tannin
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Tannins (or tannoids) are a class of , ic s that bind to and s and various other organic compounds including s and s. The term ''tannin'' (from ''tanner'', from ''tannāre'', from ''tannum'', ) refers to the use of oak and other bark in animal into . By extension, the term ''tannin'' is widely applied to any large ic compound containing sufficient s and other suitable groups (such as s) to form strong complexes with various s. The tannin compounds are widely distributed in many species of plants, where they play a role in protection from (including as ) and might help in regulating plant growth. The 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 s ranging from 500 to over 3,000 ( s) and up to 20,000 (s).


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 motif that characterizes tannins. Typically, tannin molecules require at least 12 hydroxyl groups and at least five phenyl groups to function as protein binders. s (oligo- or polystilbenes) are oligomeric forms of s 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 , unlike hydrolysable and condensed tannins, and cannot be used as tanning compounds. Some examples of pseudo tannins and their sources are:


History

, , and were first discovered by chemist in 1831. was the first person to synthesize ellagic acid by heating with or silver oxide. studied natural phenols and tannins found in different plant species. Working with , he prepared from and certain other s in 1905. He suggested its formation from - by ' in 1915. is an enzyme that Nierenstein used to produce m- from s. He proved the presence of in s in 1931. He showed in 1945 that , a molecule present in the myrobalanitannin, a tannin found in the fruit of ', is an intermediary compound in the synthesis of . At these times, molecule formulas were determined through . The discovery in 1943 by Martin and Synge of 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 and at . In 1966, 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.


Occurrence

Tannins are distributed in species throughout the . They are commonly found in both s and s. Mole studied the distribution of tannin in 180 families of s and 44 families of s (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: , , , , , , , , , for dicot and and in Monocot. To the family of the oak, , 73% of the species tested contain tannin. For those of acacias, , only 39% of the species tested contain tannin, among rate drops to 6% and 4% for the . Some families like the , , contain no tannin-rich species. The most abundant polyphenols are the s, found in virtually all families of plants, and comprising up to 50% of the dry weight of leaves.


Cellular localization

In all vascular plants studied, tannins are manufactured by a -derived , the . Tannins are mainly physically located in the s 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. Tannins are classified as s, 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. s 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 (') fruits, tannin is accumulated in the vacuole of tannin cells, which are idioblasts of parenchyma cells in the flesh.


Presence in soils

The 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 ('), 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 s.


Presence in water and wood

The of highly tannins from decaying vegetation and leaves along a stream may produce what is known as a . Water flowing out of s has a characteristic brown color from dissolved tannins. The presence of tannins (or ) in can make it smell bad or taste bitter, but this does not make it unsafe to drink. 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 in water several times, discarding the water each time. Using peat as an 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 to the water to raise its will accelerate the process of leaching, as the more solution can draw out from the wood faster than the pH-neutral water. s, while in general much lower in tannins than hardwoods, are usually not recommended for use in an aquarium so using a with a very light color, indicating a low tannin , can be an easy way to avoid tannins. Tannic 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.


Extraction

There is no single protocol for tannins from all plant material. The procedures used for tannins are widely variable.''The Tannin Handbook'', Ann E. Hagerman, 1998
book
)
It may be that in the extraction solvent increases the total yield by inhibiting interactions between tannins and s during extraction or even by breaking hydrogen bonds between tannin-protein complexes.


Tests for tannins

There are three groups of methods for the analysis of tannins: precipitation of proteins or alkaloids, reaction with phenolic rings, and depolymerization.


Alkaloid precipitation

Alkaloids such as , , or , precipitates polyphenols and tannins. This property can be used in a quantitation method.


Goldbeater's skin test

When or ox skin is dipped in , rinsed in water, soaked in the tannin solution for 5 minutes, washed in water, and then treated with 1% solution, it gives a blue black color if tannin was present.


Ferric chloride test

s for in general. Powdered plant leaves of the test plant (1.0 g) are weighed into a beaker and 10 ml of distilled water are added. The mixture is boiled for five minutes. Two drops of 5% FeCl3 are then added. Production of a greenish precipitate is an indication of the presence of tannins. Alternatively, a portion of the water extract is diluted with distilled water in a ratio of 1:4 and few drops of 10% ferric chloride solution is added. A blue or green color indicates the presence of tannins (Evans, 1989).


Other methods

The hide-powder method is used in tannin analysis for tannin and the Stiasny method for s. Statistical analysis reveals that there is no significant relationship between the results from the hide-powder and the Stiasny methods. ;Hide-powder method 400 mg of sample tannins are dissolved in 100 ml of distilled water. 3 g of slightly chromated hide-powder previously dried in vacuum for 24h over CaCl2 are added and the mixture stirred for 1 h at ambient temperature. The suspension is filtered without vacuum through a sintered glass filter. The weight gain of the hide-powder expressed as a percentage of the weight of the starting material is equated to the percentage of tannin in the sample. ;Stiasny's method 100 mg of sample tannins are dissolved in 10 ml distilled water. 1 ml of 10M HCl and 2 ml of 37% formaldehyde are added and the mixture heated under reflux for 30 min. The reaction mixture is filtered while hot through a sintered glass filter. The precipitate is washed with hot water (5× 10 ml) and dried over CaCl2. The yield of tannin is expressed as a percentage of the weight of the starting material.


