In the case of white wines, the must is left in contact with the skins for a few hours and removed with gentle pressure before fermentation. It thus picks up little tannic material or pigmentation. Rosé musts and red wine musts are partly fermented in contact with the red skins. The longer the must is in contact with skin and seeds, and the harder it is pressed, the deeper the color (whether yellow or red) and the more astringent the taste.
Before beginning the fermentation, the winemaker usually adds two substances to the must. One, sulfur dioxide, suppresses the growth of undesirable wild yeasts and bacteria, and prevents the oxidation of both flavor and pigment molecules (the same treatment is given to many dried fruits, and for the same reason). Though this treatment may sound antiseptically modern, it is centuries old. One of the natural by-products of fermentation that is increased by sulfuring is sulfites, sulfur compounds that can induce an allergic reaction in sensitive people.