Tannins are polyphenols that are obtained from various parts of different plants belonging to multiple species. Deriving it name from the technical word ‘tanning' that meant converting animal hides to leather through chemical processes; tannin is basically used for this function. It is found in abundance in the tree bark, wood, fruit, fruitpod, leaves, and roots and also in plant gall. Since earlier times, people obtained tannin for tanning from plants like wattle (Acacia sp.), oak (Quercus sp.), eucalyptus (Eucalyptus sp.), birch (Betula sp.), willow (Salix caprea), pine (Pinus sp.), quebracho (Scinopsis balansae).
Tannins are found as shapeless yellowish or light brown masses like powder, flakes or sponge. Interestingly, tannins are found almost in all plants and in all climates all over the world. Although algae, fungi and mosses do not contain much tannin. The percentage of tannins present in the plants, however, varies. While they are present in significant proportions in some plants, many others have too little of them. Tannins are usually found in large quantities in the bark of trees where they act as a barrier for micro-organisms like bacteria and fungi and protect the tree. Apart from tanning, tannins are also used in dyeing, photography, refining beer and wine as well as an astringent in medicines. Significantly, tannins form a vital element of tea!
While soluble, astringent materials are found in some plants like tea and coffee, tannins are supplemented to various processed foods, including ice-cream and caramel. They are also used as refining materials to precipitate proteins in wines and beer. As tannins often lower the absorption of some materials into the body, tannins are also often known as anti-nutrients. For example, tannins are found in tea and coffee and consuming too much of these beverages without milk may lead to calcium and iron deficiency in the body and often lead to osteoporosis (a diseases where bones become fragile) and anemia.
In order to counter these problems, it is advised that one should take tea or coffee between meals and not consecutively. In addition, adding milk or lemon juice to the tea helps in reducing or neutralizing tannins' adverse actions on iron intake. Similarly, consuming food that is rich in vitamin C also helps in neutralizing tannin's effects on iron absorption.
Tannins can be classified into two broad groups - hydrolysable tannins and condensed tannins.
Hydrolysable tannins are basically derived from simple phenolic acids like gallic acid or ellagic acid and when heated they give away pyrogallol. Pyrogallol is also known as hepatotoxic and has antiseptic as well as caustic properties. Owing to the hepatotoxic property, plants that have a concentration of tannins are not appropriate for application on open wounds. When out in the open air hydrolysable tannins normally change to a brownish color and are accountable for the brown color of many plant dyes.
On the other hand, condensed tannins, also known as non-hydrolysable tannins, do not split easily and hence it is difficult to analyze these. Condensed tannins are basically flavonoid dyes formed through bio-synthesis of flavins and catechins. When these non-hydrolysable tannins are heated up in acids they synthesize to yield a red insoluble substance known as tannin reds or phlobaphenes. Phlobaphenes are flushed precipitates found in some plants that have reddish tints and this is an indication that these plants have rich concentration of condensed tannins. When condensed tannins are heated, catechol emerges as the final product. Unlike hydrolysable tannins, condensed tannins do not possess any trace of hepatotoxicity or any adverse side effects and hence are favorable for use.
In addition to hydrolysable and condensed tannins, other examples of polyphenols include arbutin derived from uva ursi (Arctostaphylos uva-ursi), rugosin-D obtained from meadowsweet (Filipendula ulmaria) and sanguin H-6 from raspberry leaves (Rubus idaeus).
Interestingly, all tannins have several common properties amongst them. While the tannins are soluble in water and alcohol, they do not dissolve in organic solutions. Again, when reacted with nitrogenous bases, polysaccharides, some alkaloids, few glycosides and proteins all tannins form precipitates. Medically, tannins are used as antidotes to poisoning by alkaloids depending on their capacity to form insoluble tannates. However, only dilute solutions of tannins are applied for this work. Finally, almost all tannins consumed remain exuded during the digestive process, and different quantities of it enter the body fluids and are emitted by the kidney.
The above-mentioned properties of tannins should always be kept in mind while applying extracts from tannin-rich plants for medicinal purpose. Tannin is basically an astringent that means that it tauten the pores and pulls out liquids from plants. In plants, tannins are large astringents molecules that easily attaches with proteins. To find the truth about these properties of tannins you may try a few small experiments. If you put tannin on your skin, you will witness it to shrink and if you apply if on your face you will notice wrinkles appearing. At the same time, tannins help to draw out all irritants from the skin. These properties impart medicinal qualities to tannin which is applied on the skin to pull out poisons from bee stings or poison oak bringing in instant relief.
The other remedial values of tannins include application on burns to heal the injury and on cuts to stop bleeding. Tannin's ability to form a strong ‘leather' resistance on the exposed tissues helps in protecting the wounds from being affected further. While it stops infection from above, internally tannin continues to heal the wound. In case of third degree burns using strong tannin sources will not only prevent septicemia, but also help to save life. This traditional method has been practiced by most medicos in all countries. On the other hand, when a tannin-rich solution is poured on the flesh, it generates a sealing 'eschar' that often helps in growing new skin albeit temporarily. This technique requires repeated washing of the wound with tannins and this helps to eliminate the bacteria too. Hence, tannins are also said to have antiseptic properties. Interestingly, this practice is still followed in the primary health care centers in China and is also recommended as a first-aid treatment at places where emergency medical services are still inadequate or faulty.
Tannins can also be effective in curbing hemorrhages as well as restrict bare swellings. While tannins are proved haemostatics, they are also beneficial when applied on mucosal coating in mouth. Hence, herbs possessing tannins are widely used as mouthwashes, eyewashes, snuff and even as vaginal douches and also treat rectal disorders.
