The genus Quercus covers several hundred species and natural hybrids, distributed mainly over the temperate zones of the northern hemisphere (subgenus euquercus), as well as in tropical and subtropical regions of Asia (subgenus cyclobalanopsis). Oaks may be evergreen or deciduous trees, arborescent shrubs, or bushes. Their longevity often exceeds 400 years ().
The different tissues of oak trees often accumulate large amounts of poly-phenols. These polyphenols are mainly tannins. The bark of Quercus robur and Quercus petraea in Europe (known as tan) (Meunier and Vaney 1903), and that of Quercus velutina and Quercus prinus in the United States (), were used on a large scale in the leather industry until the end of the last century.
Bark, acorn cups, and galls from oaks, all rich in tannins, have been traditionally used in pharmacology for their astringent, hemostatic, and antiseptic properties. Tannic acid produced from galls of Quercus infectoria has been most commonly used; acorns from Quercus robur and galls from Quercus suber, as well as barks from both species, have also been used (). The main therapeutic applications of these tannins were externally to heal wounds, burns, dermatosis, hemorrhoids, etc., and internally in the treatment of diarrhea and gastric ulcers.
More recently, the effect of tannin structure on different biological activities of medicinal interest has been examined, i.e., in radical scavenging, inhibition of enzymes, enhancement of immune response, replication inhibition of human immunodeficiency viruses, or inhibition of carcinogen mutagenicity (). The nature and extent of the biological activities have been shown to vary markedly with different purified tannins used.
Oak materials often contain tannins as main extractives. The tannin content commonly reaches 5 to 10% of dry matter. They may be condensed or hydrolyzable tannins. The structure of the main molecules so far identified in oak materials are described. Techniques of propagation (micropropagation and somatic embryogenesis), as well as techniques of tissue and cell culture, are reviewed. The production of tannins by in vitro oak materials is examined.
Cell cultures will undoubtedly provide a valuable tool to study the biosynthesis of ellagitannins and its regulation, which may help to elucidate the biosynthesis of a particular tannin molecule with useful properties. It is remarkable that physicochemical properties of the C-glucosidic ellagitannin vescalagin, such as solubility (), affinity for proteins, polysaccharides (), or anthocyanin () differ markedly from those of other non-C-glucosidic ellagitannins. Oak materials may thus in the future be expected to provide extracts with unique biological or pharmacological properties.
Selections from the book: “Medicinal and Aromatic Plants V”, 1993.