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, a type of tannin]] A tannin (or tannoid) is an astringent, polyphenolic biomolecule that binds to and precipitates proteins and various other organic compounds including amino acids and alkaloids. The term tannin (from tanna, an Old High German word for oak or fir tree, as in ) refers to the use of wood tannins from oak in tanning animal hides into leather; hence the words "tan" and "tanning" for the treatment of leather. However, the term "tannin" by extension is widely applied to any large polyphenolic compound containing sufficient hydroxyls and other suitable groups (such as carboxyls) to form strong complexes with various macromolecules. The tannin compounds are widely distributed in many species of plants, where they play a role in protection from predation, and perhaps also as pesticides, 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 or red wine or tea. Likewise, the destruction or modification of tannins with time plays an important role when determining harvesting times. Tannins have molecular weights ranging from 500 to over 3,000 ( gallic acid esters) and up to 20,000 ( proanthocyanidins).

Structure and classes of tannins

There are three major classes of tannins: Shown below are the base unit or monomer of the tannin. Particularly in the flavone-derived tannins, the base shown must be (additionally) heavily hydroxylated and polymerized in order to give the high molecular weight polyphenol motif that characterizes tannins. Typically, tannin molecules require at least 12 hydroxyl groups and at least five phenyl groups to function as protein binders. Notes on Tannins from PharmaXChange.info Oligostilbenoids (oligo- or polystilbenes) are oligomeric forms of stilbenoids and constitute a class of tannins.

Pseudo tannins

Pseudo tannins are low molecular weight compounds associated with other compounds. They do not change color during the Goldbeater's skin test, unlike hydrolysable and condensed tannins, and cannot be used as tanning compounds. Some examples of pseudo tannins and their sources are:


Ellagic acid, gallic acid, and pyrogallic acid were first discovered by chemist Henri Braconnot in 1831. Julius Löwe was the first person to synthesize ellagic acid by heating gallic acid with arsenic acid or silver oxide. Löwe, Zeitschrift für Chemie, 1868, 4, 603 Maximilian Nierenstein studied natural phenols and tannins{{Cite journal | last1 = Drabble | first1 = E. | last2 = Nierenstein | first2 = M. | title = On the Rôle of Phenols, Tannic Acids, and Oxybenzoic Acids in Cork Formation | journal = Biochemical Journal | volume = 2 | issue = 3 | pages = 96–102.1 | year = 1907 | pmid = 16742048 | pmc = 1276196 }} found in different plant species. Working with Arthur George Perkin, he prepared ellagic acid from algarobilla and certain other fruits in 1905. He suggested its formation from galloyl- glycine by Penicillium in 1915.{{Cite journal | last1 = Nierenstein | first1 = M. | title = The Formation of Ellagic Acid from Galloyl-Glycine by Penicillium | journal = The Biochemical Journal | volume = 9 | issue = 2 | pages = 240–244 | year = 1915 | pmid = 16742368 | pmc = 1258574 | doi=10.1042/bj0090240 }} Tannase is an enzyme that Nierenstein used to produce m- digallic acid from gallotannins.{{Cite journal | last1 = Nierenstein | first1 = M. | title = A biological synthesis of m-digallic acid | journal = The Biochemical Journal | volume = 26 | issue = 4 | pages = 1093–1094 | year = 1932 | pmid = 16744910 | pmc = 1261008 | doi=10.1042/bj0261093 }} He proved the presence of catechin in cocoa beans 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.{{Cite journal | last1 = Nierenstein | first1 = M. | last2 = Potter | first2 = J. | title = The distribution of myrobalanitannin | journal = The Biochemical Journal | volume = 39 | issue = 5 | pages = 390–392 | year = 1945 | pmid = 16747927 | pmc = 1258254 | doi=10.1042/bj0390390 }} 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.


