Tannins are complex molecules based on phenol monomers. These substances are ubuquitious in the plant kingdom but are not found in the animal kingdom. Tannins obtained their name because these are phenol-rich plant extracts that are used to convert animal hides into leather. They are best known for their tanning capability in leather preparation. The word tan has its origin in the Old English word meaning oak bark. Bark from various species of oak, pine, spruce and black locust are still among the important sources of commercially used tanning.
This terminology was established before the chemical nature of these compounds was known, which often leads to confusion about the difference between tannins and lignins. One of the major function of tannins in plants is thought to be a barrier to animals tha eat them. They are also effective in inhibiting fungal and bacterial growth.
Many legume seeds including fava beans contain tannins. Colored seed coats of grains is often an indication of the presence of tannin. The best-known characteristic of tannins in foods is the astringency caused by the precipitation of salivary mucoproteins. Tannins are partly responsible for flavor in beer, tea, and some fruit juices. Apples, cranberries and grapes have high tannin levels. In general they contribute to astringency, color, flavor and storage stability.
The astringency tends to decrease in ripening bananas, persimmons, apples, peaches, plums, and stone fruits in general. In general, the lower the amount of tannins the more palatable is the plant. During fruit ripening, tanning molecules disintegrate and are replaced by increasing amounts of sugar.
Tannins have been traditionally thought to affect metabolism by binding to proteins thereby deactivating them and causing their precipitation. It is now known that in some cases tannin-protein complexes are soluble.
Tannins also bind to some other cellular components such as iron. They also scavenge free radicals of oxygen
Classification of Tannins
Three classes of tannins exist: condensed tannins (proanthocyanidins), and hydrolyzable tannins (such as gallic acid) and pseudotannins.
Tannin Health Benefits
Tannic acid (TA), also known as gallotannic acid, is a glucoside of gallic acid polymer that has been shown to possess anti-bacterial, anti-enzymatic, anti-tumor and astringent properties.4
Ellagitannins are bioactive polyphenols that have antioxidant and anti-inflammatory bioactivities. Pomegranate juice has the highest concentration of ellagitannins of any commonly consumed juice and contains the unique ellagitannin, punicalagin. Punicalagin is the largest molecular weight polyphenol known. Ellagitannins are not absorbed intact into the blood stream but are hydrolyzed to ellagic acid. They are also metabolized by gut flora into urolithins which are conjugated in the liver and excreted in the urine. These urolithins are also bioactive and inhibit prostate cancer cell growth.5
Tannins exist widely in plants, but because they precipitate proteins, scientists frequently ignore them in search of bioactive components. Recently it has been found that strawberry, cloudberry, arctic bramble, and the raspberry extracts share common polyphenol constituents, especially the ellagitannins, which have been shown to be effective antiproliferative agents.6
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- Tannic acid-induced apoptosis and -enhanced sensitivity to arsenic trioxide in human leukemia HL-60 cells.Chen KS, Hsiao YC, Kuo DY, Chou MC, Chu SC, Hsieh YS, Lin TH.
- Multitargeted therapy of cancer by ellagitannins.Heber D.
- Berry extracts exert different antiproliferative effects against cervical and colon cancer cells grown in vitro.McDougall GJ, Ross HA, Ikeji M, Stewart D.