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

• "Some patients who take witch hazel internally develop irritation of the stomach.Witch hazel tannins may cause liver damage (McGuffin et al., 1997)." ^{[Bascom IHM]}
• "...polyphenols, such as tannins, have been implicated in the reduction of digestibility of proteins, depression in nutritive value of feedstuffs, and the increase in requirements for sulphur amino acids (Bressani and Elias, 1980)" ^{[Smartt FFLO]} "Also, it is known that tannins found in plants destroy thiamin." ^{[Omaye FNT]}
• "Consumption of tannins may lead to reduced absorption of proteins and other nutrients (Johns 1990:63). This can also lead to problems in compounding herbal medicines since there is a tendency to cause precipitates in some cases. For the same reason it is not considered wise to drink tea with meals; however, herb teas are generally low in tannins (Blake 1993)." ^{[Pengelly TCMP]}

Uses

• Mordant: "A number of tannin containing substances are employed as mordants in the dyeing of textile fibers" ^{[Singh HNDP]}
  • "Tannic acid is the best tannin for the mordanting of cotton since it is the purest of all, and does not contain the natural impurities, which are partly ineffective, partly injurious in mordanting and dyeing. Tannic acid is extensively used in the dyeing of light and brilliant shades. For dark shades, extracts of gall nuts, sumach, and myrabolan are largely employed. For light shades, 2–5% (o.w.f.) tannic acid is used; while for dark shades, 5–10% tannin is required. The material is worked in the bath for some time and is allowed to soak overnight." ^{[Singh HNDP]}
• Tanning: "Tannins are primarily used in the preservation of leather" ^{[Singh HNDP]}
• Resin Additive: "The use of tannin-based resins in the wood composite industry has been of more interest in the southern hemisphere than in the northern hemisphere due to the limited raw material supply. Nonetheless, tannins have been used widely in current commercial resins systems, albeit as hardeners or extenders, and specifically in plywood applications. Tannin-based resins using formaldehyde or the copolymerization of tannins with phenolic or amino resins have been widely reported in the literature [43–48]. The importance of the flavonoid bond cleavage reaction in the preparation of cold set Glulam adhesives was demonstrated by Hemmingway et al. [47]." ^{[Pilato PRACP]}

Pharmacology

• " The antiviral effect of commercial juices is thought to be caused by polyphenols, including tannins, which form complexes with viruses (24)." ^[HPCP] "Large phenolic compounds and tannins are capable of binding to proteins of diverse origins, sometimes in a non-specific manner (Moore and Pizza, 1992). They have been reported to inhibit viral enzymes (Brinkworth et al., 1992; Fesen et al., 1994; Nonaka et al., 1990; Tan et al., 1991; Robinson et al., 1996) and infection (Nakashima et al., 1992; Kashiwada et al., 1995) as well as cellular DNA polymerases (Parker et al., 1989)." ^[BCNS]
• "Because of their astringent properties, many of the tannin­containing herbs described in this book have been used medicinally to treat minor burns and wounds, hemorrhoids, and diarrhea. According to Launert (1981), the antibiotic effect of tannins is due to their precipitating protein, thereby depriving bacteria of nutrition." ^{[Small CH]}
• "Their ability to precipitate proteins into insoluble complexes enables humans to ‘tan’ animal hides and convert them to leather. This ability is also the basis of their astringent effects. Due to protein precipitation, the tannins exert an inhibitory effect on many enzymes, hence contributing a protective function in bark and heartwoods of woody plant species. Tannins also form precipitates with polysaccharides and some alkaloids including caffeine." ^{[Pengelly TCMP]}

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]}