Proanthocyanidins

The proanthocyanidins are condensation products of fla-vanols, chiefly the catechins. Acid hydrolysis under stringent conditions ruptures carbon-carbon bonds connecting flavanol moities. Anthocyanidins and catechins are the final products. Alkaline hydrolysis results in the formation of quinone intermediates that may then be converted to anthocyanidins (3). The acid hydrolysis reaction is demonstrated in Figure 2.

The proanthocyanidins are almost invariably associated with their corresponding flavanols (catechins) from which they are derived. During flavanol biosynthesis a portion of the cation intermediate becomes attached to an already synthesized flavanol molecule to form a dimer rather than undergoing enzymic reduction to the monomer. Tri-mers and higher oligomers (condensation products of flavanols or other tannin units) are formed in an analogous fashion (4). The proportion of the various species produced is largely a function of the availability of the necessary reducing enzyme.

It has also been suggested that quinone, rather than cation, intermediates participate in proanthocyanidin formation (3). The quinones involved also react with proteins in a process that has been called quinone tanning. The reactions are irreversible with the formation of new covalent bonds. Insects exploit quinone tanning to the ultimate extent. Their bodies are covered with a hard exoskeleton consisting of quinone-protein adducts that provide protection from desiccation and predators and that account for their biological success (5).

Formation of a dimeric condensation product by the cation mechanism is illustrated in Figure 3. Procyanidins are the most frequently encountered group of the proanthocyanidins. They are derived from the simple catechins (R = H). Prodelphinidins are not as widespread. They are derived from the gallocatechins (R = OH).

Biosynthesis of the parent flavanol proceeds through a complex pathway starting with the conversion of phenylalanine to cinnamic acid. Incorporation of acetate units leads to the formation of chalcones. Ring closure results in flavonoid structures from which catechins are derived (4).

The procyanidins in foods occur in soluble free forms and in insoluble combinations with polysaccharides. Such adducts are the result of covalent bonds formed during the biosynthesis of the procyanidins. Dimeric procyanidins, barely within the molecular size range of tannins, and tri-meric forms are widely distributed in berries and in the leaves of many fruit plants at levels of 50 to 500 mg/100 g fresh weight (6). They are often accompanied by dimeric prodelphinidins, which predominate in strawberries. Fruits and grains are also rich in higher oligomers of catechins. As molecular size increases, the solubility of the proanthocyanidins decreases. Varieties of sorghum, one of the most widely consumed of all grains by humans, differ in tannin levels by a factor of 100. Some fodder varieties contain as much as 6% tannin (7).

Proanthocyanidins and their parent flavanols that occur in human food undergo changes during preparation and storage that often lead to the formation of highly colored polymeric material. Enzymic browning occurs when susceptible polyphenols come in contact with plant oxidases in the presence of atmospheric oxygen as a result of cell disruption caused by spoilage, insect infestation, or other mechanical damage. This effect is exploited in the manufacture of black tea. The cell structure of fresh green tea leaf is disrupted mechanically to promote contact between the abundant cellular flavanol supply and oxidizing enzymes. Proanthocyanidins constitute one fraction of the oxidized catechin group and are responsible for some of the desirable organoleptic properties of black tea. Procyanidins are also important constituents of cocoa. They undergo many changes during the processing of the bean that impact on flavor and color (6). Gallotannins are also present and constitute over 4% of the dried fermented cotyledons.

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