Tannin

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 are used in medicine primarily because of their astringent properties, which are due to the fact that they react with the proteins of the layers of tissue with which they come into contact. Tannins are known to tan the outermost layer of the mucosa and to render it less permeable and more resistant to chemical and mechanical injury or irritation however, the correlation between the molecular structures of tannins and the astringent/antiulcer activity is not known. 

Several plants with antiulcer active which contain tannins have been reported (Table 64.3). 

Plants Containing Tannins with Antiulcer Activity 

A crude extract of Linderae umbellatae exhibited antipeptic and antiulcerogenic activity, and these effects were considered ascribable to the presence of tannins or related compounds. Nine condensed tannins (monomers, dimers, trimers, and tetramers) have been isolated and their antipeptic and antiulcer activity confirmed experimentally (pylorus ligated in rats and stress-induced gastric lesions in mice). 

Tannins represent the largest gronp of polyphenols. They are widely distribnted in the bark of trees, insect galls, leaves, stems and frnit. Tannins were originally isolated from the bark and insect galls of oak trees. They are the chief plant constitnents responsible for astringency. 

Tannins are high-molecnlar-weight componnds (500-5000) containing snfficient phenolic hydroxyl gronps to permit the formation of stable cross-links with proteins, and as a resnlt of this cross-linking enzymes may be inhibited. Almost all tannins are classified as either hydrolysable tannins or condensed tannins—some plants contain both kinds 

Hydrolysable tannins are derived from simple phenolic acids, particnlarly gallic acid, which is linked to a sngar by esterfication. The gallic acid gronps are nsnally bonded to form dimers snch as ellagic acid. 

Actions of hydrolysable tannins Actions of hydrolysable tannins include  

Drying effect on mucous membranes, reduces hypersecretions 

Tannins can be extracted from various plants. The component analyses of several tannins are displayed as follows  

Component %) Chestnut Nutgall bark Hemlock bark Quebracho bark Mangrove bark 

The depressant Quebracho is widely applied in foreign flotation industry. Quebracho is extracted from quebracho bark. The molecular formula of Quebracho is C22H20O9 or C23H2o08- 

In general, tannins are used to depress dolomite, calcite, and quartz. The depressing performance of quercus tannin in the flotation of bornite and quartz using, respectively, xanthate and oleic acid as collector is shown in Fig. 4.1. The depressing performances of quebracho tannin and willow tannin in the flotation of cassiterite and quartz are shown in Fig. 4.2. 

Tannin can also be used as the depressant of hematite in the reverse flotation of quartz activated by Ca using anionic collector. 

The tannins are synthesized by plants and are abundantly distributed In many different forms of plant life. Common sources of tannins include for example, the bark, leaves, fruit and roots of many plants most tannins, however, have been and are still derived from the bark of a few trees and shrubs, such as oak, chestnut, hemlock, mangrove, quebracho, and wattle, from which they are generally extracted with water. 

All the tannins readily react with proteins, forming insoluble, stable compounds when they react with collagen, the main constituent of animal skin, they form leather, a material that is resistant to hydrolysis, oxidation, and biological attack and therefore stable to weathering and resistant to decomposition. Since tannins from different plant sources have different chemical compositions, each tannin used for tanning skin produces a leather having slightly different properties and color. Tannins that have 

Trees from Which Tannins Have Been Extracted 

Bark Fruits Fruit pods Galls Wood 

Mineral tanning was probably first practiced in ancient Mesopotamia and then spread to Egypt, the Middle East, and the Mediterranean Sea area (Levey 1958). Mineral-tanned leather is soft to handle, has a velvety texture, and is almost white, a color practically impossible to achieve by other tanning processes. It is, however, very sensitive to humidity and water under wet conditions the alum in the leather is hydrolyzed (decomposed by water), forming sulfuric acid, a very strong acid that attacks the leather and causes its rapid decay. Mineral-tanned leather that has been humid or wet for a more or less extended period of time loses some of its characteristic properties, such as softness, pliability, and strength, and becomes hard, horny, and brittle. 

Depsides and Tannins. In forming depsides (poljnmers of phenol carboxylic adds) and tannin (sugar esters of phenol carboxylic acids), Fischer prepared the alkyl carbonates of the phenol carboxylic acids. Treatment of these products with phosphorus pentachloride gave the corresponding acid chlorides the latter were useful in preparing depsides and tannins. Using such reagente. 

Fischer synthesized hcanoric acid, a depside found in lichen substances. 

A tannin-like material was prepared by treating tricarbonaethoxygalloyl chloride with finely divided glucose in quinoline and chloroform. The resulting 

During the investigation of tannin-like substances, Fischer prepared a hepta (tribenzoyl gallpyl)-p-iodophenyl-maltosazone, having a moleculai weight greater than 4000. 

