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Tannins (or tannoids) are a class of astringent, that bind to and precipitate and various other organic compounds including and . The term tannin is widely applied to any large compound containing sufficient and other suitable groups (such as ) to form strong compounds with various .
The term tannin (from scientific French tannin, from French tan "crushed oak bark", tanner "to tan", cognate with English language tanning, Medieval Latin tannare, from Proto-Celtic *tannos "oak") refers to the abundance of these compounds in oak bark, which was used in tanning animal hides into leather.
The tannin compounds are widely distributed in many species of plants, where they play a role in protection from predation (acting as ) and might help in regulating plant growth. The astringency from the tannins is what causes the dry and puckery feeling in the mouth following the consumption of unripened fruit, red wine or tea. Likewise, the destruction or modification of tannins with time plays an important role when determining harvesting times.
Tannins have ranging from 500 to over 3,000 (gallic acid ) and up to 20,000 daltons ().
(oligostilbenes or polystilbenes) are oligomeric forms of and constitute a minor class of tannins.
| Rhubarb |
| Tea, acacia, catechu, Theobroma cacao, guarana |
| Nux-vomica, coffee, mate |
| Carapichea ipecacuanha |
Maximilian Nierenstein studied natural phenols and tannins found in different plant species. Working with Arthur George Perkin, he prepared ellagic acid from algarobilla and certain other in 1905. He suggested its formation from galloyl-glycine by Penicillium in 1915. Tannase is an enzyme that Nierenstein used to produce m-digallic acid from . He proved the presence of catechin in in 1931. He showed in 1945 that luteic acid, a molecule present in the myrobalanitannin, a tannin found in the fruit of Terminalia chebula, is an intermediary compound in the synthesis of ellagic acid.
At these times, molecule formulas were determined through combustion analysis. The discovery in 1943 by Martin and Synge of paper chromatography provided for the first time the means of surveying the phenolic constituents of plants and for their separation and identification. There was an explosion of activity in this field after 1945, including prominent work by Edgar Charles Bate-Smith and Tony Swain at Cambridge University.
In 1966, Edwin Haslam proposed a first comprehensive definition of plant polyphenols based on the earlier proposals of Bate-Smith, Swain and Theodore White, which includes specific structural characteristics common to all phenolics having a tanning property. It is referred to as the White–Bate-Smith–Swain–Haslam (WBSSH) definition.
The most abundant polyphenols are the , found in virtually all families of plants, and comprising up to 50% of the dry weight of leaves. (1990). 9780849368110, CRC Press. ISBN 9780849368110
Tannins are classified as ergastic substances, non-protoplasm materials found in cells. Tannins, by definition, precipitate proteins. In this condition, they must be stored in organelles able to withstand the protein precipitation process. are isolated plant cells which differ from neighboring tissues and contain non-living substances. They have various functions such as storage of reserves, excretory materials, pigments, and minerals. They could contain oil, latex, gum, resin or pigments. They also can contain tannins. In Japanese persimmon ( Diospyros kaki) fruits, tannin is accumulated in the vacuole of tannin cells, which are idioblasts of parenchyma cells in the flesh.)
Leaf litter and other decaying parts of kauri ( Agathis australis), a tree species found in New Zealand, decompose much more slowly than those of most other species. Besides its acidity, the plant also bears substances such as waxes and phenols, most notably tannins, that are harmful to .
Tannins leaching from an unprepared driftwood decoration in an aquarium can cause pH lowering and coloring of the water to a tea-like tinge. A way to avoid this is to boil the wood in water several times, discarding the water each time. Using peat as an aquarium substrate can have the same effect. Many hours of boiling the driftwood may need to be followed by many weeks or months of constant soaking and many water changes before the water will stay clear. Raising the water's pH level, for example by adding baking soda, will accelerate the process of leaching.
Tannins in water can lead to feather staining on wild and domestic Anseriformes which frequent the water; , which are typically white in colour, can often be observed with reddish-brown staining as a result of coming into contact with dissolved tannins, though dissolved iron compounds also play a role.
