Tanning agents are chemical substances that are used to tan animal hides. A peeled animal skin is converted into leather by the treatment with tanning agents , which z. B. prevents putrefaction . There are naturally occurring (natural) tannins and artificially produced (synthetic) tannins.
Tanning agents are parts of plants or mixtures of substances that contain one or more tanning agents. However, the terms tanning agent and tanning agent are often used as synonyms .
A tanning extract is an extract of a tanning agent. After the extraction, the tannin is present in a higher concentration, without any interfering components of the starting material. Tanning extracts are themselves tanning agents.
Vegetable tannins (so-called tannins ) are also used in medicine , and diverse beneficial effects have been proven. In addition, tannins are known as a flavor component of wine and tea .
Tannins are often found in plants (see also useful plants ), for example in leaves , woods , bark , fruits and roots of chestnuts , bananas , oaks , spruces , mimosas , quebracho , tea and coffee . Plant breakdown products such as peat also contain tannins. Vegetable tannins are also known as vegetable tannins or tannins .
Due to their chemical structure, vegetable tanning agents can be divided into two groups:
- Hydrolyzable tannins, e.g. B. Gallotannins. The basic building blocks are tannic acids, e.g. B. gallic or ellagic acid in connection with glucose.
- Condensed tannins, e.g. B. Pyrocatechins. The basic building blocks are aromatic polyhydroxy compounds, e.g. B. catechin .
The chemical detection of tannins is possible with the vanillin-HCl reaction (red color).
In addition to the vegetable tanning agents, the natural tanning agents also include the cross- linking fatty tanning agents in the form of polyunsaturated fats , e.g. B. Trane . Trane are oils obtained from fish or marine mammals . For a tanning effect they must contain sufficient polyunsaturated fatty acids . Fats with only monounsaturated fatty acids have no tanning effect. The oxidation of these Trane produces reactive degradation products with a tanning effect, which lead to cross-linking of the collagen.
Other natural products such as urine, egg yolk, flour, various animal innards such as brain or liver show a certain preservative effect when used appropriately, but they cannot achieve a real tanning effect.
Artificial tanning agents
Mineral tanning agents. The main representatives are:
- Aluminum salts, especially in the form of alums (alum salts also occur partly as natural minerals, but are mostly artificially produced)
- trivalent chromium salts (the poisonous hexavalent chromium compounds have no tanning effect)
- Zirconium salts
- Some other metal salts such as B. iron, zinc, titanium and even gold salts are known to have a tanning effect.
- Aldehydes . The tanning effect of aldehydes has long been known. Widely used examples are formaldehyde and glutaraldehyde , which are not only used for leather production, but also for fixation in histology and taxidermy and as germicidal disinfectants .
- Synthetic tanning agents (syntans) based on phenol derivatives. They were developed at the beginning of the 20th century and are based on the vegetable tanning agents in a simplified form. Ferric chloride or the Emerson reaction can be used to test for phenols .
- Polymer tanning agents ( resin tanning agents ). The basic components of polymer tanning agents include acrylates and polyurethanes . Aldehydes, isocyanates , isothiocyanates and succinimide esters are used as reactive groups .
- Aliphatic paraffin sulfochlorides
There are also a number of man-made organic compounds that have some tanning effect, e.g. B. dyes or preservatives .
Effect of tannins
Tannins combine with proteins on contact , which changes their properties:
- The water bound in the proteins is displaced by the tannins.
- From now on, the proteins cannot be broken down by microorganisms or only with great difficulty.
- The swelling capacity in water, acids and alkalis is greatly reduced.
- The temperature resistance is increased.
- Biologically active proteins are denatured by tannins and are no longer biologically active.
Structured proteins, e.g. B. the collagen of the skin, keep their natural structure. However, the structures are more or less strongly cross-linked by the tannins. Unstructured proteins, e.g. B. proteins from a chicken egg are precipitated.
Tannins in medicine
The group of tannins occupies an important place among the therapeutically active ingredients from medicinal plants one (for further plant active ingredient groups see herbal medicine ). The tannin content in drugs can be determined photometrically , iodometrically or using the so-called hide powder method .
Plants or parts of plants that contain medically effective tannins
- Gall apples ( Gallae ) from Quercus infectoria
- Witch hazel leaves ( Hamamelidis folium ) from Hamamelis virginiana = witch hazel
- Walnut leaves ( Juglandis folium ) from Juglans regia
- Oak bark ( Quercus cortex ) from Quercus robur and Quercus
- Ratanhia root ( Ratanhiae radix ) from Krameria lappacea
- Bloodroot root ( Tormentillae rhizoma ) of erecta Potentilla
- Blueberries ( Myrtilli fructus ) from Vaccinium myrtillus
Folk medicine uses also:
- Blackberry leaves from Rubus fruticosus
- Fingerweed from Potentilla anserina
- Strawberry leaves of Fragaria vesca
- Scotswort Agrimonia eupatoria
- Lady's mantle herb from Alchemilla xanthochlora
- Plantain leaves from Plantago major
- Plantain leaves from Plantago lanceolata
- Rose petals from Rosa gallica
- Button leaves from Sanguisorba officinalis
Tannins can also condense living tissue on the surface and form a protective membrane, e.g. B. on a mucous membrane . Due to their contracting ( astringent ) and drying effect, they remove the breeding ground for bacteria that have settled on the skin or a mucous membrane. The penetration of bacteria and fungi into the tissue is made more difficult ( antimicrobial effect). Pain and wound secretion are reduced, inflammation is inhibited and capillary bleeding is stopped.
Tannins are used for stomach and intestinal inflammation, slight diarrhea (constipating effect), inflammation in the mouth and throat, as a hemostatic agent, for rapid wound healing and for minor burns and frost damage. They also act as an antidote to heavy metal or alkaloid poisoning, as they can dissolve heavy metal ions and alkaloids from their compounds.
Tannins also have a "medicinal" effect in living plants by protecting the plant from rot .
With long-term use, tannins can be hepatotoxic (damage to the liver). If the doses are too high, it can lead to inflammation of the stomach lining or nausea.
Tannins reduce the absorption of basic drugs and some minerals such as iron .
Other uses of tanning agents
Tannins are also the basis for the synthesis of resins , cork , anthocyanins and flavonoids .
Some tannins form very stable color complexes with metal salts, mainly iron salts, which were previously used for the production of writing inks .
Tannins in wine
Grapes contain tannins as components of stems, seeds and berry skins that contribute to the taste of the wine , especially with red wine . The tannin content is thus a quality factor of the wine, provided it is in a balanced relationship to the other taste components (acidity, residual sugar) and aromas.
- ^ Duden online: Tannin
- ↑ Duden online: Gerbmittel
- ↑ Duden online: Gerbextrakt
- ↑ WebWalking am Kiischpelt: Die Lohe in Handwerk, Industrie und Medizin , accessed on June 3, 2013.
- Otto Th. Schmidt, Walter Mayer: Natural tannins . In: Angewandte Chemie . tape 68 , no. 3 , 1956, pp. 103-115 , doi : 10.1002 / anie.19560680305 .
- H. Wagner: Medicinal drugs and their ingredients . Wissenschaftliche Verlagsgesellschaft., Stuttgart 1999, ISBN 3-8047-1605-9 .