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Tannic acid is a specific form of tannin, a type of
polyphenol Polyphenols () are a large family of naturally occurring organic compounds characterized by multiples of phenol units. They are abundant in plants and structurally diverse. Polyphenols include flavonoids, tannic acid, and ellagitannin, some ...
. Its weak acidity ( pKa around 6) is due to the numerous
phenol Phenol (also called carbolic acid) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile. The molecule consists of a phenyl group () bonded to a hydroxy group (). Mildly acidic, it r ...
groups in the structure. The
chemical formula In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...
for commercial tannic acid is often given as C76H52O46, which corresponds with decagalloyl glucose, but in fact it is a mixture of polygalloyl glucoses or polygalloyl quinic acid esters with the number of galloyl moieties per molecule ranging from 2 up to 12 depending on the plant source used to extract the tannic acid. Commercial tannic acid is usually extracted from any of the following plant parts: Tara pods (''
Caesalpinia spinosa ''Tara spinosa'', commonly known as ''tara'' (Quechua), also known as Peruvian carob or spiny holdback, is a small leguminous tree or thorny shrub native to Peru. ''T. spinosa'' is cultivated as a source of tannins based on a galloylated quinic ...
''), gallnuts from '' Rhus semialata'' or ''
Quercus infectoria ''Quercus infectoria'' or the Aleppo oak is a species of oak well known for producing galls (called manjakani in Malaysia, majuphal in India) that have been traditionally used for centuries in Asia medicinally while also used in softening leather ...
'' or Sicilian sumac leaves ('' Rhus coriaria''). According to the definitions provided in external references such as international pharmacopoeia,
Food Chemicals Codex The Food Chemicals Codex (FCC) is a collection of internationally recognized standards for the purity and identity of food ingredients. Scope The FCC features more than 1,250 monographs, including food-grade chemicals, processing aids, foods (suc ...
and FAO-WHO tannic acid monograph only tannins obtained from the above-mentioned plants can be considered as tannic acid. Sometimes extracts from
chestnut The chestnuts are the deciduous trees and shrubs in the genus ''Castanea'', in the beech family Fagaceae. They are native to temperate regions of the Northern Hemisphere. The name also refers to the edible nuts they produce. The unrel ...
or oak wood are also described as tannic acid but this is an incorrect use of the term. It is a yellow to light brown
amorphous In condensed matter physics and materials science, an amorphous solid (or non-crystalline solid, glassy solid) is a solid that lacks the long-range order that is characteristic of a crystal. Etymology The term comes from the Greek language, Gr ...
powder. While tannic acid is a specific type of tannin (plant polyphenol), the two terms are sometimes (incorrectly) used interchangeably. The long-standing misuse of the terms, and its inclusion in scholarly articles has compounded the confusion. This is particularly widespread in relation to green tea and black tea, both of which contain many different types of tannins not just exclusively tannic acid. Tannic acid is not an appropriate standard for any type of tannin analysis because of its poorly defined composition.


Quercitannic and gallotannic acids

Quercitannic acid is one of the two forms of tannic acid found in oak bark and leaves. The other form is called gallotannic acid and is found in oak galls. The quercitannic acid molecule is also present in quercitron, a yellow dye obtained from the bark of the Eastern black oak (''Quercus velutina''), a forest tree indigenous in North America. It is described as a yellowish-brown amorphous powder. In 1838,
Jöns Jacob Berzelius Baron Jöns Jacob Berzelius (; by himself and his contemporaries named only Jacob Berzelius, 20 August 1779 – 7 August 1848) was a Swedish chemist. Berzelius is considered, along with Robert Boyle, John Dalton, and Antoine Lavoisier, to be o ...
wrote that quercitannate is used to dissolve morphine. In 1865 in the fifth volume of "A dictionary of chemistry", Henry Watts wrote :
It exhibits with ferric salts the same reactions as gallotannic acid. It differs however from the latter in not being convertible into
gallic acid Gallic acid (also known as 3,4,5-trihydroxybenzoic acid) is a trihydroxybenzoic acid with the formula C6 H2( OH)3CO2H. It is classified as a phenolic acid. It is found in gallnuts, sumac, witch hazel, tea leaves, oak bark, and other plants ...
, and not yielding pyrogallic acid by dry distillation. It is precipitated by sulfuric acid in red flocks. ( Stenhouse, Ann. Ch. Pharm. xlv. 16.)
According to Rochleder (ibid lxiii. 202), the tannic acid of black tea is the same as that of oak-bark.
In 1880, Etti gave for it the molecular formula C17H16O9. He described it as an unstable substance, having a tendency to give off water to form anhydrides (called phlobaphenes), one of which is called ''oak-red'' (C34H30O17). For him, it was not a glycoside. In Allen's "Commercial Organic Analysis", published in 1912, the formula given was C19H16O10. Other authors gave other molecular formulas like C28H26O15, while another formula found is C28H24O11. According to Lowe, two forms of the principle exist – "one soluble in water, of the formula C28H28O14, and the other scarcely soluble, C28H24O12. Both are changed by the loss of water into oak red, C28H22O11." Quercitannic acid was for a time a standard used to assess the phenolic content in
spice A spice is a seed, fruit, root, bark, or other plant substance primarily used for flavoring or coloring food. Spices are distinguished from herbs, which are the leaves, flowers, or stems of plants used for flavoring or as a garnish. Spices a ...
s, given as ''quercitannic acid equivalent''. In an interesting historical note, the inventor of
carborundum Silicon carbide (SiC), also known as carborundum (), is a hard chemical compound containing silicon and carbon. A semiconductor, it occurs in nature as the extremely rare mineral moissanite, but has been mass-produced as a powder and crystal si ...
, Edward G. Acheson, discovered that gallotannic acid greatly improved the plasticity of clay. In his report of this discovery in 1904 he noted that the only known historical reference to the use of organic material added to clay is the use of straw mixed with clay described in the Bible, Exodus 1:11 and that the Egyptians must have been aware of his (re-)discovery. He stated "This explains why the straw was used and why the children of Israel were successful in substituting stubble for straw, a course that would hardly be possible, were the fibre of the straw depended upon as a bond feasible for the clay, but quite reasonable where the extract of the plant was used."