Distribution of Hydrolyzable Tannins Proanthocyanidins or Condensed Tannins Flavan-3,4-diols Flavan-3-ols Procyanidin Dimers 

Condensed Units Derived from Proanthocyadin Units Biological Activity of Tannins Binding of Tannins to Protein Tannins in the Diets of Animals The Cost of Tannin Defenses and Resource Allocation Other Proposed Functions of Tannins Medicinal Properties of Tannins Correlation of Tannin Consumption and Throat Cancer Tannins and the Production of Leather Other Types of Tannins Analysis of Tannins References 

Plant polyphenols, which have the ability to precipitate protein, collectively, are called tannins. These compounds have been used for millenia to convert raw animal hides into leather. In this process, tannin molecules cross-link the protein and make it more resistant to bacterial and fungal attack. Molecules in the molecular weight range of 500-2000 (3000) are most effective, but the ability to bind effectively varies with different tannin structures. Today, however, many substances considered to be tannins by virtue of their structure and bios5mthetic origin have limited, if any, ability to make leather (Hagerman and Butler, 1981). 

Although vegetable tannins are invariably pol3rphenolic substances, not all phenolic materials are tannins. Gallic acid 

Tannins usually are located in the vacuoles of plant cells and, in many cases, make up a sizable proportion of the dry weight of plants. Tannins often are concentrated in epidermal tissues and in the bark of woody plants, but these phenolic compounds may be found in leaves, roots, stems, fruits, bark, wood, or other plant parts. Tannin content may vary widely during the growing season in a particular plant part. 

Within the realm of polymer science and technology, the major application of tannins is in adhesives, mostly for wood, because of their aptitude to react with formaldehyde, although considerable research has been devoted to reduce the use of the latter by optimizing the conditions favoring tannin self-condensation and/or reactions with other aldehydes, as reviewed recently by Pizzi [50], the most authoritative expert in the field. 

A major development in the use of tannins in macromolecular materials has taken place in the last few years and again this original contribution comes from Pizzi s laboratory. Condensed tannins were found to crosslink in polycondensation reactions with furfuryl alcohol and small amounts of formaldehyde, thus 

Further work on these remarkable materials involved their carbonization and the thorough characterization of the ensuing carbon foams [53], as well as their subsequent chemical activation, which produced a dramatic increase in surface area, reaching 1800 m2g when ZnCl2 was used as promoter [54]. 

In conclusion, it is heartening to witness the lively rebirth of tannins as precursors of promising materials. As with other natural monomers and macromonomers, it appears that the possible interest of polymer bioprocesses could also be assessed on tannins, a type of study that has not been published thus far. 

aliphatic, straight, or branched chain Lignoceric G24H4502 

Source Francis, W., Coal Its Formation and Composition, Edward Arnold Ltd., London, U.K., 1961, P-168. 

FIGURE 3.13 Abiotic acid—a constituent of natural resins. 

FIGURE 3.15 Gallic acid and ellagic acid—constituents of tannins. 

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Bismarck brown, Basic Brown 1 Basic azo-dyestuff, dyes wool (reddish brown), used for cotton with tannin as mordant. Used as hair dye. 

C7H6O5. Colourless crystals with one molecule of water, m.p. 253" C, sparingly soluble in water and alcohol. It occurs free in woody tissue, in gall-nuts and in tea, and is a constituent of the tannins, from which it can be obtained by fermentation or by acid hydrolysis. It gives a blue-black colour with Fe and is used in the manufacture 6f inks. On heating it gives pyrogallol. 

C7H6O4. Crystallizes with IH2O m.p. 199" C. It occurs in the free state in the onion and other plants is a constituent of one group of tannins, and is a product of the alkaline decomposition of resins. 

Natural sensitizers Natural tannins Natural vs synthetic dyes Nature ofinvention Nausea N.S. Nautilus Naval brass... 

Fig. 1. An amplified outline scheme of the making of various wiaes, alternative products, by-products, and associated wastes (23). Ovals = raw materials, sources rectangles = wines hexagon = alternative products (decreasing wine yield) diamond = wastes. To avoid some complexities, eg, all the wine vinegar and all carbonic maceration are indicated as red. This is usual, but not necessarily tme. Similarly, malolactic fermentation is desired in some white wines. FW = finished wine and always involves clarification and stabilization, as in 8, 11, 12, 13, 14, 15, 33, 34, followed by 39, 41, 42. It may or may not include maturation (38) or botde age (40), as indicated for usual styles. Stillage and lees may be treated to recover potassium bitartrate as a by-product. Pomace may also yield red pigment, seed oil, seed tannin, and wine spidts as by-products. Sweet wines are the result of either arresting fermentation at an incomplete stage (by fortification, refrigeration, or other means of yeast inactivation) or addition of juice or concentrate.

