The termtannin (from scientific Frenchtannin, from Frenchtan "crushed oak bark",tanner "to tan", cognate withEnglishtanning,Medieval Latintannare, from Proto-Celtic*tannos "oak") refers to the abundance of these compounds inoakbark, which was used intanning animalhides intoleather.
The tannin compounds are widely distributed in many species of plants, where they play a role in protection frompredation (acting aspesticides) and might help in regulating plant growth.[1] Theastringency from the tannins is what causes the dry and puckery feeling in the mouth following the consumption of unripened fruit, red wine or tea.[2] Likewise, the destruction or modification of tannins with time plays an important role when determining harvesting times.
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 weightpolyphenol motif that characterizes tannins. Typically, tannin molecules require at least 12 hydroxyl groups and at least five phenyl groups to function as protein binders.[4]
Pseudo-tannins are low molecular weight compounds associated with other compounds. They do not change color during theGoldbeater's skintest, unlike hydrolysable and condensed tannins, and cannot be used as tanning compounds.[4] Some examples of pseudo tannins and their sources are:[7]
At these times, molecule formulas were determined throughcombustion analysis. The discovery in 1943 by Martin and Synge ofpaper 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 byEdgar Charles Bate-Smith andTony Swain atCambridge University.[16]
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.[17][self-published source?]
The most abundant polyphenols are thecondensed tannins, found in virtually all families of plants, and comprising up to 50% of the dry weight of leaves.[19][20]
In all vascular plants studied, tannins are manufactured by achloroplast-derivedorganelle, thetannosome.[21] Tannins are mainly physically located in thevacuoles 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 asergastic 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.[22]
Theconvergent 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.[23]
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 tomicroorganisms.[24]
Theleaching of highlywatersoluble tannins from decaying vegetation and leaves along a stream may produce what is known as ablackwater river. Water flowing out ofbogs has a characteristic brown color from dissolvedpeat tannins. The presence of tannins (orhumic acid) inwell water can make it smell bad or taste bitter, but this does not make it unsafe to drink.[25]
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 thewood in water several times, discarding the water each time. Using peat as anaquarium 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'spH level, e.g. by addingbaking soda, will accelerate the process of leaching.[26]
Tannins in water can lead to feather staining on wild and domesticwaterfowl which frequent the water;mute swans, 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 dissolvediron compounds also play a role.[27]
Softwoods, while in general much lower in tannins than hardwoods,[28] are usually not recommended for use in an aquarium[29] so using ahardwood with a very light color, indicating a low tannincontent, can be an easy way to avoid tannins. Tannicacid 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.[30]
Tannin-rich fresh water draining into Cox Bight from Freney Lagoon, Southwest Conservation Area, Tasmania, Australia
There is no single protocol forextracting tannins from all plant material. The procedures used for tannins are widely variable.[31] It may be thatacetone in the extraction solvent increases the total yield by inhibiting interactions between tannins andproteins during extraction[31] or even by breaking hydrogen bonds between tannin-protein complexes.[32]
There are three groups of methods for the analysis of tannins: precipitation of proteins or alkaloids, reaction with phenolic rings, and depolymerization.[33]
Whengoldbeater's skin or ox skin is dipped inHCl, rinsed in water, soaked in the tannin solution for 5 minutes, washed in water, and then treated with 1%FeSO4 solution, it gives a blue black color if tannin was present.[35]
The following describes the use offerric chloride (FeCl3) tests forphenolics 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.[36] 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).[37]
The hide-powder method is used in tannin analysis forleather tannin and the Stiasny method forwood adhesives.[38][39] Statistical analysis reveals that there is no significant relationship between the results from the hide-powder and the Stiasny methods.[40][41]
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.
