Phytochemicals
Index

Tannins

Tannins are complex, astringent phenols, the active part being gallic acid. They are used commercially in tanning leather. They bind up proteins, including enzymes, and thus can quickly stop all cell functions. They occur in many tree barks and other plant structures but are seldom a problem for humans. They are, however, present in the acorns and leaves of oaks (Quercus spp.), making them extremely bitter and potentially toxic. Even edible types of acorns, before they can be safely consumed, usu- ally must have their tannins removed through leaching. [CPPlantMush]

"Tannins are responsible for the sensation of astringency of certain foods, such as tea. The puckering of the mouth resulting from tannins is different from the sensation produced by plant acids. Tannins are defensive compounds that counteract bacteria and fungi by complexing with their proteins (McGee 1984). " [Small CH]

"Tannins were first isolated chemically from oak bark, and it is thought that “tannin” came from a Celtic term for oak. Oak, tannins, and leather tanning have always been synonymous." [BackMed]

Tannins "...occur as two major types – the hydrolysable tannins and the non-hydrolysable (condensed) tannins." [HerbalMed3]

Hydrolysable Tannins

"Berries and pomegranates also contain ellagitannins, which are very large polyphenolic molecules called hydrolysable tannins. There are few studies on these, although ellagitannins do contribute to the antioxidant content of fruits. " [Skinner BIF] "The hydrolysable tannins are esters of sugars with phenolic acids and they are either gallotannins (galloyl esters of glucose), e.g. pentagalloyl glucose, or ellagitannins (hexahydrodiphenic acid, derived from two units of gallic acid, esters with glucose), e.g. agrimoniin from agrimony."[HerbalMed3] "They are readily soluble in water and alcohol. Botanicals containing hydrolysable tannins include cranesbill (Geranium maculatum) and agrimony (Agrimonia eupatoria)." [Pengelly TCMP]

Non-hydrolysable tannins

"Non-hydrolysable tannins, also known as condensed tannins or proanthocyanidins, are polymers of catechin or gallocatechin linked by C–C bonds (e.g. cola tannins). Examples of some chemical structures of hydrolysable and non-hydrolysable tannins are given in Figure 10." [HerbalMed3] "Upon hydrolysis condensed tannins form insoluble red residues or phlobaphenes. They are only partially soluble in water and alcohol; the addition of glycerine aids solubility. The basic catechin-type molecules are of low molecular weight—hence they are not true polyphenols and have low astringency." [Pengelly TCMP]

"Proanthocyanidins, or condensed tannins, are polyflavonoid in nature, consisting of chains of flavan-3-ol units. They are widely distributed in food such as apple, grape, strawberry, plum, sorghum and barley (8). Proanthocyanidins have relatively high molecular weights and have the ability to complex strongly with carbohydrates and proteins." [Huang Phenolic 1]


Hazards

Uses


Pharmacology


Examples of tannins in plants

Rose petals (Rosa Spp.) "The astringent effect, particularly of the petals, is a result of high tannin levels, which help make rose useful in staunching bleeding and unwanted discharges. There is an effect too on the digestive system, cutting over-acidity and overactivity in the stomach, as well as reducing the spasms involved in diarrhea, colitis, and dysentery." [BackMed]

"From their study of West African lima beans,....The concentration of tannic acid varies from 0.71 to 0.93%, with a mean of 0.77%. These workers suggest that tannin content may be linked with consumer acceptability of lima bean varieties, since consumers always inherently prefer the low tannin varieties." [Smartt FFLO]

"Brown algae contain certain tannins which make them very biter if they are overcooked" [Mouritsen Seaweeds]


Remediation

"Other resources of organic amendments for metal immobilization are: straws of cotton, rice and maize and rice hulls used to fix Cd, Cr, Pb, bagasse of sugarcane for Pb, waste water of paper mills for Cd, Cu, Hg, Ni, Pb and Zn and bark and sawdust. The latter are effective because of their high tannin content. The phenolic hydroxyl groups of tannin easily form chelates with metals when ion exchange happens. Both mechanisms can favorise the fixation of Cd, Hg, Cu and Pb of contaminated soils. Peats have a great affinity for Cd, Cu, Pb and Zn (Kumpiene et al. 2008). A disadvantage of peat is its high affinity for water and a tendency to shrink and swell by drying and weeting, producing bad stability." [Bini PHE]


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