Tannins are bitter and astringent compounds found in most plant species. There are many hundreds of individual compounds that vary greatly in their size and complexity.
All tannins bind to and precipitate protein and various other organic compounds and macromolecules. This property of binding is the basis of their role in tanning hides for leather and gave rise to their name, as well as to the term astringent from the Latin, ad astringere, ‘to bind’.
Tannins are involved in plant growth regulation and play two defensive roles, one against micro-organisms and the other against herbivores. Tannins bind to cell walls and other phenolics to create a physical defensive barrier when cell integrity is lost and, being bitter and astringent; their taste is a deterrent to herbivores.
Managing grape tannin in the vineyard may not translate directly into desirable quantities of tannin in wine. Increasing evidence suggests that tannins are located in the cell walls of grape skin during berry development, as part of the plant’s defence mechanism against micro-organisms. Rather than being released into the wine during winemaking, grape cell wall material binds additional tannins, which accounts for the discrepancy between grape and wine tannin levels. In the seed the tannins are located in the seed coat and this layer of cells dies as the seed matures. When this happens, tannins bind to other phenolics, cell wall proteins and polysaccharides.
Tannins contribute bitterness, astringency and complexity and affect the colour stability of wine. Their astringency is what gives the dry and puckery feeling in the mouth following tasting of red wine as well as unripe fruit or tea. Red wines generally contain more tannin than white wine because the extended contact of the grape skins with the juice allows tannins to be extracted into the wine.
Structurally similar to tannins are anthocyanins, which give grapes and young red wines their colour. During winemaking, anthocyanins break down or interact with other elements to form new compounds, some of which are coloured. The effect of site and season on tannin levels, coupled with the effects of within-vineyard variability, has always been an issue in vineyards. Managing that variation requires an understanding of what causes it. Bunch exposure and canopy management have long been touted as tools for managing colour in some grape varieties.