Substances, acidic astringent

Tannins are amorphous substances, which produce colloidal acidic aqueous solutions with astringent taste. With iron salts (FeCls) they form dark blue or greenish black water-soluble compounds. Tannins form insoluble and indigestible compounds with proteins, and this is the basis of their extensive use in the leather industry (tanning process), and for the treatment of diarrhoea, bleeding gums and skin injuries. 

Tannins—Tannic acid——322.—Quite a number of different substances of vegetable origin, principally derived from barks, leaves, and seeds. Tliey are amorphous, soluble in water, astringent, capable of precipitating albumen, and of forming imputrescible compounds with the gelatinoids. They are, with one possible exception, glucosides. 

Guinotannic acid—a tannin existing in cinchona barks, probably in combination with the alkaloids. It is a light yellow substance soluble in w ater, alcohol, and ether its taste is astringent, but not bitter. Dilute HjSOi decomposes it, at a boiling temperature, into glucose and a red substance—quinova red. 

Water absorbs about 100 times its volume of this gas, and acquires a pale yellow colour, and the astringent, not acid, taste and peculiar smell of the gas. It bleaches powerfully, and is easily decomposed by all substances which have a strong affinity for chlorine, or for oxygen. 

Phytochemistry Leaves, flowers, bark and seeds contain glycosides (amygdalin, prulaurasin, and prunasin). Prussic acid is found in the bark and leaves. Fruits contain malic and citric acids, sugar, astringent substances, ascorbic acid, and flavonoids (Maznev 2004 Deineka et al. 2004). 

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