According to Seniewaska and Taj (2017), the
classic definition of tannin is from the use of tannin as an agent of tanning
process of animal skin into leathers. This process permanently alters the protein structure of skin, making it more durable
and less susceptible to decomposition, and also possibly coloring it.

Chemically, tannins are classified into
two types. The first type is hydrolysable tannin (pyrogallol class), with
esters of glucose and acids such as chebulic, ellagic, gallic and m-digallic.
The other type is condensed tannins (catechol tannins), based on
leuco-anthocyanidin and like substances (Ethuk, Okeudo, Esonu and Udedibie,
2012).

Tannin is described as a class of
antioxidant polyphenols that may impair the digestion of various nutrients. As
mentioned by Chung, Wei and Johnson (1998), tannins are considered
nutritionally undesirable because they precipitate proteins. This tannin
activity also be stated in which have done a study on betel nuts. The
predominant tannins in betel nuts is gallotannic acid, which is present in the
outer part of the nut. In addition, minor amounts of gallic acid, D-catechol
and phiobatannin are also present in the inner part of the nut. These tannins
are capable of precipitating the proteins.

Besides, tannins also inhibit digestive
enzymes and affect the utilization of vitamins and minerals (Chung, Wei and
Johnson, 1998). According to War et al. (2012), tannins also precipitate the
proteins such as the digestive enzymes of herbivores hence this fact supports
the 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 concentration
of tannin in dietary intake will also depress the activity of microbial enzyme
such as cellulose. It also can leads to the despair of intestinal digestion
(Gemede and Ratta, 2014).

Furthermore, tannins also reported to reduce
the digestion of protein in animals. As stated by War et al. (2012), tannins
precipitate proteins nonspecifically, by hydrogen bonding or covalent bonding
of protein-NH2 groups. When ingested, tannins reduce the digestibility of the
proteins thereby decrease the nutritive value of plants and plant parts to
herbivores. Gemede and Ratta (2014) also agreed that tannins decrease the
protein quality of foods.

Besides, tannins also reported to
interfere the dietary iron absorption. Hydrolysable
tannin act as metal ion chelators considerably inhibiting non-heme iron
absorption. The examples of the non-heme iron sources are plant foods and iron
supplements. Animal sources of iron are not affected by the hydrolysable tannin
(Gemede and Ratta, 2014). War et
al. (2012) also stated the same idea on the relation of iron absorption and
tannins in which they reported that the tannins’ mechanism of chelating the
metal ions. This mechanism leads to the reduction of iron bioavailability to
herbivores. Tadele (2015) also supported that
tannin interferes the dietary iron absorption.

Apart
from being an anti-nutritional for certain nutrients and enzymes, the tannins
also have the carcinogenic effect on health. In accordance to Chung, Wei and
Johnson (1998), carcinogenic effect of tannin is portrayed by Far East people
who like to chew betel nuts after dinner time. These nuts which are
tannin-containing foods is believed to be the one that are responsible for the
high incidence of cheek and esophageal cancers among the Far East population. Accoding
to Sharan, Mehrotra, Choudhury and Asotra (2012), the prolonged use of betel
nuts is stated to be associated with the increased risk for the development of
oral malignancy such as oral squamous cell carcinoma. Furthermore, Sharan et al.
(2012) also highlighted that betel nuts causes the oral leukoplakia and oral
submucous fibrosis. The leukoplakia is the definition of visible whitish so
this condition is characterized by predominantly white patch or plaque on the
oral mucosa.  On the other hand, oral submucous fibrosis
happen to the oral cavity, oro- and hypopharynx and the upper third of the
oesophagus.

Besides,
another story regarding the Caribbean natives who are heavy consumers of herb
tea and sorghum is mentioned by Chung, Wei and Johnson (1998). These two
tannin-rich food items are thought to contribute to the high incidence of esophageal
cancer in Carribean islanders. According to Chung, Wei and Johnson (1998), the
underlying reason why tannins are claimed to be the culprit of these issues is
the idea of their ability to cause irritation and cellular damage rather than
the action on DNA mutation. However, tannins are concluded by Chung, Wei and
Johnson (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 grazing
animals. A study was done on mice by using tannic acid. An injection of this
acid to the mice as the procedure to see any reaction to the liver cell of the
subjects. 700 milligram of tannic acid per kilogram body weight of the mouse was
injected subcutaneously to each mouse and the results were recorded. The study
reported a significant breakdown of polyribosomes in mouse liver and inhibited
the incorporation of amino acids into hepatic proteins. Tannins were shown to
bind epithelial proteins and cause precipitation. They then penetrated through
the superficial cells and induced liver damage (Chung, Wei and Johnson, 1998).

Although
high tannins content is proved to amplify many adverse effects to health, the
fact that tannins also give benefits cannot be denied. Despite from being a
carcinogenic agents, tannins are reported to have the anticarcinogenic
activity. From Chung, Wei and Johnson (1998), the inhibitory effect of green
tea on cancer formation has been stated to be well documented. Moreover, tannins
also reported to have the antimicrobial activity as tannins can inhibit
filamentous fungi such as Aspergillus niger, Botrytis cinerea, Chaetomium
cupreum, 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-borne
bacteria. 

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