When applied internally, tannins affect the walls of the stomach and other digestive parts. They sour the mucus secretions and contract or squeeze the membranes in such a manner that secretions from the cells are restricted. The good thing is that tannins' anti-inflammatory effect helps to control or curb all indications of gastritis, enteritis, oesophagitis and irritating bowel disorders. This action is possible by involving lymph stasis and neutralizing the autolytic enzymes. Conventionally, tannins have also been used to cure diarrhea. In most rural areas diarrhea is caused due to the irritation of the enteritis or the small intestine and is the reason for many deaths worldwide. Although diarrhea initially affects the large bowel, but a reflex action origination higher up aims at eliminating the disturbing material in the system as early as possible. Diarrhea many be considered to be a healthy action as it helps to remove the unwanted or disturbing substance from the system, but if it prolongs, it may lead to dehydration and nausea often resulting to death. Thus, in order to control the fierceness of diarrhea, application of an effective astringent medicine is recommended. An effective astringent does not stop the flow of the disturbing substance in the stomach, but helps in controlling the irritation in the small intestine.
In earlier days, bowel looseness was a result of infection in the small bowel. But in the contemporary society, there are other reasons than this for the disorder. Today looseness of bowel can occur even due to bowel tremor and worsening irritating diseases in the small or large bowel. This means that the application of tannins is restricted in bowel disorder cases these days. However, they can still be used to reduce irritations and control the ferocity of diarrhea. These days agents such as kaolin and morphine are also used to treat the looseness of bowel. Tannins, however, differ from them as they are purely applied on the basis of symptoms and have a healing effect on the bowel wall. At the same time, tannins restraint the entry of any unwanted pathogenic substances.
Although tannins are beneficial in treating bowel irritations and many other ailments, there is a need to restrict or limit the wanton use of tannins in order to avoid unwanted problems. It is a well-known fact that when reacted with all proteins (including dietary proteins) tannins form precipitates. But these tannin-proteins are too complex to be soaked into the blood stream and while they are in the digestive system they intend to restrict or retard the secretion of enzymes. Hence, it is important to restrict the usage of the medicines rich in tannins. This is particularly important when consuming tea and coffee without milk. As the astringent in these beverages retard the metabolism process, it is always advised that tea and coffee should be taken only lightly brewed. And it is even better to consume tea with milk or lemon juice that counteracts the effects of astringent.
In fact, the issue is pretty complex. For example caffeine is an alkaloid and is compounded by tannin. And hence it is available to the body more through coffee than tannin-rich tea. Nevertheless, the tannin effect has its advantages too. When milk is added to tea it not only forms an altogether different complex, but milk being a protein comes into action with the tannin in tea. So when we drink tea with milk, the tannin in tea is engaged with milk protein and hence does not affect any other protein in the digestive system.
There is a strong possibility that intrusion may take place between tannins and alkaloids as well as other pharmacological substances. Hence there can be lesser movement in plants with high degree of tannins or if tannin-rich medicines are supplemented to them. In fact, tannins also behave like mucilages as they too function as astringents only when they come in contact with the skin/ flesh surface or the digestive system's wall at some point. However, when they are applied internally, tannins do not have any noticeable astringent action on the body organs or tissues. This is despite the fact that the astringent plants may include other ingredient with this function.
While obtaining tannins, it is best to collect the tannin-rich tree barks in the spring for this is the time when the juices begin to come up in the trees and the leaves being to sprout. During this time, you need not work hard to peel off the bark; they will come out without much effort. Although people may use bark collected during any time of the year, during spring the concentration of tannin is maximum in the bark. The concentration of tannin is more in the inner bark (cambium layer) of the tree. In addition, an older tree has more tannin than a younger one and there is more tannin in the lower parts of the tree than the upper parts. Then there are again different timings for de-barking different trees for tannin. It is said that a fir tree can be de-barked for tannin only when it is 30 years old, while it is best to de-bark an oak tree between 15 and 30 years. However, there is difference of opinion in this regard, as many others are of the view that the best time to de-bark oak trees for tannin is between 30 and 35 years.
However, if one is interested in obtaining tree barks for tannin, it is best to buy the shredded barks sold at the saw mills. But before you purchase such shredded barks from the saw mills you must ascertain that the logs or the barks had not been left in the open to be soaked in the rain. Rain-soaked shredded bark is not of much use as it does not contain high tannin levels as tannin is soluble in water and if left in the rain will drain out with other sap. For best results, the barks should be dried in the sun and stored dry. When the bark is stored dry, it can be used for an indefinite period without these losing their tannin content. Besides, dry bark is always easy to grind and extract tannin.
Tannin has several industrial uses as preservatives. In dry wood and leather, tannin averts rotting, changing of shape and decay by bacteria and fungi. It also repels insects and herbivores and protects the plants. Bacteria and fungi cannot grow on plants or wood containing tannin by restricting the flow of extra-cellular microbial enzymes. As a result of this, micro-organisms are not allowed to develop colonies in plants containing tannin or timber and leather treated with the substance. Most significantly, tannins destroy the microbe's metabolism process by depriving them of iron and other metal ions through restriction of oxidative phosphorylation.
Before concluding, it is worthwhile to mention that the exclusive properties and actions of tannin may initiate many to undertake research in industries related to paint manufacture, construction, wood preservation, bath and tiles as well as leather. The most important properties of tannin that can be taken up for innovation is of course its preservative action.