By removing the grape skins after red grapes are pressed. Many people believe that red wine is made from red grapes and white wine from green grapes. They are wrong. When grapes are pressed, the juice is white, no matter what variety of grape is used. But if the juice is left in contact with red grape skins, pigments leach out of the skins and color the wine. These pigments belong to a family of compounds called anthocyanins which are responsible for the coloration of many fruits and vegetables. If red grapes are pressed and the juice is allowed to ferment after being separated from the skins, the result is white wine, referred to as “blanc de noirs” meaning a white wine from dark grapes.
A well-known example is a variety of champagne produced from the Pinot Noir grapes. White wines can also be made from white grapes with the skins being removed before fermentation starts. Since there is no need for color, there is no point in having the juice be in contact with the skins. Such contact would only serve to leach out tannins, compounds that add astringency and a bitter flavor to the wine. In the case of red wines, contact with the skins of course is necessary to infuse the fermenting juice with the colored anthocyanins. But this means that tannins are also being leached out. So a prime question in red wine production is when to transfer the wine from the fermentation vessel where it is contact with skins to barrels where it is not. Leaving the juice in contact with the skins after color has fully developed can yield too high a tannin content and is a no-no. Now, tannins are not totally undesirable. They add body to the wine and a certain “pucker factor” that contributes to the wine-drinking experience. Tannins are complex polymers of molecules called phenols which have the capability of increasing the viscosity of saliva by linking together some of its naturally occurring proteins. Indeed, tannins extracted from plants are used to “tan” leather, a process that cross-links proteins, turning soft animal skin into a substance tough enough for shoes, belts and furniture. Vintners know that color is a marker of the fermentation process and have traditionally used its development to judge how long to let grape juice sit in contact with skins. Early experiments to put such decisions on a scientific footing resulted in the production of “wine color cards” that featured a series of circles with different red hues for comparison with the color of a fermenting batch of wine. The colors were based on experiments that had shown the optimal time for transfer of the fermenting juice to barrels.
Today various sophisticated scientific instruments are available to help vintners produce high quality wine of consistent flavour. Spectrometers can measure subtle differences in color and pressure transducer sensors can monitor the extent to which sugar has been converted to alcohol. Amazingly, though, the best instrument for distinguishing the subtleties of wine is still the human nose which can detect the presence of some chemicals in concentrations that defy detection by the most sensitive instrumental techniques.