TANNINS
Dr. John J. lal
Tannins, one of the major groups of bioactive phytochemicals present in plants are abundant in tea, herbs, fruits etc. and possess a wide range of biological activities. Tannin rich foods are once considered to as antinutrients or with little nutritional value however, recent findings indicate that the antinutrient effect of tannins is not due to its inhibition on food consumption or digestion but rather the decreased efficiency in converting the absorbed nutrients to new body substances.
In India, consumption of betel nuts is very popular and incidence of oesophageal cancer rate is very high and it has been suggested that tannins present in these plants are carcinogenic. But certain recent studies suggest that the carcinogenic activity of tannins may be related to components associated with tannins rather than tannins themselves. At the same time, several studies obviously indicate a protective role of tea tannins to cancers. It has been established that polyphenols and many tannin components are suggested to be anticarcinogenic. There are also reports that tannin molecules have the ability to decrease the mutagenic activity of a number of mutagens. Many of these mutagens produce free radicals and these free radicals interact with cellular macromolecules. Here tannins cab act as antioxidants and they protect the cell organelles. Thus, it is a very important protective measure against oxidative damage. Tannins also exhibit antimicrobial properties. Presences of tannins inhibit the growth of many microorganisms like fungi, yeasts, bacteria, and viruses. Certain studies also proved that tannic acid and propyl gallate (but not gallic acid), are inhibitory to food borne bacteria, aquatic bacteria, and off-flavor-producing microorganisms. It has been suggested that the antimicrobial property is due to the hydrolysis of ester linkage between gallic acid and polyols hydrolyzed after ripening of many edible fruits. Tannins in these fruits thus serve as a natural defense mechanism against microbial infections. The antimicrobial activity of tannic acid can also be used in food processing to increase the shelf life of certain foods like catfish fillets. Tannins have also been reported to exert other physiological effects, like acceleration of blood clotting, reduction of blood pressure and serum lipid level, and modulate immune responses. In short the properties of tannins can be attributed as anticarcinogenic, antimicrobial and antioxidant and this is the cheapest alternative to protect the foodstuffs, cell organelles and to increase the shelf life of processed foods.