Tannins (polyphenols, catechins).
General information, chemical structure
Tannins were originally used to tan animal hides and in leather manufacturing processes. They are water-soluble polyphenols.
A distinction is made between hydrolysable tannins (gallotannins and ellagitannins) and condensed catechin polymers, which can only be cleaved to a very limited extent by hydrolysis, if at all.
Hydrolysable tannins consist of sugars or sugar alcohols that have been estified with gallic acids. The condensed tannins are based on catechin, a flavan-3-ol and its isomers. Catechin is related to the flavonoids in biogenetic terms. Other tannins are derived from phloroglucinol (brown algae) and caffeic acid units. The parent substances (monomers) do not demonstrate any tannin characteristics. These are only demonstrated by the oligomers or more highly condensed compounds. High-polymer compounds are insoluble in water and therefore no longer demonstrate any tanning effect (phlobaphenes, tannin reds).
There are various intermediate forms between the gallotannins and the catechin tannins, such as the catechin gallates and epicatechin gallates found in black and green tea.
Chlorogenic acid, rosmarinic acid (lamiaceac tannin) are also tannins in a broader sense of the word, as are condensed phloroglucinol ethers (phlorotannins, algal tannins), which have little tanning effect.
Characteristics, effects
Tannins are soluble in hot water, methanol and ethanol. They form slightly soluble complexes with proteins, heavy metals and alkaloids. Like all polyphenols, they are sensitive to oxidation; air oxidation, enzymatic condensation and acid condensation cause the colourless compounds to polymerise and become darkly coloured, physiologically inert compounds.
The tanning effect of tannins is caused by an interaction with the collagen fibres in the hide. Only molecules with a molecular weight between 500 and 3000 have tanning properties. The phenols react with the acid amide groups of the collagen to form covalent bonds (leather). The term astringent is used, on the other hand, if less stable hydrogen bridges and ionic bonds are formed.
Tannins and polyphenols demonstrate a multitude of pharmacological effects, not least because of their antioxidative and adsorptive properties:
- antisecretive and antiperistaltic effects in the treatment of diarrhoea,
- antibacterial, antiviral, anticariogenic,
- antiphlogistic, antiexudative and antiallergic by inhibiting hyaluronidase / intervening in the arachidonic acid metabolism,
- antioxidative, like many other polyphenols. A chemopreventive effect is ascribed to them in the treatment of ischaemia, cancer, arterial sclerosis and rheumatic arthritis because of their radical scavenging properties.
Analysis
Total polyphenol determination procedures make use of the compound class's oxidation capability as a rule. An aqueous extract of the analysed material is transformed in the alkaline medium with phenol reagent (Folin & Ciocalteu, molybdate-tungsten reagent) for this; the absorption of the solution is determined photometrically and is calculated as a defined reference substance (gallic acid, pyrogallol). (Examples: black tea and green tea in accordance with ISO/DIS 14502-1; vine leaves in accordance with Ph. Franc. X.)
Apart from the total determination procedure described above, tannin determination (e.g. Ph. Eur. 2.8.14; German Pharmaceutical Codex (DAC), Ph. Helv.) also involves adding hide powder to a fraction of the extract, that is binding the tannins. The polyphenols remaining in the solution undergo photometric determination analogously and the tannin content is calculated from the difference between the total phenol content and the content of phenols, which is not adsorbed by hide powder (e.g. hamamelitannins in hamamelis leaves, tannins in oak bark and rathania roots). Earlier pharmacopoeias describe the gravimetric determination of the tannins that can be adsorbed by hide powder. The Ph. Eur. also outlines a procedure for the photometric determination of hydroxycinnamic acid derivatives, calculated as rosmarinic acid in melissa leaves.
Today, the low-molecular polyphenols are usually determined selectively by means of HPLC. This means that a number of different methods are now available to determine chlorogenic acid and rosmarinic acid in diverse matrices. Products made from black tea, green tea and grape seed have gained considerable significance in the market for food supplements and lifestyle products. They include products for which the quality can be assured by means of HPLC determination of catechin, epicatechin, epicatechin gallate, epigallocatechin and epigallocatechin gallate in accordance with ISO TC34/SC8, ISO/WDCD 14502-2 and INA 122.001.
All of these determination procedures can be performed properly and professionally by PhytoLab, the specialist laboratory. PhytoLab is Europe's leading external laboratory in the field of analysing plant constituents, such as the polyphenols and tannins described above. PhytoLab's scientists also specialise in registration and market authorisation procedures for herbal medicinal products. Officially acknowledged in accordance with § 14 of the German Drug Law (AMG) and accredited in accordance with Directive 93/99/EEC, PhytoLab specialises in the aforementioned analytical procedures with state-of-the-art equipment and highly qualified personnel.
