What reduction means
Reduction is the process of simmering or boiling a liquid, uncovered, so that some of its water evaporates as steam. As the liquid reduces in volume, its flavours become more concentrated and intense, and it often becomes thicker and glossier, especially if it contains gelatine from stock or sugar. The word also describes the finished concentrated liquid, as in a red wine reduction or a balsamic reduction. Reduction is among the most important techniques in sauce making.
Stocks are reduced to concentrate their flavour and body, and in classic French cooking very reduced stocks become demi-glace and glace. Pan sauces are made by deglazing a pan with wine or stock and reducing it until syrupy, then often finishing with butter or cream. Wine, cider and spirits are reduced to cook off the alcohol and concentrate their fruit, vinegars are reduced to make sweet-sour glazes, and fruit juices and syrups are reduced for desserts. Cream sauces are reduced to thicken them naturally.
A wide, shallow pan speeds up evaporation, and the heat should be steady but not so fierce that the liquid catches. Salt is best added at the end, because it becomes more concentrated as the liquid reduces. Reducing a liquid until almost nothing is left is called reducing à sec. You will see reduction in sauce, stock and dessert recipes.