Reaction with phenolic rings

The bark tannins of ' have been revealed by the usual color and precipitation reactions and by quantitative determination by the methods of Löwenthal-Procter and of DeijsChemical study of bark from Commiphora angolensis Engl. Cardoso Do Vale, J., Bol Escola Farm Univ Coimbra Edicao Cient, 1962, volume 3, page 128
abstract
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(- method). Colorimetric methods have existed such as the Neubauer-Löwenthal method which uses as an oxidizing agent and 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 , 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 , instead of potassium permanganate, and indigo sulfate.


Food items with tannins


Pomegranates


Accessory fruits

contain both hydrolyzable and condensed tannins.


Berries

Most berries, such as , and , contain both hydrolyzable and condensed tannins.


Nuts

vary in the amount of tannins they contain. Some species of acorns of contain large amounts. For example, acorns of and in were found to contain 2.4–5.2% and 2.6–4.8% tannins as a proportion of dry matter, but the tannins can be removed by leaching in water so that the acorns become edible. Other nuts – such as , , , and – contain lower amounts. Tannin concentration in the crude extract of these nuts did not directly translate to the same relationships for the condensed fraction.


Herbs and spices

s, , , , , and all contain tannins.


Legumes

Most s contain tannins. Red-colored beans contain the most tannins, and white-colored beans have the least. s without shells have a very low tannin content. s (garbanzo beans) have a smaller amount of tannins.


Chocolate

contains about 6% tannins.


Drinks with tannins

Principal human dietary sources of tannins are tea and coffee. Most wines aged in s possess tannins absorbed from the wood. Soils high in clay also contribute to tannins in wine grapes.Oz Clarke ''Encyclopedia of Grapes'' pp. 155–162 Harcourt Books 2001 This concentration gives wine its signature . Coffee pulp has been found to contain low to trace amounts of tannins.


Fruit juices

Although citrus fruits do not contain tannins, orange-colored juices often contain tannins from food colouring. Apple, grape and berry juices all contain high amounts of tannins. Sometimes tannins are even added to juices and ciders to create a more astringent feel to the taste.


Beer

In addition to the s extracted from to provide bitterness in , condensed tannins are also present. These originate both from malt and hops. Trained brewmasters, particularly those in Germany, consider the presence of tannins to be a flaw. However, in some styles, the presence of this astringency is acceptable or even desired, as, for example, in a . In lager type beers, the tannins can form a precipitate with specific haze-forming proteins in the beer resulting in 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 , haze-forming proteins by using or tannic acid.


Properties for animal nutrition

Tannins have traditionally been considered , but it is now known that their beneficial or antinutritional properties depend upon their chemical structure and dosage. The new technologies used to analyze molecular and chemical structures have shown that a division into condensed and hydrolyzable tannins is too simplistic. Recent studies have demonstrated that products containing tannins included at low dosages (0.15–0.2%) in the diet of chickens may be beneficial. Some studies suggest that chestnut tannins have positive effects on quality in the , in particular reducing s (non-protein nitrogen) in the lowest wilting level. Improved fermentability of nitrogen in the may occur. Studies conducted in 2002 on ''in vitro'' ammonia release and dry matter degradation of soybean meal comparing three different types of tannins (, and chestnut) demonstrated that chestnut tannins are more efficient in protecting soybean meal from ''in vitro'' degradation by rumen bacteria. 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 ). Many tannin-consuming animals secrete a tannin-binding protein () in their saliva. Tannin-binding capacity of salivary mucin is directly related to its proline content. Salivary s (PRPs) are sometimes used to inactivate tannins. One reason is that they inactivate tannins to a greater extent than do dietary proteins resulting in reduced fecal nitrogen losses. PRPs additionally contain non-specific nitrogen and non-essential amino acids making them more convenient than valuable dietary protein. s, another type of s, also precipitate tannins from solution, thus preventing alimentary adsorption.


Tannin market

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 s, which were invented in response to a scarcity of vegetable tannins during World War II. At that time, many small tannin industry sites closed. Vegetable tannins are estimated to be used for the production of 10–20% of the global leather production. 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 ). For large quantities, the most cost-effective method is . is used worldwide as clarifying agent in alcoholic drinks and as aroma ingredient in both alcoholic and soft drinks or juices. Tannins from different botanical origins also find extensive uses in the wine industry.


Uses

Tannins are an important ingredient in the process of tanning leather. from , , chestnut and has traditionally been the primary source of tannin, though inorganic are also in use today and account for 90% of the world's leather production. Tannins produce different colors with (either blue, blue black, or green to greenish-black) according to the type of tannin. is produced by treating a solution of tannins with . Tannins can also be used as a , and is especially useful in ing of s 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 developed jointly by the Tanzania Industrial Research and Development Organization and Forintek Labs Canada. ' tannins has been investigated for the production of . s, e.g., quebracho tannin, and s, e.g., chestnut tannin, appear to be able to substitute a high proportion of synthetic phenol in phenol-formaldehyde resins for wood . Tannins can be used for production of anti- primer, sold under brand-name "Nox Primer" for treatment of rusted steel surfaces prior to painting, rust converter to transform oxidized steel into a smooth sealed surface and rust inhibitor. The use of s made of tannins has been investigated to remove and from solution. Immobilized tannins have been tested to recover from seawater.


See also

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References


External links



*   * {{Authority control Phytochemicals