Minerals, particularly Bentonite, ate used to remove proteins that tend to cause haze in white wines. The natural tannin of ted wines usually removes unstable proteins from them. Excess tannin and related phenols can be removed and haze from them prevented by addition of proteins or adsorbents such as polyvinylpyttohdone. Addition of protein such as gelatin along with tannic acid can even be used to remove other proteins from white wines. Egg whites or albumen ate often used to fine ted wines. Casein can be used for either process, because it becomes insoluble in acidic solutions like wines. 

Com, wheat, and rice are the most desirable common grains and are used extensively ia pet foods. Oats and barley often tend to have excess fiber, which can be objectionable. However, barley is a preferred grain for moisture absorption and form ia caimed foods because the turgid white form is desired ia some canned dog foods. Milo has enormous variations ia tannin content which can influence digestibiUty and acceptabiUty, thus limiting its use ia pet foods (see Wheat and other cereal grains). 

Investigations have focused on the content of polyphenoHcs, tannins, and related compounds in various foods and the influence on nutrient availabiHty and protein digestibiHty. It has been estabHshed that naturally occurring concentrations of polyphenoloxidase and polyphenols in products such as mushrooms can result in reduced iron bio availabiHty (75). Likewise, several studies have focused on decreased protein digestibiHty caused by the tannins of common beans and rapeseed (canola) (76—78). 

Alginates, alkaloids, glycerides, gutta, phenols, rosins, rubber, saponins sterols, tall oils, tannins, terpenes, waxes... 

The quahty of individual gums is mainly deterrnined by color and taste or odor. Many gums are colorless when secreted, but darken on aging. Most gums are usually tasteless unless contaminated by the bitter flavors of tannins which precludes their use in foods. 

Plants and microorganisms produce unique and diverse chemical stmctures, some of which act as immunomodulators (18—28). Of specimens used in traditional medicine, approximately 450 plant species have shown antiviral activity out of 4000 plants screened (19). Several tannins (20) exhibit strong inhibition of tumor promotion experimentally. Pretreatment of mice with small amounts of tannins for several days strongly rejected transplanted tumors. This activity has been claimed to be effected through enhancement of host-mediated antitumor activity. 

The apphcation of vegetable tanning materials has an additive effect on the leather. The more vegetable tannins appHed the more the leather becomes like vegetable-tanned leather. The color is changed, the fullness of feel increases, and the leather can be worked and embossed like vegetable leather. 

In retanning, vegetable tannins may be used in conjunction with or may be entirely replaced by synthetic tanning agents called syntans. The syntans and other specialty chemicals allow the creation of leathers not possible using vegetable tannins alone. 

The mechanism of the tarmage is accepted to be largely one of replacement of the bound water molecules by the phenoHc groups of the tannin and subsequent formation of hydrogen bonds with the peptide bonds of the protein. The effect of this bonding is to make the leather almost completely biorefractive. 

Niobic Acid. Niobic acid, Nb20 XH2O, includes all hydrated forms of niobium pentoxide, where the degree of hydration depends on the method of preparation, age, etc. It is a white insoluble precipitate formed by acid hydrolysis of niobates that are prepared by alkaH pyrosulfate, carbonate, or hydroxide fusion base hydrolysis of niobium fluoride solutions or aqueous hydrolysis of chlorides or bromides. When it is formed in the presence of tannin, a volurninous red complex forms. Freshly precipitated niobic acid usually is coUoidal and is peptized by water washing, thus it is difficult to free from traces of electrolyte. Its properties vary with age and reactivity is noticeably diminished on standing for even a few days. It is soluble in concentrated hydrochloric and sulfuric acids but is reprecipitated on dilution and boiling and can be complexed when it is freshly made with oxaHc or tartaric acid. It is soluble in hydrofluoric acid of any concentration. 

A considerable quantity of oil can be extracted from waste material from shelling and processing plants, eg, the inedible kernels rejected during shelling and fragments of kernels recovered from shells. About 300 t of pecan oil and 300—600 t of English walnut oil are produced aimuaHy from such sources. The oil is refined and used for edible purposes or for the production of soap the cake is used in animal feeds (see Feeds and feed additives). Fmit-pit oils, which closely resemble and are often substituted for almond oil, are produced on a large scale for cosmetic and pharmaceutical purposes (143). For instance, leaves, bark, and pericarp of walnut may be used to manufacture vitamin C, medicines, dyes and tannin materials (144). 

Leather Taiming. Oxahc acid is used as a pH modifier in leather tanning by tannin and basic chromium sulfate. It also functions as a bleaching agent for leather (qv). 

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