Initially used as a method to preserve some wines, fortification is the addition of brandy or a neutral spirit in order to boost a wine’s ALCOHOL content. Fortified wines generally have between 17 and 21 percent alcohol. Many fortified wines are sweet because by adding ALCOHOL to a level (15 to 16 percent) above which the YEAST cannot work, the yeast quits converting the natural sugar. The earlier this fortification process occurs the more RESIDUAL SUGAR is left and the sweeter the wine. Some of the better-known examples are PORT, SHERRY, MADEIRA, MÁLAGA, and MARSALA.