Ho O

Figure 1. Flavones: the basic structure of flavonoids.

Carbohydrates

Acetyl-CoA +C02

3-Malonyl-CoA

Phosphoenolpyruvate — + D-Erythrose-4-phosphate

Heptulosonic acid

Shikimic acid

Phenylalanine

4-Coumaroyl-CoA

5-Dehydroquinic acid

Cinnamic acid p-Coumaric acid

4,2',4',6'-Tetrahydroxychalcone

Naringenin (Flavanone)

Isoflavones (genistein) Flavones (apigenin) Dihydroflavonols (dihydrokaempferol)

Flavonols Catechins Proanthocyanidins Anthocyanidins

Figure 2. Biosynthetic pathways of phenolic compounds.

most reliable method for the determination of total phenols in food analysis laboratory is based on oxidation with the Folin-Ciocalteu reagent, which contains sodium phos-phomolybdate and sodium tungstate. The intensity of the resulting blue complex can be estimated with a colorimeter or with a spectrophotometer Umax at 725 nm). The colorless procoanthocyanidins can be estimated by heating them with acid and converting them to colored anthocyanidins. Individual phenolic compounds have been determined qualitatively by paper chromatography and semi-quantitatively by densitometric analysis of the colored spots obtained by spraying two-dimensional chromato-gram with a suitable reagent. Thin layer chromatography has been widely employed for polyphenols, because it is a highly effective, convenient, and inexpensive technique, especially for the separation of anthocyanin.

Today, it is possible to separate and quantify the individual polyphenols of fruits and vegetables by means of high-performance liquid chromatography (HPLC) with great success (13-15). HPLC has advantages of sensitivity, speed, and ease of use compared with other chromatographic procedures. When coupled with a diode array detector, HPLC provides an ideal procedure for accurately analyzing complex mixtures of polyphenols. Hydroxycin-namic acid esters in fruits and vegetables have been successfully separated by using a polyamide column (5). However, before using these procedures, the polyphenols must first be fractionated into several chemical groups to separate the individual polyphenols effectively (15,16). In ad dition, proton magnetic resonance, 13C nuclear magnetic resonance (NMR), and mass spectrophotometry have been extensively used to determine the structures of polyphenols. Noteworthy achievements are the electron impact, chemical ionization, and fast atom bombardment (FAB) mass spectrometry that have been employed for the identification of various flavonoids, oligomeric hydrolyzable tannins, polyphenolic glycosides, and procyanidins (17). Previously, it had been difficult to obtain mass spectra of anthocyanins because they are not volatile and often present as salts. FAB mass spectrometry, however, gives molecular ions directly as (M)+, and, therefore, its use has been extensive. The early use of CCl4-soluble flavonoid tri-methylsilyl ether derivatives for *H NMR spectroscopy and the advent of more sophisticated, higher-field spectrometers that have the capability to run 13C as well as XH NMR spectra have been well reported (18,19). More recently, two-dimensional (2-D) homonuclear and heteronuclear spectra were produced for various flavonoids and its glycosides.

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