Antioxidant activity and phenolic compounds of 112 traditional Chinese medicinal plants associated with anticancer.

Tannins (commonly referred to as tannic acid) are water-soluble polyphenols that are present in many plant foods. They have been reported to be responsible for decreases in feed intake, growth rate, feed efficiency, net metabolizable energy, and protein digestibility in experimental animals. Therefore, foods rich in tannins are considered to be of low nutritional value. However, recent findings indicate that the major effect of tannins was not due to their inhibition on food consumption or digestion but rather the decreased efficiency in converting the absorbed nutrients to new body substances. Incidences of certain cancers, such as esophageal cancer, have been reported to be related to consumption of tannins-rich foods such as betel nuts and herbal teas, suggesting that tannins might be carcinogenic. However, other reports indicated that the carcinogenic activity of tannins might be related to components associated with tannins rather than tannins themselves. Interestingly, many reports indicated negative association between tea consumption and incidences of cancers. Tea polyphenols and many tannin components were suggested to be anticarcinogenic. Many tannin molecules have also been shown to reduce the mutagenic activity of a number of mutagens. Many carcinogens and/or mutagens produce oxygen-free radicals for interaction with cellular macromolecules. The anticarcinogenic and antimutagenic potentials of tannins may be related to their antioxidative property, which is important in protecting cellular oxidative damage, including lipid peroxidation. The generation of superoxide radicals was reported to be inhibited by tannins and related compounds. The antimicrobial activities of tannins are well documented. The growth of many fungi, yeasts, bacteria, and viruses was inhibited by tannins. We have also found that tannic acid and propyl gallate, but not gallic acid, were inhibitory to foodborne bacteria, aquatic bacteria, and off-flavor-producing microorganisms. Their antimicrobial properties seemed to be associated with 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 property of tannic acid can also be used in food processing to increase the shelf-life of certain foods, such as catfish fillets. Tannins have also been reported to exert other physiological effects, such as to accelerate blood clotting, reduce blood pressure, decrease the serum lipid level, produce liver necrosis, and modulate immunoresponses. The dosage and kind of tannins are critical to these effects. The aim of this review is to summarize and analyze the vast and sometimes conflicting literature on tannins and to provide as accurately as possible the needed information for assessment of the overall effects of tannins on human health.

Tannins and Human Health: A Review

Proanthocyanidins (syn condensed tannins) are complex flavonoid polymers naturally present in cereals, legume seeds and particularly abundant in some fruits and fruit juices. They share some common structural features—phenolic nature and high molecular weight—with phenolic polymers found in black tea and red wine (called here tannin-like compounds). The polymeric nature of proanthocyanidins makes their analysis and estimation in food difficult. For this reason, little is known about their consumption, although they likely contribute a large part of the daily polyphenol intake. They also share common physicochemical properties: they form stable complexes with metal ions and with proteins and are, like other polyphenols, good reducing agents. Many of their biological effects of nutritional interest derive from these properties. As metal ion chelators, they influence the bioavailability of several minerals. The nutritional significance of the non-specific complexation of proteins is less clear. As reducing agents, they may participate in the prevention of cancers, both of the digestive tract and inner organs. They may also protect LDLs against oxidation and inhibit platelet aggregation and therefore prevent cardiovascular diseases. In vitro, animal and human studies on the prevention of these chronic diseases are reviewed with particular attention to wine and tea polyphenols. The lack of data on their bioavailability and the paucity of human studies are emphasised.

© 2000 Society of Chemical Industry

Proanthocyanidins and tannin-like compounds – nature, occurrence, dietary intake and effects on nutrition and health

Bitter taste, phytonutrients, and the consumer: a review

Potential toxicity of flavonoids and other dietary phenolics: significance for their chemopreventive and anticancer properties.

Vegetable tannins are water-soluble polyphenols that are present in many plant foods. They can be divided into hydrolysable and condensed tannins. Tannins are considered nutritionally undesirable because they precipitate proteins, inhibit digestive enzymes and affect the utilization of vitamins and minerals. Tannin components have also been implicated in the high levels of cheek and oesophageal cancers in certain regions of the world. However, many tannin molecules have been reported to reduce the mutagenicity of a number of mutagens. Tannins were also reported to have anticarcinogenic activity. The growth of fungi, bacteria, and viruses has been inhibited by tannins. Tannins in food plants serve as a natural defence mechanism against microbial infections. Thus, tannins can theoretically serve as natural regulators of the microbial population in different habitats including the human gastrointestinal tract. Tannins have also been reported to produce other physiological effects such as immune response, hepatotoxicity and lipid metabolism. Ingestion of large quantities of tannins may result in adverse health effects. However, the intake of a small quantity of the right kind of tannins may be beneficial to human health.

Are tannins a double-edged sword in biology and health?

Mutagenicity studies of kojic acid

Mutagenicity and toxicity studies of p-phenylenediamine and its derivatives

Immunoassay techniques using the highly specific and sensitive nature of immunological reactions have been developed and applied in the food industry for detecting the naturally occurring constituents, antibiotics, pesticide residues, microorganisms, and fragments of microbial constituents related to food analysis, food production, food processing, and food safety. Both polyclonal and monoclonal antibodies are employed for the development of the various immunoassay systems, including enzyme-linked immunoassay (ELISA) and radioimmunoassay (RIA). Immunoassay techniques provide complementary and/or alternate approaches in reducing the use of costly, sophisticated equipment and analysis time, but still maintaining reliability and improved sensitivity. Immunoassay techniques in their most simple forms provide excellent screening tools to detect adulteration and contaminations qualitatively. The application of immunoassay techniques contributes tremendously to the quality control and safety of our food supply.

C.I. Wei

Papers

Tannins and Human Health: A Review

Are tannins a double-edged sword in biology and health?

Mutagenicity studies of kojic acid

Mutagenicity and toxicity studies of p-phenylenediamine and its derivatives

Application of immunoassay in the food industry