Simmering a sauce to evaporate some of the water thickens the sauce because only the water evaporates, not the solids. As the solids become more concentrated, the sauce becomes thicker.This technique has always been important for finishing sauces (see the next section). It has become more important as a basic thickening technique as modern chefs use less starch for thickening.
Use caution when reducing stock-based sauces. If such a sauce is reduced too much, the concentration of gelatin may give it a gluey or sticky texture, and it will con geal quickly on plates. Also, the sauce may have a heavily cooked taste that is not as appealing as the fresher,livelier taste of a stock that has not been cooked as much.
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