Tannins are complex chemical substances derived from phenolic acids (tannic acid). The word tannin comes from the old German word tanna meaning oak. Tannins are a class of astringent, polyphenolic biomolecules that bind to and precipitate proteins and various other organic compounds including amino acids and alkaloids. Tannins are water-soluble polyphenols that are present in many plant foods. They are responsible for decreases in feed intake, growth rate, feed efficiency, net metabolizable energy, and protein digestibility in experimental animals. Some of the richest and most common dietary sources of tannins include coffee, tea, wine, and chocolate. The astringent and bitter flavors that are characteristic of these foods and beverages are usually attributable to their abundant supply of tannins.
Tannin is most common in gymnosperm and angiosperm plants. It protects the plants from predation and helps in the synthesis of plant hormones. Fruits such as grapes, oranges, walnuts, hazelnuts, pecans, blueberries, strawberries, and cranberries have rich content of tannin. Thus, tannins have been used for ages in the production of many flavoring compounds for foods. It is used as a spice and for its aromatic and flavoring properties. One group of tannins known as resveratrol is mostly found in red grapes, red wine, and peanuts. In addition to being bitter, tannins can be found in some fruits. These berries and other fruits contain a small amount of tannin, but the majority of the tannin content is from the resveratrol.
Tannins have a unique property of being able to penetrate the cell membrane of any living thing it comes in contact with, and since they are lipid soluble they can be transported across the cell membrane with other molecules and waste products. This property has made it very useful as a natural substance for many medicinal uses. When used as an antibiotic it has been found to inhibit the growth of certain pathogenic microorganisms by killing them. It has also been found to inhibit growth and spread of tumor cells and cancer cells in laboratory animals. Other medical properties of tannin include a characteristic smell similar to that of cloves, a smell that is pleasant and a flavor resembling that of burnt tea or coffee.
Moreover, the results suggest that tannins have remarkable activity in cancer prevention and anticancer. It has wealthy foreground for developing new cancer prevention agents and/or new anticancer drugs screening among tannin compounds. However, incidences of certain cancers, such as esophageal cancer, have been reported to be related to the consumption of tannins-rich foods such as herbal teas and betel nuts, suggesting that tannins might be carcinogenic. While, in large amounts, tannic acid can cause side effects such as vomiting, nausea, stomach irritation, and liver damage. Regular consumption of herbs with high tannin concentrations seems to be associated with an increased chance of developing nose or throat cancer.
Tannins are found commonly in the bark of trees, wood, leaves, buds, stems, fruits, seeds, roots, and plant galls. The positive health benefits of tannin come from its anti-carcinogenic and anti-mutagenic properties, mostly due to its anti-oxidizing nature. Tannins also remove harmful microbes from the body, and fight against harmful bacteria, viruses, and fungi. Moreover, they are medicinally significant due to their astringent properties. Therefore, there is an increasing demand for tannins across the globe.