Tannins

TANNINS are widely distributed in the plant kingdom, as they are contained in various parts of very many plants (e.g., in the bark of oak and willow, blueberries, tea leaves, rhubarb rhizome, and grape skins). The common properties of all tannins are: an astringent taste, solubility in water, the ability to precipitate proteins, gelatin, alkaloids, and salts of heavy metals from solution, and the ability to produce a black or green coloration with ferric salts. Tannins are divided by their chemical nature into two large groups: 1) hydrolyzable, i.e., those that add water under the influence of dilute mineral acids and certain enzymes, splitting into simpler molecules, and 2) condensed tannins, which are incapable of undergoing hydrolytic cleavage. Both groups are very closely related to phenol acids, and although the second group is more common in nature (e.g., the tannin of oak bark belongs to it), the chemical nature of the first group is better studied; a characteristic and important representative of this group is tannin, otherwise known as tannic acid (Tanninum s. Acidum tannicum). The exact chemical structure of tannin has not yet been established, but the hypothesis of Fischer and Freudenberg (E. Fischer, K. Freudenberg) must be considered very probable; they, having shown that tannin yields glucose and meta-digallic acid upon hydrolysis, consider it a penta-ester of glucose and digallic acid, as shown by the following formula: C₃₄H₂₈O₂₂[CO - C₆H₃(OH)₂OCOC₆H₃(OH)₂]₅. The pharmacological action of tannins is due to their ability to precipitate the proteins of the protoplasm of tissue cells and intercellular substance, whereby tannins exert a locally irritating or astringent effect on mucous membranes depending on the concentration of the solution. When an excessively large dose of tannins is taken internally, the irritating effect may manifest as nausea, vomiting, and diarrhea. For therapeutic purposes, tannins are used as astringents, as well as hemostatics, since tannins, by precipitating blood proteins, cause it to coagulate. Among the plant preparations containing tannins are various parts of many plants of the temperate and tropical zones, for example: nutgalls—Gallae turcicae—in the form of Tinctura Gallarum (per se for swabbing, e.g., for stomatitis); oak bark from Quercus pedunculata—Cortex Quercus (5–10% decoction); sage leaves from Salvia officinalis—Folium Salviae (infusion 10.0–15.0 : 200.0); Radix Ratanhiae from the Peruvian Krameria triandra (per se 0.5–1.0 in powders and pills, externally—decoction 5.0–15.0 : 100.0); Tinctura Ratanhiae (10–20 drops, externally per se for swabbing the gums); Extractum Ratanhiae (0.5–1.5 in pills and mixture, externally—10% ointment for healing wounds); Catechu—dried juice of the East Indian Acacia Catechu (0.5–2.0); Tinctura Catechu (like Tinctura Ratanhiae); Kino—dried juice from Pterocarpus marsupium (0.5–1.0, externally—decoction 3.0 to 10.0 : 150.0); Tinctura Kino (like Tinctura Ratanhiae); bark and flowers of the North American Hamamelis virginica—Extractum Hamamelidis fluidum (30–40 drops, especially for hemorrhoidal bleeding); West Indian logwood—Lignum Campechianum (decoction 20.0 : 200.0, 1 tablespoon pro dosi); bistort rhizome—Rhizoma Polygoni bistortae (decoction 1.0 : 15.0 or Extractum Bistortae fluidum); tormentil root—Rhizoma Tormentillae from Potentilla tormentilla (decoction—a folk remedy against dysentery); blueberries—Fructus Myrtilli (in the form of a decoction or liquid soup). For tannins in manufacturing, see Leather industry.