TANNINS, PRESENTATION AND TYPOLOGIES
WHAT ARE THEY ?
Tannins are natural substances with a clear antioxidant activity and a capacity to capture free radicals. Regarding these health aspects, the "French Paradox"
thesis is noteworthy, according to which there are fewer cardiopathies in communities that regularly use red wine - well-known to be rich in tannins - in their daily diet.
The generic term "tannin" includes substances having common characteristics like: astringent taste, reacting with proteins and yielding a blue-black colour together with iron.
The astringent taste is due to the tannins reacting with the proteins of saliva. This characteristic is important in oenology for inhibiting enzymes (denaturation of their protein fraction and their insolubility) and for contributing to the protein stability in wines.
There are preparations that often differ from one to another in their constitution, which depends on the raw material from which they are extracted, the extraction technique and how they are desiccated.
Oxidation can interfere during the preparation phase, which could cause a rapid degradation of the product. It seems evident that it is not sufficient to know only its botanical origin to determine the action of a tannin ; it is necessary to know its extraction/desiccation methods and to consider its reactivity case by case. For winemakers, it is crucial to test the tannin for the sensory modifications that will result when it is added to the must or wine.
TYPOLOGIES AND PRESENCE IN THE PLAN KINGDOM
There are hydrolyzable tannins (ellagic and gallic) and condensed tannins (catechinic).
Traditionnally, oenological tannins are extracted from gallnuts, which are the result of pathological excrescences that develop on the trunk of certain trees under the influence of insect stings (gallic tannins). This type if tannin is also found in grape stems, seeds ans skins. The most common form, as determined by the polymer structure, seems to be pentadigalloylglucose (gallic acid esterifies the five free hydroxyls of the glucose).
Ellagic tannins are not present in grapes, but they are constituents of the wood. European oak contains four monomers and four dimer ellagitannins. The american species does not contain the dimers.
The condensed tannins are polymers: the basic units are the (+)catechin and the (-)epicatechin. They are found in grapes and in some exotic wood.
In the past, tannins were used mainly as clarifiers, together with protein clarifiers (gelatin, albumin, caseinates,..). Nowadays, a wide range of tannins is available with different technological aims:
- for colour stabilization
- as a catalyst for redox reactions (including the prevention of the so-called "taste of light")
- for elimination of thiol compounds
- as an antioxidant
- for an anti-oxidative effect (it reacts with the part of the oxidative enzymes)
- to reach protein stability
- for their effect on sensory characteristics
- for structuration of wines
- to prevent reduction
- indirectly with SO2 for a bacteriostatic effect
- for clarifications