For plants this is a defence mechanism against predators, i.e. animals that eat them, since, as well as the astringency, which can be dissuasive for some animals, tannins can also combine with enzymes involved in digestion, and as a consequence render them inactive and thus affect digestion.
Certain herbivores have developed, through the course of evolution, protective mechanisms, notably the production of salivary proteins that can neutralise tannins. These proteins contain a high proportion of a particular amino acid2 called proline. Research scientists have noticed an interesting fact: the production of salivary proteins rich in proline increases in guinea pigs submitted to a diet rich in tannins. In the absence of such an adaptation, the guinea pigs can die.
| The strong affinity between proline-rich proteins and tannins is central to research being done by one of the groups at UMR 1083 Oenology Science at Montpellier. Tannins in grapes contribute to the organoleptic3 qualities of wine, notably the astringency that they will give it. This astringency can be more or less pleasant to the wine-taster, depending on the quantities of the different tannins present in the wine. Wine growers would therefore like to reduce the number of “bad tannins” and replace them with “good” ones to improve the flavour of their wines. Given that everything depends on a few percentage points, procedures that cause tannins to precipitate can have a big impact on the organoleptic qualities of a wine, even if the quantities of tannins eliminated are low. This is the aim, for instance, of “collage”: due to additives such as gelatine, the organoleptic qualities of wine can be modified, in addition to being clarified. This balance of good/bad tannins depends on the ”tannin maturity” of the grapes before picking. Certain wine growers can judge this just by tasting the grapes, but it would be better if this very imprecise and empirical method could be standardised.
It is the long-term aim of experiments carried out by Aude Vernhet and Francis Canon on two of the SOLEIL beamlines, SWING and DISCO, to establish links between the form and structure of tannin and the astringency it provokes when combined with salivary proteins, so as to be able to differentiate between “good” and “bad” tannins. | 
Diagram of a dimer (2 molecules) of the tannin epicatechin |
Tannins are often flexible molecules that are difficult to study using methods such as crystallography and X-ray diffraction. Salivary proteins are also flexible and therefore not easily studied either using “classical” structural methods. There is therefore little information available on the mechanisms whereby salivary protein/tannin complexes are formed.
Scientists have thus turned to two other techniques available at SOLEIL: small-angle X-ray scattering (SWING beamline) and mass spectrometry (DISCO beamline).
Small-angle X-ray scattering provides information on the structure of large molecules or molecular complexes, even when they are not in the form of crystals but in solution.
On SWING, Aude Vernhet has studied, in succession, different tannins dissolved in ethanol. The aim was to follow the modifications of structure and conformation of these molecules during their oxidation. This oxidation can, for example, lead to their polymerisation4. Oxidation reactions such as these take place during transformations that accompany the manufacture of consumables containing tannin (e.g. wine, or other fruit-based foodstuffs). New molecules formed during these reactions have, a priori different physical properties to the “original” tannins, so possibly having an influence on the gustatory qualities of the product. If these processes are better known, this may help in controlling the organoleptic properties of the foodstuff under study.
On DISCO, Francis Canon, an INRA PhD student, came to prepare model mixtures to mimic wine: 12° ethanol solution containing tannin (several different tannins were successively tested). To these wine models were added an aqueous solution of human salivary protein so that tannin-protein complexes could form, as in the mouth. Small samples of these solutions were taken and injected in the ion source of the mass spectrometer.
Mass spectrometry can be compared to extremely sensitive “molecular scales”. When a tannin combines with a salivary protein, the mass of the complex is greater than for the protein alone. This difference, although extremely small (certain tannins have very small molecular weights), can be detected by the mass spectrometer: these are found as peaks at different positions between the spectrum of the protein alone and that of the protein + tannin. One therefore gets information on the stoichiometry5 of the complex.
In addition, the affinity between the salivary protein and different tannins has been studied by means of two different experiments. One approach is to put tannins in competition with salivary protein (the “simplified wine” contains at that point several tannins). The idea is as follows: the stronger the affinity shown by a protein for a tannin, the more abundantly the complex it forms with it will be found in solution. In addition, the peak corresponding to this complex will be high. By comparing the peaks of the different protein-tannin complexes, the tannins can be classified in an increasing order of affinity for the salivary protein.
In the second approach, only one tannin is studied at a time. The experiment consists of fragmenting the protein-tannin complexes formed by collisions with gas atoms. The following are measured in parallel: the energy given by the “atom fragmentors” and the amount of complex that is broken up. In addition, energy has to be provided to break up the complex and the more stable the latter, the greater the tannin’s affinity to the salivary protein under study. By studying several tannins in succession, one can obtain a tannin/salivary protein relative affinity scale.
These affinity scales are a first step in unravelling the still little known complexation mechanisms.
However, the flavour of wine will, no doubt, still retain much of its mystery for a long time to come.
1Precipitation: in chemistry, describes the phenomenon whereby a solid separates from a liquid in which it had dissolved. A “precipitate” thus forms as an insoluble deposit.
2Amino acids: the “elementary building” bricks of proteins. There are twenty different amino acids, found in the composition of proteins throughout the living world.
3Organoleptic: affecting the sense organs. The organoleptic qualities of food: taste, odour, colour, consistency…
4Polymerisation: the assembly of several molecules to form a larger molecule. For example, proteins are polymers of amino acids.
5Stoichiometry: calculation of the respective quantities of reagents and products in a chemical reaction. Here it is the number of molecules of a given tannin combining with one molecule of the salivary protein under study.
