Tannin is present in many types of plant material. Natural tannin is mostly made up of the chemical compound gallotannin. It has a complex molecule with the basic structure of 1 glucose molecule bonded to as many as 5 molecules of digallic acid. In water solution, the tannin molecule hydrolizes to give glucose and digallic acid.

The carboxyl hydrogen in the digallic acid molecule is the source of the acidic properties of tannic acid. The substance is in effect a monoprotic acid, inasmuch as the OH groups attached to the benzene rings do not readily yield protons.

The anion that results from the dissociation of tannic acid is

C6H2 (OH) 3COOC6H2 (OH) 2COO-1

This anion can combine with ferrous iron to form complexes of the type AFe+1 or A2Fe. Tannic acid complexes with ferric iron may consist of large highly colored molecules that behave as colloids. The well-known black material produced by mixing tannic acid and ferric iron solutions is a ferric complex.

Solutions of tannic (digallic) acid at concentrations of 5 or 50 parts per million reduce dissolved ferric iron to the ferrous state when the pH is less than 4. In solutions with a pH of 4 or more, a black material containing ferric iron and tannic acid is precipitated. In a solution containing 500 parts per million of tannic acid, a ferrous complex forms at a pH of more than 5. This complex is oxidized at a slow rate, and some ferrous iron remains in solution after a month of storage in contact with air. Uncomplexed ferrous iron is oxidized and precipitated from solution in a few hours or less at pH levels of more than 5. Concentrations of tannic acid that markedly affect oxidation of ferrous iron are probably higher than the concentrations usually found in surface or ground water.

for detail you read following article

Hyperfine Interactions (Impact Factor: 0.21). 01/2001; 134(1):109-114. DOI: 10.1023/A:1013838600599

Tea inhibits iron absorption in studies in which tea is given with radiolabeled iron to humans as a single dose. Our objective was to test the hypothesis that proline-rich proteins (PRPs) may act as a defense against this effect by forming complexes with tannins, thereby preventing them from inhibiting iron absorption. Two studies were conducted. In study 1, rats were given test solutions containing 59FeCl3 in water, tea, or tea + gelatin (T/G). In study 2, the rats were divided into 3 groups and assigned to one of 3 nutritionally complete diets: control, tea (5 g tea tannin/kg diet), or T/G (5 g tea tannin + 60 g gelatin/kg diet). Rats were fed the respective diets for 5 d and then given a single 59Fe-labeled meal of the diet. Iron absorption was measured by whole-body retention of the 59Fe over a 2-wk period. Iron absorption in study 1 was lower in the tea group (24 ± 9.6%, P < 0.05) than in the T/G (42 ± 19.4%) or water groups (50 ± 7.5%). In study 2, iron absorption did not differ among the groups. Rats fed the tea diet had dramatic hypertrophy of the parotid salivary glands. Adding gelatin as a proxy for salivary PRPs to the tea eliminated the inhibitory effect of tea on iron absorption. The results suggest that PRPs, whether from salivary glands or diet, can protect against the inhibition of iron absorption by tea.

online - http://jn.nutrition.org/content/135/3/532.full

http://www.krepublishers.com/02-Journals/JHE/JHE-18-0-000-000-2005-Web/JHE-18-1-000-000-2005-Abst-PDF/JHE-18-1-013-020-2005-1251-Das-P/JHE-18-1-013-020-2005-1251-Das-P-Full-Text.pdf