavones, catechins, proanthocyanidins, and antho-
cyanins. Most flavonoids occur as glycosides in which the aglycone moiety is
esterified with various sugars. The point of attachment of the sugar varies for different
flavonoids, and to add to the complexity, it may be acylated. 72 The pale yellow
flavonols frequently occur in leaves. 72
Flavones differ from flavonols by lacking the 3-hydroxyl group on the hetero-
cyclic ring. Apigenin and luteolin are common in angiosperms, while tricin is present
in grasses 32 (Figure 6.11).
Anthocyanidins all possess the basic flavylin ring structure and normally exist
as glycosides, the latter referred to as anthocyanins ( Figure 6.12 ). These are widely
distributed in leaves, flowers, and fruits of higher plants, with cyanidin (red) being
the most common. The substitution patterns on the B ring modify the color. 72