Tannins

The term ''tannin'' is commonly used in wine circles, but many people aren't really sure exactly what it means.

Why is the subject of tannins an important one for the wine trade at large, and not just winemakers and anoraks? First, suspect that whatever your involvement in the trade, you’ll be familiar with the term ''tannin'' and it’s a word that you’ll have used frequently, perhaps, without a clear idea of what you are referring to. Second, it’s a field of active current research, and data that are only now just accumulating are pointing towards a very different understanding of the role of tannins in red wines than that traditionally espoused by wine textbooks. 

Introducing tannins

The term ''tannin'' is an old one, and comes from the practice of using extracts from plants to cure leather (the process referred to as ‘tanning’). This process exploits one of the key properties of tannins: they have a strong tendency to link up with a range of other chemical entities, most particularly proteins.

Tannins are therefore defined functionally. They are polyphenolic compounds that bind to and precipitate proteins. It’s a slightly complicated picture: not all polyphenols can act as tannins, and not all phenolics that bind proteins are tannins, but it’s still a useful definition.

Tannins and red wine colour

Here’s another story that could do with some revision. Researchers are now beginning to understand the nature of colour in red wines, and the picture emerging is challenging traditional understanding in this area. Colour in red wines actually falls into three categories. First we have the anthocyanins, the primary pool of colour from the grape. Young wine is packed with anthocyanins, which are very reactive: they interact with both sulphur dioxide and oxygen, which bleaches them. Their colour is also influenced by the pH of the must. At lower (more acidic) pH they are redder; at higher (less acidic) pH they are bluer. It turns out that anthocyanins are unstable, and aren’t that important for the long-term colour of red wines. In addition to anthocyanins there are two major fermentation-derived colour groups. The first of these is the pigmented polymers. These are formed by the chemical linkage between tannins and anthocyanins. This is a covalent (strong) linkage and is very important in forming stable colour in wines. The evidence suggests that most of the pigmented polymer formation occurs during fermentation. The third group is called the anthocyanin-derived pigments, which arise from reactions between anthocyanins and other phenolics and aldehydes. This is a massive, complicated class of non-bleachable pigments, and is an area of intense current research, with new members are being added all the time. The anthocyanin-derived pigments are still quite reactive and they can go and form further combinations with tannins to form pigmented polymers. There’s also current interest in the phenomenon known as copigmentation. This is the stable combination of anthocyanins with phenolic ''copigments''—colourless molecules which combine with the anthocyanins to increase colour intensity.