According to Seniewaska and Taj (2017), theclassic definition of tannin is from the use of tannin as an agent of tanningprocess of animal skin into leathers. This process permanently alters the protein structure of skin, making it more durableand less susceptible to decomposition, and also possibly coloring it. Chemically, tannins are classified intotwo types.

The first type is hydrolysable tannin (pyrogallol class), withesters of glucose and acids such as chebulic, ellagic, gallic and m-digallic.The other type is condensed tannins (catechol tannins), based onleuco-anthocyanidin and like substances (Ethuk, Okeudo, Esonu and Udedibie,2012).Tannin is described as a class ofantioxidant polyphenols that may impair the digestion of various nutrients. Asmentioned by Chung, Wei and Johnson (1998), tannins are considerednutritionally undesirable because they precipitate proteins. This tanninactivity also be stated in which have done a study on betel nuts. Thepredominant tannins in betel nuts is gallotannic acid, which is present in theouter part of the nut. In addition, minor amounts of gallic acid, D-catecholand phiobatannin are also present in the inner part of the nut. These tanninsare capable of precipitating the proteins.

Besides, tannins also inhibit digestiveenzymes and affect the utilization of vitamins and minerals (Chung, Wei andJohnson, 1998). According to War et al. (2012), tannins also precipitate theproteins such as the digestive enzymes of herbivores hence this fact supportsthe report from Chung, Wei and Johnson in their 1998 study. Gemede and Ratta(2014) also stated that tannins are known to inhibit the activities of trypsin,chemotrypsin, amylase and lipase. Apart from these enzymes, high concentrationof tannin in dietary intake will also depress the activity of microbial enzymesuch as cellulose. It also can leads to the despair of intestinal digestion(Gemede and Ratta, 2014).

Furthermore, tannins also reported to reducethe digestion of protein in animals. As stated by War et al. (2012), tanninsprecipitate proteins nonspecifically, by hydrogen bonding or covalent bondingof protein-NH2 groups. When ingested, tannins reduce the digestibility of theproteins thereby decrease the nutritive value of plants and plant parts toherbivores. Gemede and Ratta (2014) also agreed that tannins decrease theprotein quality of foods.Besides, tannins also reported tointerfere the dietary iron absorption. Hydrolysabletannin act as metal ion chelators considerably inhibiting non-heme ironabsorption. The examples of the non-heme iron sources are plant foods and ironsupplements.

Animal sources of iron are not affected by the hydrolysable tannin(Gemede and Ratta, 2014). War etal. (2012) also stated the same idea on the relation of iron absorption andtannins in which they reported that the tannins’ mechanism of chelating themetal ions. This mechanism leads to the reduction of iron bioavailability toherbivores. Tadele (2015) also supported thattannin interferes the dietary iron absorption.

Apartfrom being an anti-nutritional for certain nutrients and enzymes, the tanninsalso have the carcinogenic effect on health. In accordance to Chung, Wei andJohnson (1998), carcinogenic effect of tannin is portrayed by Far East peoplewho like to chew betel nuts after dinner time. These nuts which aretannin-containing foods is believed to be the one that are responsible for thehigh incidence of cheek and esophageal cancers among the Far East population. Accodingto Sharan, Mehrotra, Choudhury and Asotra (2012), the prolonged use of betelnuts is stated to be associated with the increased risk for the development oforal malignancy such as oral squamous cell carcinoma.

Furthermore, Sharan et al.(2012) also highlighted that betel nuts causes the oral leukoplakia and oralsubmucous fibrosis. The leukoplakia is the definition of visible whitish sothis condition is characterized by predominantly white patch or plaque on theoral mucosa.  On the other hand, oral submucous fibrosishappen to the oral cavity, oro- and hypopharynx and the upper third of theoesophagus.Besides,another story regarding the Caribbean natives who are heavy consumers of herbtea and sorghum is mentioned by Chung, Wei and Johnson (1998). These twotannin-rich food items are thought to contribute to the high incidence of esophagealcancer in Carribean islanders.

According to Chung, Wei and Johnson (1998), theunderlying reason why tannins are claimed to be the culprit of these issues isthe idea of their ability to cause irritation and cellular damage rather thanthe action on DNA mutation. However, tannins are concluded by Chung, Wei andJohnson (1998) to be carcinogenic only in the presence of other carcinogens.Therefore, tannins are perceived to be the promoters in inducing skin carcinogenesis.

Besides,tannin also has the ability to produce hepatic necrosis in humans and grazinganimals. A study was done on mice by using tannic acid. An injection of thisacid to the mice as the procedure to see any reaction to the liver cell of thesubjects.

700 milligram of tannic acid per kilogram body weight of the mouse wasinjected subcutaneously to each mouse and the results were recorded. The studyreported a significant breakdown of polyribosomes in mouse liver and inhibitedthe incorporation of amino acids into hepatic proteins. Tannins were shown tobind epithelial proteins and cause precipitation. They then penetrated throughthe superficial cells and induced liver damage (Chung, Wei and Johnson, 1998).Althoughhigh tannins content is proved to amplify many adverse effects to health, thefact that tannins also give benefits cannot be denied. Despite from being acarcinogenic agents, tannins are reported to have the anticarcinogenicactivity. From Chung, Wei and Johnson (1998), the inhibitory effect of greentea on cancer formation has been stated to be well documented.

Moreover, tanninsalso reported to have the antimicrobial activity as tannins can inhibitfilamentous fungi such as Aspergillus niger, Botrytis cinerea, Chaetomiumcupreum, Colletotrichum graminicola, Coniophora olivacea, Coriolus versicolor,Crinipellis perniciosa, Fomes annosus, Gloeophyllum trabeum, Meruliuslacrymans,Penicillium species, Poria monticola, Trametes hirsuta and Trichoderma viride.In addition, various yeasts are proved to be sensitive to tannins. Furthermore,tannic acid and propyl gallate are shown to inhibit the growth of food-bornebacteria. 


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