Astringent

File:AlumCrystal.jpg

A crystal of alum, an astringent

An astringent (also spelled adstringent) substance is a chemical compound that tends to shrink or constrict body tissues, usually locally after topical medicinal application. The word "astringent" derives from Latin adstringere, meaning "to bind fast". Two common examples are calamine lotion and witch hazel.

Astringency is also the dry, puckering mouthfeel caused by tannins found in many fruits such as blackthorn, chokecherry, bird cherry, quince and persimmon fruits, and banana skins. The tannins denature the salivary proteins, causing a rough "sandpapery" sensation in the mouth. Astringency tastes unpleasant to many mammals (including humans), which tend to avoid eating astringent fruit; conversely, birds do not taste astringency and readily eat these fruit. It is thought that fruit astringency gives a selective advantage to some plant varieties because birds are better than mammals at long-distance seed dispersal, often flying a great distance before passing the seeds in their droppings.

Tannins, which are widespread astringent compounds, are found in some red wines and teas. A small amount of astringency is expected in some wines, especially young red wines made from grapes such as cabernet sauvignon.

File:Bottle of tannic acid.jpg

A bottle of tannic acid, an astringent

Uses

Astringent medicines cause shrinkage of mucous membranes or exposed tissues and are often used internally to check discharge of blood serum or mucous secretions. This can happen with a sore throat, hemorrhages, diarrhea, or with peptic ulcers. Externally applied astringents, which cause mild coagulation of skin proteins, dry, harden, and protect the skin. Acne sufferers are often advised to use astringents if they have oily skin. Astringents also help heal stretch marks and other scars. Mild astringent solutions are used in the relief of such minor skin irritations as those resulting from superficial cuts, allergies, insect bites, or fungal infections such as athlete's foot.

Some common astringent agents include alum, oatmeal, acacia, sage, yarrow, witch hazel, bayberry, distilled vinegar, very cold water, and rubbing alcohol. Astringent preparations include silver nitrate, potassium permanganate, zinc oxide, zinc sulfate, Burow's solution, tincture of benzoin, and vegetable substances such as tannic and gallic acids. Balaustines are the red rose-like flowers of the pomegranate, which are very bitter to the taste. In medicine, its dried form has been used as an astringent. Some metal salts and acids have also been used as astringents. Eye drops such as Rohto Ice are used as an astringent.

References

ca:Astringent zh-yue:收濕藥料