Tannins comprise a group of compounds with a wide diversity in structure that share their ability to bind and precipitate proteins. The name tannins refers to the process of tanning animal skin to form leather. This process has been known since prehistoric times, when animal hides were treated with animal fat and brain tissue. Chemically this resulted in the cross-linking of the collagen chains in the hide. Throughout much of history the tanning process was performed with tannins derived from plants, until minerals such as aluminum and chromium replaced the use of plant tannins during the last century. As part of Japanese and Chinese natural medicine tannins have been used as anti-inflammatory and antiseptic compounds. They have also been used to treat a wide array of illnesses, including diarrhea and tumors in the stomach or duodenum (Khanbabaee and Van Ree, 2001). Another application of tannins is in wine and beer production, where they are used to precipitate proteins.
Tannins are abundant in many different plant species, in particular oak (Quercus spp.), chestnut (Castanea spp.), staghorn sumac (Rhus typhina), and fringe cups (Tellima grandiflora). Tannins can be present in the leaves, bark, and fruits, and are thought to protect the plant against infection and herbivory.
Tannins can be classified in three groups: condensed tannins, hydrolysable tannins, and complex tannins (Khanbabaee and Van Ree, 2001). These groups can then be further divided, as shown in Figure 1-3.
3.13.1 Condensed tannins
Condensed tannins are also referred to as proanthocyanidins. They are oligomeric or polymeric flavonoids consisting of flavan-3-ol (catechin) units. Hydrolysis under harsh conditions, such as heating in acid, yields anthocyanidins. An example of a condensed tannin is procyanidin B2 (epicatechin-(4p^8')-epicatechin; 1.90). In this case the interflavanyl linkage is between C4 of the 'lower' unit, and C8 of the 'upper' unit. The linkage can also be between C4 of one unit and C6 of the second unit.
Polymers are formed through the action of acids or enzymes. Polymers made up of between two and ten residues are called flavolans. Polymers made up of more than 50 catechin units have been identified (Khanbabaee and Van Ree, 2001).The degree of polymerization affects the ability to precipitate proteins. This is of importance in wine making, where a high level of condensed tannins, especially in red wines, can result in the dry feeling on the inside of the mouth.
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