They are found in plants, seeds, tree bark, leaves and skin of red fruits. They are responsible for acer / bitter taste of fruit, tea, chocolate ...We can recognize them by sensation of "shrinkage" that they creates in our mouth (astringent property).
Tannins are colored compounds. They can be orange, amber and yellow hues. Because of tannin, an older red wines take on brown and shades of color bricks. In young (black) wines purple shades predominate. Compounds responsible for this color are similar to tannins, and they are called anthocyanins. With time, these molecules are deposited on the bottom of the bottle. Therefore, the wine loses its original color and takes on the color of the bricks ie tannins.
TANNINS IN THE WINE
Tannins act as natural preservatives. This means that because of tannins wine can keep its good properties for a longer time. They give the wine a desirable sensation of dryness and cleanliness. However, if there are too many tannins in the wine, the sensation can be uncomfortable. The dose of tannins in wine is an important factor of its quality.
Red wines are rich in tannins, and white are poor. The reason is simple: most of the tannins are located in the skin of the berries. During fermentation of the red wines, there is contact between the skin and juice, so tannins can cross from the skin to juice. In the fermentation of white wines, there is no skin and juice contact. Besides the grape, tannins can get to the wine via oak barrels as well. So, white wines which have been aged in these barrels, also contain some of the tannins.