Tannins are distributed in species throughout the plant kingdom. They are commonly found in both gymnosperms as well as angiosperms. Mole studied the distribution of tannin in 180 families of dicotyledons and 44 families of monocotyledons (Cronquist). Most families of dicot contain tannin-free species (tested by their ability to precipitate proteins). The best known families of which all species tested contain tannin are: Aceraceae, Actinidiaceae, Anacardiaceae, Bixaceae, Burseraceae, Combretaceae, Dipterocarpaceae, Ericaceae, Grossulariaceae, Myricaceae for dicot and Najadaceae and Typhaceae in Monocot. To the family of the oak, Fagaceae, 73% of the species tested (N = 22) contain tannin. For those of acacias, Mimosaceae, only 39% of the species tested (N = 28) contain tannin, among Solanaceae rate drops to 6% and 4% for the Asteraceae. Some families like the Boraginaceae, Cucurbitaceae, Papaveraceae contain no tannin-rich species. The most abundant polyphenols are the condensed tannins, found in virtually all families of plants, and comprising up to 50% of the dry weight of leaves. Tannins of tropical woods tend to be of a cathetic nature rather than of the gallic type present in temperate woods.Tannin in Tropical Woods. Doat J, Bois. For Tmp., 1978, volume 182, pages 34-37 There may be a loss in the bio-availability of still other tannins in plants due to birds, pests, and other pathogens.

Localization in plant organs

Tannins are found in leaf, bud, seed, root, and stem tissues. An example of the location of the tannins in stem tissue is that they are often found in the growth areas of trees, such as the secondary phloem and xylem and the layer between the cortex and epidermis. Tannins may help regulate the growth of these tissues.

Cellular localization

In all vascular plants studied so far, tannins are manufactured by a chloroplast-derived organelle, the tannosome."The tannosome is an organelle forming condensed tannins in the chlorophyllous organs of Tracheophyta". Jean-Marc Brillouet, Charles Romieu, Benoît Schoefs, Katalin Solymosi, Véronique Cheynier, Hélène Fulcrand, Jean-Luc Verdeil and Geneviève Conéjéro, Annals of Botany, September 11, 2013, Tannins are mainly physically located in the vacuoles or surface wax of plants. These storage sites keep tannins active against plant predators, but also keep some tannins from affecting plant metabolism while the plant tissue is alive; it is only after cell breakdown and death that the tannins are active in metabolic effects. Tannins are classified as ergastic substances, i.e., 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. Idioblasts 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 etc. 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.Identification of Molecular Markers Linked to the Trait of Natural Astringency Loss of Japanese Persimmon (Diospyros kaki) Fruit. Shinya Kanzaki, Keizo Yonemori and Akira Sugiura, J. Amer. Soc. Hort. Sci., 2001, 126(1), pages 51–55 ( article)

Presence in soils

The convergent evolution of tannin-rich plant communities has occurred on nutrient-poor acidic soils throughout the world. Tannins were once believed to function as anti-herbivore defenses, but more and more ecologists now recognize them as important controllers of decomposition and nitrogen cycling processes. As concern grows about global warming, there is great interest to better understand the role of polyphenols as regulators of carbon cycling, in particular in northern boreal forests. 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 microorganisms.Eric Verkaik, Anne G. Jongkindet, Frank Berendse "Short-term and long-term effects of tannins on nitrogen mineralisation and litter decomposition in kauri (Agathis australis (D. Don) Lindl.) forests". Plant And Soil 2006; Volume 287, Numbers 1–2, pages 337–345

Presence in water and wood

The leaching of highly water soluble tannins from decaying vegetation and leaves along a stream may produce what is known as a blackwater river. Water flowing out of bogs has a characteristic brown color from dissolved peat tannins. The presence of tannins (or humic acid) in well water can make it smell bad or taste bitter, but this does not make it unsafe to drink. Tannins, lignins and humic acids in well water on www.gov.ns.ca 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. Adding baking soda to the water to raise its pH level will accelerate the process of leaching, as the more alkaline solution can draw out tannic acid from the wood faster than the pH-neutral water. Preparing Driftwood for Your Freshwater Aquarium Softwoods, while in general much lower in tannins than hardwoods, Polyflavonoid tannins — a main cause of soft-rot failure in CCA-treated timber are usually not recommended for use in an aquarium Driftwood Do's & Don'ts so using a hardwood with a very light color, indicating a low tannin content, 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 southern yellow pine, cedar, redwood, red oak, etc.) tend to contain a lot of tannin. Tannin and hardwood flooring
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