What à sec means
À sec means “dry” in French, and in the kitchen it describes reducing a liquid until almost none of it is left. You will usually see it in the phrase “reduce à sec” or “reduce almost à sec”. The liquid, which might be wine, vinegar, stock or a mixture, is simmered until the water has boiled away and only a thick, sticky glaze coats the bottom of the pan. This leaves an intensely flavoured base for a sauce.
The classic example is beurre blanc, where white wine, vinegar and shallots are reduced almost à sec before cold butter is whisked in. Béarnaise and many pan sauces start the same way. The point is concentration, packing the taste of a large amount of liquid into a spoonful. The tricky part is the end, because once most of the liquid has gone, the sugars and solids can catch and burn quickly.
A wide, heavy pan helps the liquid reduce evenly, and lowering the heat as it thickens gives you more control. Despite the name, the pan should not be completely dry. You will meet the term in French recipes, classic sauce instructions and professional cookery books, usually as the opening stage of a sauce.