The word “Tannin” originally refers to the early use of oak and other barks in tanning animal hides into leather. Tannins (commonly referred as tannic acid) are water-soluble polyphenols commonly distributed in plants (The Editors of Encyclopedia Britannica 2016). The presence of tannins has been noted in both gymnosperms as well as angiosperms. The plant families that have been found to produce tannins are Actinidiaceae, Anacardiaceae, Aceraceae, Burseraceae, Bixaceae, Combretaceae, Dipterocarpaceae, Ericaceae, Grossulariaceae, and Myricaceae in case of dicots and Najadaceae and Typhaceae in case of monocots (Mole 1993). Additionally, tannins have been found to occur in numerous plant parts for instance: roots, barks, wood, fruits, and leaves. Apart from these they are also reported in galls resulting from the insect attacks. They can be present in any form ranging from a group of pale-yellow to light-brown amorphous powdery substances, flakes, or a spongy mass (The Editors of Encyclopedia Britannica 2016).
Tannins have high molecular weights ranging from 500 to 20,000 and possess low nutritional value (Swain and Bate-Smith 1962). Due to the presence of a high number of hydroxyls and other suitable groups (like carboxyls), they have the tendency to bind and precipitate proteins, amino acids, and alkaloids.
Hydrolysable tannins: Tannins that form gallic or ellagic acids on heating with hydrochloric or sulfuric acids are termed as hydrolysable tannins or pyrogallol-type tannin. These tannins are generally present in different vegetable plants like myrobalan (Terminalia chebula), sumac (Rhus coriaria), gallnuts (Quercus infectoria and Rhus semialata), Aleppo gallnuts (Andricus kollari), tara pods (Caesalpinia spinosa), chestnut wood (Castanea sativa), and oak wood (Quercus robur, Quercus petraea, and Quercus alba) (Haslam 1989; Hemingway and Laks 2012).
Condensed tannins: The polymers resulting from the condensation of the flavans are called as condensed tannins. Sugar residues are absent in the condensed tannins. They are present in uebracho wood (Schinopsis lorentzii), mimosa bark (Acacia mollissima), grape seeds (Vitis vinifera), pine barks, and spruce barks (Haslam 1989; Ping et al. 2011).
Pseudotannins: Pseudotannins unlike the hydrolysable and condensed tannins do not change color during Goldbeater’s skin test. Furthermore, they cannot be used as tanning compounds and are present in Rhubarb, acacia, catechu, tea, coffee etc. (Kar 2003).
Properties of Tannins: Tannins function as protectors and pesticides in plants. The astringent taste of the tannins causes a dry and pucker feeling in the mouth similar to that felt after the consumption of tea, unripened fruit, or red wine. This unpleasant feeling wards off the predators and the insects (McGee 2004). Tannins are extensively used in tanning leather, dyeing fabric, making ink, and in various medical applications. Apart from protection, tannins are also involved in the regulation of the plant growth (Corcoran et al. 1972).
Studies have shown that tannins possess anticarcinogenic and antimutagenic activity. These polyphenolic compounds are known to reduce the level of reactive oxygen species present in the body. This anti-oxidative property of tannins is assumed to be the underlying reason for its anti-cancer property. Many reports have been published claiming the antimicrobial nature of tannins against fungi, bacteria and viruses. They are also known to possess certain physiological properties like accelerating the clotting of the blood, reduction in the levels of the serum lipid and blood pressure, modulation of immune response, and also causing liver necrosis. They function as good preservatives and are also used for clearing Japanese sake. Tannins have the ability to produce different colors with ferric chloride. Along with the use in the formation of corrosion free primers, it is also used in wood and particleboard adhesives (Chung et al. 1998).
Tannins are polyphenolic plant-based compounds playing a pivotal role in the protection of the plants. Although, the tannins are low in nutritive value, they have a wide range of applications in the human health as well as in combating diseases. More research in this field would unravel the true potential of tannins and uncover more applications for the betterment of the human health.
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- The Editors of Encyclopedia Britannica (2016). Tannin. Encyclopedia Britannica. https://www.britannica.com/science/tannin. Accessed 22 Feb 2019.