The bark tannins ofCommiphora angolensis have been revealed by the usual color and precipitation reactions and by quantitative determination by the methods of Löwenthal-Procter and of Deijs[42] (formalin-hydrochloric acid method).[43]
Colorimetric methods have existed such as the Neubauer-Löwenthal method which usespotassium permanganate as an oxidizing agent andindigosulfate as an indicator, originally proposed by Löwenthal in 1877.[44] 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 crystallisedoxalic 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.[45][46] A modified method, proposed in 1903 for the quantification of tannins in wine, Feldmann's method, is making use ofcalcium hypochlorite, instead of potassium permanganate, and indigo sulfate.[47]
Nuts vary in the amount of tannins they contain. Some species of acorns ofoak contain large amounts. For example, acorns ofQuercus robur andQuercus petraea inPoland were found to contain 2.4–5.2% and 2.6–4.8% tannins as a proportion of dry matter,[51] but the tannins can be removed by leaching in water so that the acorns become edible.[52] Other nuts – such ashazelnuts,walnuts,pecans, andalmonds – contain lower amounts. Tannin concentration in the crude extract of these nuts did not directly translate to the same relationships for the condensed fraction.[53]
Mostlegumes contain tannins. Red-colored beans contain the most tannins, and white-colored beans have the least.Peanuts without shells have a very low tannin content.Chickpeas (garbanzo beans) have a smaller amount of tannins.[54]
Principal human dietary sources of tannins are tea and coffee.[56] Most wines aged incharredoak barrels possess tannins absorbed from the wood.[57] Soils high in clay also contribute to tannins in wine grapes.[58] This concentration gives wine its signatureastringency.[59]
Coffee pulp has been found to contain low to trace amounts of tannins.[60]
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.[61]
In addition to thealpha acids extracted fromhops to provide bitterness inbeer, 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[citation needed]. However, in some styles, the presence of this astringency is acceptable or even desired, as, for example, in aFlanders red ale.[62]
In lager type beers, the tannins can form a precipitate with specific haze-forming proteins in the beer resulting inturbidity 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 usingPVPP, haze-forming proteins by usingsilica or tannic acid.[63]
Tannins have traditionally been consideredantinutritional, depending upon their chemical structure and dosage.[64]
Many studies suggest that chestnut tannins have positive effects onsilage quality in theround bale silages, in particular reducingNPNs (non-protein nitrogen) in the lowest wilting level.[65]
Improved fermentability ofsoya meal nitrogen in therumen may occur.[66] 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, seeplant defense against herbivory).Histatins, another type ofsalivary proteins, also precipitate tannins from solution, thus preventing alimentary adsorption.[67]
Legume fodders containing condensed tannins are a possible option for integrated sustainable control of gastrointestinalnematodes in ruminants, which may help address the worldwide development of resistance to syntheticanthelmintics. These include nuts, temperate and tropical barks, carob, coffee and cocoa.[68]
Tannins have been used since antiquity in the processes of tanning hides for leather, and in helping preserve iron artifacts (as with Japanese iron teapots).
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).[69]
There was a collapse in the vegetable tannin market in the 1950s–1960s, due to the appearance ofsynthetic tannins, which were invented in response to a scarcity of vegetable tannins during World War II. At that time, many small tannin industry sites closed.[70] Vegetable tannins are estimated to be used for the production of 10–20% of the global leather production.[citation needed]
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 instanceglycerin). For large quantities, the most cost-effective method is hot water extraction.
Tannic acid 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.[citation needed]
Tannins are an important ingredient in the process of tanning leather.Tanbark fromoak,mimosa, chestnut andquebracho tree has traditionally been the primary source oftannery tannin, though inorganictanning agents are also in use today and account for 90% of the world's leather production.[71]
Tannins produce different colors withferric 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 withiron(II) sulfate.[72]
Tannins can also be used as amordant, and is especially useful innatural dyeing ofcellulose fibers such as cotton.[73] 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 industrialparticleboardadhesive developed jointly by the Tanzania Industrial Research and Development Organization and Forintek Labs Canada.[74]Pinus radiata tannins has been investigated for the production ofwood adhesives.[75]
Tannins can be used for production of anti-corrosive primers for treating rusted steel surfaces prior to painting, converting rust to iron tannate and consolidating and sealing the surface.
The use ofresins made of tannins has been investigated to removemercury andmethylmercury from solution.[76] Immobilized tannins have been tested to recoveruranium from seawater.[77]
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