the practice of adding a spirit, usually grape spirit, to a wine to arrest its fermentation, increase its alcoholic strength and/or ensure future microbiological stability. Most bacteria and yeast strains are incapacitated above alcohol levels of 15.5 – 18%.
The stage at which the spirit is added varies depending on the style of fortified wine produced. The earlier it is added, the sweeter the resulting wine will be. For Vins de Liqueur (mistelles) such as Pineau de Charantes, grape spirit is added to sweet, unfermented or barely fermented grape juice. For most port-style wines, including most Madeiras, and for all Vins Doux Naturels (Banyuls, Maury, Rivesaltes), fortification arrests the fermentation and a portion of the natural grape sugar is preserved in the wine. (In France, this process is called mutage.) In the case of most Sherrys and Marsalas, fortifying spirit is not added until after the wine has completed its fermentation.