|Class/Polymer:||Hydrolyzable Tannins||Condensed Tannins|
The leaching of tannins from the decaying leaves of vegetation adjoining a stream may produce what is known as a blackwater river.
The tea plant (Camellia sinensis) is an example of a plant said to have a naturally high tannin content. When any type of tea leaf is steeped in hot water it brews a "tart" (astringent) flavor that is characteristic of tannins. This is due to the catechins and other flavonoids. Tea "tannins" are chemically distinct from other types of plant tannins such as tannic acid and tea extracts have been reported to contain no tannic acid. Black tea and peppermint tea are more inhibitory of iron than herb teas like chamomile, vervain, lime flower and pennyroyal.
Tannins (mainly condensed tannins) are also found in wine, particularly red wine. Tannins in wine can come from many sources and the tactile properties differ depending on the source. Tannins in grape skins and seeds (the latter being especially harsh) tend to be more noticeable in red wines, which are fermented while in contact with the skins and seeds to extract the colour from the skins. The stems of the grape bunches also contain tannins, and will contribute tannins if the bunches are not de-stemmed before pressing and fermentation. Tannins extracted from grapes are condensed tannins, which are polymers of proanthocyanidin monomers. Hydrolysable tannins are extracted from the oak wood the wine is aged in. Hydrolysable tannins are more easily oxidised than condensed tannins.
Modern winemakers take great care to minimize undesirable tannins from seeds by crushing grapes gently when extracting their juice, to avoid crushing the seeds. Pressing the grapes further results in press wine which is more tannic and might be kept separately. De-stemming is also widely practiced. Wines can also take on tannins if matured in oak or wood casks with a high tannin content. Tannins play an important role in preventing oxidation in aging wine and appear to polymerize and make up a major portion of the sediment in wine.
Recently, a study in wine production and consumption has shown that tannins, in the form of proanthocyanidins, have a beneficial effect on vascular health. The study showed that tannins suppressed production of the peptide responsible for hardening arteries. To support their findings, the study also points out that wines from the regions of southwest France and Sardinia are particularly rich in proanthocyanidins, and that these regions also produce populations with longer life spans.
Tannins in wine have been described, particularly by novice drinkers, as having the effect of making wine difficult to drink compared to a wine with a lower level of tannins. Tannins can be described as leaving a dry and puckered feeling with a "furriness" in the mouth that can be compared to a stewed tea, which is also very tannic. This effect is particularly profound when drinking tannic wines without the benefit of food.
Many oenophiles see natural tannins (found particularly in varietals such as Cabernet Sauvignon and often accentuated by heavy oak barrel aging) as a sign of potential longevity and ageability. As tannic wines age, the tannins begin to decompose and the wine mellows and improves with age, with the tannic "backbone" helping the wine survive for as long as 40 years or more. A strongly tannic wine is also well-matched to very fatty food courses, in particular steaks; the tannins help break down the fat, with a salutary impact on both the wine and the steak. In many regions (such as in Bordeaux), tannic grapes such as Cabernet Sauvignon are blended with lower-tannin grapes such as Merlot or Cabernet Franc, diluting the tannic characteristics. Wines that are vinified to be drunk young typically have lower tannin levels.
Pomegranates contain a diverse array of tannins, particularly hydrolysable tannins. The most abundant of pomegranate tannins are called punicalagins. Punicalagins have a molecular weight of 1038 and are the largest molecule found intact in rat plasma after oral ingestion and were found to show no toxic effects in rats who were given a 6% diet of punicalagins for 37 days. Punicalagins are also found to be the major component responsible for pomegranate juice's antioxidant and health benefits.
Several dietary supplements and nutritional ingredients are available that contain extracts of whole pomegranate and/or are standardized to punicalagins, the marker compound of pomegranate. Extracts of pomegranate are also Generally Recognized as Safe (GRAS) by the United States Food and Drug Administration. It has been recommended to look for pomegranate ingredients that mimic the polyphenol ratio of the fruit, as potent synergistic effects have been observed in 'natural spectrum' extracts, especially pomegranate concentrate normalized to punicalagins.
Tannin is also found in chocolate. The usual concentration is around 10mg per ml in the liquid form. You would have to eat 100 bars of chocolate to consume the equivalent amount found in a bottle of wine.
If ingested in excessive quantities, tannins inhibit the absorption of minerals such as iron and calcium which may, if prolonged, lead to anemia or osteoporosis. This is because tannins are metal ion chelators, and tannin-chelated metal ions are not bioavailable. This may not be bad for someone with an infection, as iron is mopped up by the immune system to keep microorganisms from properly multiplying. Tannins have been shown to precipitate proteins, which inhibits in some ruminant animals the absorption of nutrients from high-tannin grains such as sorghum. Tannins only reduce the bioavailability of plant sources of iron, also known as non-heme. Animal sources, or heme iron absorption will not be affected by tannins. Tannic acid does not affect absorption of other trace minerals such as zinc, copper, and manganese in rats.
Tannins are phenolic compounds and interfere with iron absorption through a complex formation with iron when it is in the gastrointestinal lumen which decreases the bioavailability of iron. There is an important difference in the way in which the phenolic compounds interact with different hydroxylation patterns (gallic acid, catechin, chlorogenic acid) and the effect on iron absorption. The content of the iron-binding galloyl groups may be the major determinant of the inhibitory effect of phenolic compounds. However, condensed tannins do not interfere with iron absorption.
In order to prevent these problems, it is advised to drink tea and coffee between meals, not during. Foods rich in vitamin C help neutralize tannin's effects on iron absorption. Adding lemon juice to tea will reduce the negative effect of tannins in iron absorption as well. Adding milk to coffee and tea has very little to no influence on the inhibitory effect of tannins.
In sensitive individuals, a large intake of tannins may cause bowel irritation, kidney irritation, liver damage, irritation of the stomach and gastrointestinal pain. With the exception of tea, long-term and/or excessive use of herbs containing high concentrations of tannins is not recommended. A correlation has been made between esophogeal or nasal cancer in humans and regular consumption of certain herbs with high tannin concentrations.
The anti-inflammatory effect of tannins help control all indiccations of gastritis, esophagitis, enteritis, and irritating bowel disorders. Diarrhea is also treated with an effective astringent medicine that does not stop the flow of the disturbing substance in the stomach; rather, it controls the irritation in the small intestine.
Tannins not only heal burns and stop bleeding, but they also stop infection while they continue to heal the wound internally. The ability of tannins to form a protective layer over the exposed tissue keeps the wound from being infected even more. Tannins are also beneficial when applied to the mucosal lining of the mouth.
Tannins can also be effective in protecting the kidneys. Tannins have been used for immediate relief of sore throats, diarrhea, dysentery, hemorrhaging, fatigue, skin ulcers and as a cicatrizant on gangrenous wounds. Tannins can cause regression of tumors that are already present in tissue, but if used exessively over time, they can cause tumors in healthy tissue. Tannins are used indirectly as molluscicides to interrupt the transmission cycle of schistosomiasis. They have also reported to have anti-viral affects. When incubated with red grape juice and red wines with a high content of condensed tannins, the poliovirus, herpes simplex virus, and various enteric viruses are inactivated.
Tannins can also be used to pull out poisons from poison oak or from bee stings, causing instant relief. The tannins help draw out all irritants from the skin because tannin is an astringent that tightens pores and pulls out liquids.
Tannins produce different colors with ferric 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 with iron(II) sulfate.
Tannins, including gallo and ellagic acid (epigallitannins), are inhibitors of HIV replication.