, while in general much lower in tannins than hardwoods, are usually not recommended for use in an aquarium so using a hardwood with a very light color, indicating a low tannin Concentration, can be an easy way to avoid tannins. Tannic acid is brown in color, so in general white woods have a low tannin content. Woods with a lot of yellow, red, or brown coloration to them (like cedar, redwood, red oak, etc.) tend to contain a lot of tannin.
Colorimetric methods have existed such as the Neubauer-Löwenthal method which uses potassium permanganate as an oxidizing agent and Indigo dye sulfate as an indicator, originally proposed by Löwenthal in 1877. The difficulty is that the establishing of a titer for tannin is not always convenient since it is extremely difficult to obtain the pure tannin. Neubauer proposed to remove this difficulty by establishing the titer not with regard to the tannin but with regard to crystallised oxalic acid, whereby he found that 83 g oxalic acid correspond to 41.20 g tannin. Löwenthal's method has been criticized. For instance, the amount of indigo used is not sufficient to retard noticeably the oxidation of the non-tannins substances. The results obtained by this method are therefore only comparative. A modified method, proposed in 1903 for the quantification of tannins in wine, Feldmann's method, is making use of calcium hypochlorite, instead of potassium permanganate, and indigo sulfate.
Coffee pulp has been found to contain low to trace amounts of tannins.
In lager type beers, the tannins can form a precipitate with specific haze-forming proteins in the beer resulting in turbidity at low temperature. This chill haze can be prevented by removing part of the tannins or part of the haze-forming proteins. Tannins are removed using PVPP, haze-forming proteins by using silica or tannic acid.
Many studies suggest that chestnut tannins have positive effects on silage quality in the round bale silages, in particular reducing NPNs (non-protein nitrogen) in the lowest wilting level.
Improved fermentability of Soybean nitrogen in the rumen may occur. Condensed tannins inhibit herbivore digestion by binding to consumed plant proteins and making them more difficult for animals to digest, and by interfering with protein absorption and digestive enzymes (for more on that topic, see plant defense against herbivory). , another type of , also precipitate tannins from solution, thus preventing alimentary adsorption.
Legume fodders containing condensed tannins are a possible option for integrated sustainable control of gastrointestinal in ruminants, which may help address the worldwide development of resistance to synthetic . These include nuts, temperate and tropical barks, carob, coffee and cocoa.
Industrial tannin production began at the beginning of the 19th century with the industrial revolution, to produce tanning material for the need for more leather. Before that time, processes used plant material and were long (up to six months).
There was a collapse in the vegetable tannin market in the 1950s–1960s, due to the appearance of , which were invented in response to a scarcity of vegetable tannins during World War II. At that time, many small tannin industry sites closed.
The cost of the final product depends on the method used to extract the tannins, in particular the use of solvents, alkali and other chemicals used (for instance Glycerite). For large quantities, the most cost-effective method is hot water extraction.
Food-grade tannic acid is used as a clarifying agent and flavor ingredient in beverage manufacturing.
Tannins produce different colors with ferric chloride (either blue, blue black, or green to greenish-black) according to the type of tannin. Iron gall ink is produced by treating a solution of tannins with iron(II) sulfate.
Tannins can also be used as a mordant, and are especially useful in of such as cotton. The type of tannin used may or may not have an impact on the final color of the fiber.
Tannin is a component in a type of industrial particleboard adhesive developed jointly by the Tanzania Industrial Research and Development Organization and Forintek Labs Canada. Pinus radiata tannins has been investigated for the production of Wood glue.
Tannins can be used for production of anti-corrosion primers for treating rusted steel surfaces prior to painting, converting rust to iron tannate and consolidating and sealing the surface.
The use of made of tannins has been investigated to remove mercury and methylmercury from solution. Immobilized tannins have been tested to recover uranium from seawater.
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