One of the main ingredients in red wine, which gives it colour, body and texture, is a group of natural polymers called polyphenols. These molecules, mainly tannins and anthocyanins, are released from grape skins during fermentation.
Upon further ageing, the tannins evolve into ever more complex structures until the wine is eventually drunk.
Tannins are responsible for a wine’s bitter taste and astringent feel to the tongue (the viscosity of saliva drops suddenly as the tannins precipitate out the saliva proteins). The deep-red and violet anthocyanins endow red wine with its colour. In very young wines, the tannins tend to be sharp-tasting. But as the wine ages, the tannins change and the wine tastes more velvety and smooth, or fine-grained and supple. These are all terms that represent different sensations of astringency and are related to the complex structure of the tannins.
One of the important processes in the ageing of a red wine is that some tannin molecules can aggregate to create a colloidal dispersion of nano-sized particles. The amount, the size and the shape of these particles depend on the chemical composition of the tanninpolymer structures, and this changes as the wine ages.
Our aim was to explore this colloidal state and its effect on the quality of a red wine. Small Angle Neutron Scattering (SANS) offered a suitable technique. We first created a model ‘wine’ using tannins extracted from the seeds of Languedoc-Roussillon grenache grapes. Our tannin fraction had an average degree of polymerisation of 11 units, so we called it DP11. The wine was a fairly awful-tasting solution of tartaric acid in water, with 12 per cent alcohol and 5 grammes per litre of the DP11 tannin. To increase the contrast of the tannin structures we used deuterated water and alcohol.

Shape of tannin structures

We were able to identify both the polymeric tannins and nanoparticles using SANS. What was significant, however, was that the size and internal structure of the tannin particles depended sensitively on how the sample was prepared. We made two samples with the same chemical composition, but mixed in a different way.
Tannins are completely soluble in alcohol and form particles only when water is present. In the first sample, an alcoholic tannin solution was added to the acidic water to give a highly supersaturated solution which then formed small aggregates. These gradually grew into the metastable colloidal state.

The SANS spectrum revealed that the nanoparticles were about 600 nanometres across and had a branched, bushy structure. In the second sample,we gradually added small quantities of acid water to the tannin-alcohol solution, following the SANS spectrum after each addition. The spectrum revealed only small DP11 tannin polymers down to a level of 68 per cent of alcohol, when the colloidal state suddenly appeared. In this case, only the largest particles grew, with the smaller ones re-dissolving, so aggregation did not happen.

These results show that in making wine, the architecture of the tannin particles and the manner in which the tannins are released from the grapes are just as important as the overall chemical composition.

D. Zanchi et al., ILL brochure "Neutrons and soft matter", p.7

<< Back