where do the tannins in wine come from
Wine color Red drupe grapes can produce white wine if they are quickly pressed and the juice not allowed to be in contact with the skins. The color is mainly due to plant pigments, notably phenolic compounds (anthocyanidins, tannins, etc.). The color depends on the presence of acids in the wine. It is altered with wine aging by reactions between different active molecules present in the wine, these reactions generally giving rise to a browning of the wine, leading from red to a more tawny color. The use of a wooden barrel (generally oak barrels) in aging also affects the color of the wine.
Storage of wine Most wine racks are designed to allow a wine to be stored on its side. The thinking behind this orientation is that the cork is more likely to stay moist and not dry out if it is kept in constant contact with the wine. Some wineries package their wines upside down in the box for much the same reason. Research in the late 1990s suggested that the ideal orientation for wine bottles is at a slight angle, rather than completely horizontal. This allows the cork to maintain partial contact with the wine in order to stay damp but also keeps the air bubble formed by a wine's ullage at the top rather than in the middle of the bottle if the wine is lying on its side. Keeping the ullage near the top, it has been argued, allows for a slower and more gradual oxidation and maturation process. This is because the pressure of the air bubble that is the ullage space rises and falls depending on temperature fluctuation. When exposed to higher temperatures the bubble's pressure increases (becomes positive relative to the air outside of the bottle), and if the wine is tilted at an angle, this compressed gas will diffuse through the cork and not harm the wine. When the temperature falls the process reverses. If the wine is completely on its side then this action will eject some wine through the cork. Through this "breathing" which can result from variations in temperature, oxygen may be repeatedly introduced into the bottle and as a result can react with the wine. An appropriate and constant temperature is therefore preferred. Additionally, oxidation will occur more rapidly at higher temperatures and gases dissolve into liquids faster the lower the temperature.[8]