Black Tea - an "Iron Robber"

Demonstration Experiment on Video

Black Tea - an "Iron Robber"

Objective: Polyphenol-Iron Complex

Peter Keusch

German version

Supermarket products:
Ceylon India Black Tea
steel wool for cleaning

Apparatus and glass wares:
2 beakers 100 mL
beaker 250 mL
pair of tweezers
spatula spoon
hotplate

Experimental procedure:

A tea bag is placed in 250 ml of hot water. The tea extraction is allowed to steep for about three minutes. Afterwards the solution is diluted until it has a light yellow color. The diluted extraction solution is divided equally into two 100 mL beakers. A wad of steel wool is placed into one of the two beakers. The two solutions are heated to boiling for about 15 minutes. Then the steel wool is removed from the relevant beaker.

Result:

Already during heating the color of the extraction solution containing the steel wool changes. The solution becomes noticeably darker. Finally the color turns to blue-black.

Video clip (Download RealPlayer .rm file)

Discussion and back ground:

Polyphenols of the tea react with Fe³⁺ ions to produce a blue-black colored polyphenol-iron complex. The three phenolic oxygen atoms in the gallate moiety seem to be available for the coordinations with the metal ion.

A steady supply of Fe³⁺ ions is needed to form hemoglobin and healthy red blood cells. Ascorbic acid reduces Fe³⁺ taken into body to form Fe²⁺. Iron can only be resorbed in the bivalent form. Human studies have shown that drinking tea during meals sinificantly inhibits the resorption of both food iron and medicinal iron. This study provides evidence from experiments with rats that the polyphenols in the tea are responsible for the inhibition, probably by forming non-absorbable complexes with the iron within the intestinal lumen. Polyphenols act as a natural iron chelating agent. Therefore the best time for drinking tea is two hours after a meal.

The catechins present in tea are commonly called polyphenols. They are flavanol derivatives.

Green and black teas are processed differently during manufacturing. Fresh green tea leaves, which are very rich in catechins, are not fermented; they are withered, and catechin oxidation by polyphenol oxidase is prevented by steaming (Japan) or by panning (China), processes that essentially maintain the polyphenols in their structures. The major catechins found in green tea are epicatechin, epigallocatechin, epicatechin-3-gallate, and epigallocatechin-3-gallate. Black tea leaves are subjected to crushing and a full fermenting process. During the fermentation the monomeric flavanols undergo polyphenol oxidase-dependent oxidative polymerization leading to the formation of thearubigin and theaflavin. These compounds give black tea its characteristic color, aroma and taste and may have certain heart protective and anti-cancer properties. Catechins and other polyphenols have antioxidant activities. These wonder nutrients scavenge for cell-damaging free radicals in the body and detoxify them.

Structure of the polyphenols present in tea

Index of Lecture Experiments