Uses

Tannins are a basic ingredient in the chemical staining of
wood Wood is a porous and fibrous structural tissue found in the stems and roots of trees and other woody plants. It is an organic materiala natural composite of cellulose fibers that are strong in tension and embedded in a matrix of lignin ...
, and are already present in woods like oak, walnut, and
mahogany Mahogany is a straight- grained, reddish-brown timber of three tropical hardwood species of the genus '' Swietenia'', indigenous to the AmericasBridgewater, Samuel (2012). ''A Natural History of Belize: Inside the Maya Forest''. Austin: U ...
. Tannic acid can be applied to woods low in tannin so chemical stains that require tannin content will react. The presence of tannins in the bark of redwood (''Sequoia)'' is a strong natural defense against
wildfire A wildfire, forest fire, bushfire, wildland fire or rural fire is an unplanned, uncontrolled and unpredictable fire in an area of combustible vegetation. Depending on the type of vegetation present, a wildfire may be more specifically identi ...
,
decomposition Decomposition or rot is the process by which dead organic substances are broken down into simpler organic or inorganic matter such as carbon dioxide, water, simple sugars and mineral salts. The process is a part of the nutrient cycle and ...
and infestation by certain insects such as
termite Termites are small insects that live in colonies and have distinct castes ( eusocial) and feed on wood or other dead plant matter. Termites comprise the infraorder Isoptera, or alternatively the epifamily Termitoidae, within the order Blat ...
s. It is found in the seeds,
bark Bark may refer to: * Bark (botany), an outer layer of a woody plant such as a tree or stick * Bark (sound), a vocalization of some animals (which is commonly the dog) Places * Bark, Germany * Bark, Warmian-Masurian Voivodeship, Poland Arts, e ...
, cones, and heartwood. Tannic acid is a common
mordant A mordant or dye fixative is a substance used to set (i.e. bind) dyes on fabrics by forming a coordination complex with the dye, which then attaches to the fabric (or tissue). It may be used for dyeing fabrics or for intensifying stains in ...
used in the dyeing process for
cellulose Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wa ...
fibers such as
cotton Cotton is a soft, fluffy staple fiber that grows in a boll, or protective case, around the seeds of the cotton plants of the genus '' Gossypium'' in the mallow family Malvaceae. The fiber is almost pure cellulose, and can contain minor pe ...
, often combined with alum and/or
iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
. The tannin mordant should be done first as metal mordants combine well with the fiber-tannin complex. However this use has lost considerable interest. Similarly tannic acid can also be used as an aftertreatment to improve wash fastness properties of acid dyed
polyamide A polyamide is a polymer with repeating units linked by amide bonds. Polyamides occur both naturally and artificially. Examples of naturally occurring polyamides are proteins, such as wool and silk. Artificially made polyamides can be made th ...
. It is also an alternative for fluorocarbon aftertreatments to impart anti-staining properties to polyamide yarn or carpets. However, due to economic considerations currently the only widespread use as textile auxiliary is the use as an agent to improve chlorine fastness, i.e. resistance against dye bleaching due to cleaning with hypochlorite solutions in high-end polyamide 6,6-based carpets and swimwear. It is, however, used in relatively small quantities for the activation of
upholstery Upholstery is the work of providing furniture, especially seats, with padding, springs, webbing, and fabric or leather covers. The word also refers to the materials used to upholster something. ''Upholstery'' comes from the Middle English ...
flock; this serves as an anti- static treatment. Tannic acid is used in the conservation of ferrous (iron based) metal objects to passivate and inhibit corrosion. Tannic acid reacts with the corrosion products to form a more stable compound, thus preventing further corrosion from taking place. After treatment the tannic acid residue is generally left on the object so that if moisture reaches the surface the tannic acid will be rehydrated and prevent or slow any corrosion. Tannic acid treatment for conservation is very effective and widely used but it does have a significant visual effect on the object, turning the corrosion products black and any exposed metal dark blue. It should also be used with care on objects with copper alloy components as the tannic acid can have a slight etching effect on these metals. Tannic acid is also found in commercially available iron/steel corrosion treatments, such as Hammerite Kurust.


