Plant Phenolics as Pigments and Flavors

Condensed tannins (also referred to as proanthocyanidins) are flavonoid units linked by carbon-carbon bonds that are not susceptible to cleavage by hydrolysis. These compounds are responsible for the intense pink, red, purple, or blue colors of many flowers, fruits, and leaves. Flavonoids, mainly anthocyanins, are responsible for the bright autumn colors in many plant species. Other flavo-noid pigments having antioxidant properties (quercetins) also increases dramatically in plants, especially in epidermal

Table 2 Major classes of phenolic compounds in plants

No. of C atoms

Basic skeleton

Phenolic class

Examples

6

C6

Simple phenols Benzoquinones

Catechol, hydroxyquinone 2,6-Dimethoxybenzoquinone

7

C6-C1

Phenolic acids

Gallic, salicylic

8

C6-C2

Acetophenones Tyrosine derivatives Phenylacetic acids

3-Acetyl-6-methoxybenzaldehyde Tyrosol p-Hydroxyphenylacetic

9

Phenylpropenes

Coumarins

Isocoumarins

Chromones

Caffeic, ferulic Myristicin, eugenol Umbelliferone, aesculetin Bergonon Euenin

10

C6-C4

Naphthoquinones

Juglone, plumbagin

13

C6-C1-C6

Xanthones

Mangiferin

14

C6-C2-C6

Stilbenes Anthraquinones

Resveratrol Emodin

15

C6-C3-C6

Flavonoids Isoflavonoids

Quercetin, cyanidin Genistein

18

(C6-C3)2

Lignans Neolignans

Pinoresinol Eusiderin

30

(C6-C3-C6)2

Biflavonoids

Amentoflavone

Catechol melanins Flavolans

Adapted from Harborne JB (1980) Plant phenolics. In: Bell EA and Charlwood BV (eds.) Encyclopedia of Plant Physiology, Vol. 8: Secondary Plant Products, pp. 329-394. New York: Springer.

Adapted from Harborne JB (1980) Plant phenolics. In: Bell EA and Charlwood BV (eds.) Encyclopedia of Plant Physiology, Vol. 8: Secondary Plant Products, pp. 329-394. New York: Springer.

cells, upon exposure to light in the UV-B range. These pigments are thought to have a photoprotective role.

Condensed tannins account for the astringent tastes of many fruits and wines. Many plants have fruits, leaves, or roots that are rich in flavorful phenolics. Consider, for example, the tastes of grapes, teas, cranberries, grapefruit, coffee, cinnamon, ginger, and vanilla.

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