Use in food

In many parts of the world, its uses in food are permitted. In the
United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...
, tannic acid is generally recognized as safe by the
Food and Drug Administration The United States Food and Drug Administration (FDA or US FDA) is a federal agency of the Department of Health and Human Services. The FDA is responsible for protecting and promoting public health through the control and supervision of food ...
for use in baked goods and baking mixes, alcoholic and non-alcoholic beverages, frozen dairy products, soft and hard candy, meat products, and rendered animal fat. According to
EU directive The European Union (EU) is a supranational political and economic union of member states that are located primarily in Europe. The union has a total area of and an estimated total population of about 447million. The EU has often been ...
89/107/EEC, tannic acid cannot be considered as a food additive and consequently does not hold an
E number E numbers ("E" stands for "Europe") are codes for substances used as food additives, including those found naturally in many foods such as vitamin C, for use within the European Union (EU) and European Free Trade Association (EFTA). Commonly ...
. Under directive 89/107/EEC, tannic acid can be referred to as a food ingredient. The E-number E181 is sometimes incorrectly used to refer to tannic acid; this in fact refers to the INS number assigned to tannic acid under the FAO-WHO Codex Alimentarius system.


Uses as a medication

In conjunction with magnesium and sometimes activated charcoal, tannic acid was once used as a treatment for many toxic substances, such as
strychnine Strychnine (, , US chiefly ) is a highly toxic, colorless, bitter, crystalline alkaloid used as a pesticide, particularly for killing small vertebrates such as birds and rodents. Strychnine, when inhaled, swallowed, or absorbed through the e ...
, mushroom, and ptomaine poisonings in the late 19th and early 20th centuries. The introduction of tannic acid treatment of severe burn injuries in the 1920s significantly reduced mortality rates. During
World War I World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was List of wars and anthropogenic disasters by death toll, one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, ...
, tannic acid dressings were prescribed to treat "burns, whether caused by incendiary bombs,
mustard gas Mustard gas or sulfur mustard is a chemical compound belonging to a family of cytotoxic and blister agents known as mustard agents. The name ''mustard gas'' is technically incorrect: the substance, when dispersed, is often not actually a gas, ...
, or
lewisite Lewisite (L) (A-243) is an organoarsenic compound. It was once manufactured in the U.S., Japan, Germany and the Soviet Union for use as a chemical weapon, acting as a vesicant (blister agent) and lung irritant. Although the substance is colorless ...
". After the war this use was abandoned due to the development of more modern treatment regimens.


Hazards

Tannic acid could cause potential health hazards such as damage to the eye, skin, respiratory tract, and gastrointestinal tract. It may cause irritation, redness, pain, blurred vision, and possible eye damage. When tannic acid is absorbed through the skin in harmful amounts, it may cause irritation, redness, and pain. Nausea, vomiting and diarrhoea are symptoms of tannic acid ingestion and prolonged exposure may cause liver damage. Upon inhalation, tannic acid may cause respiratory tract irritation.


Crocodilian coloration

Skin color in
Crocodilia Crocodilia (or Crocodylia, both ) is an order of mostly large, predatory, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years ago in the Late Cretaceous period ( Cenomanian stage) and are the closest livi ...
(crocodiles and alligators) is very dependent on water quality. Algae-laden waters produce greener skin, while tannic acid in the water from decay of leaves from overhanging trees (which produces some types of blackwater rivers) often produce darker skin in these animals.


References


General references

''The Merck Index'', 9th edition, Merck & Co., Rahway, New Jersey, 1976. {{DEFAULTSORT:Tannic Acid Gallotannins Astringent flavors