Reduction is the most fundamental technique in the saucier's arsenal — the evaporation of water from a liquid through sustained heat, concentrating dissolved flavours, thickening through increased viscosity, and intensifying both taste and body. Every sauce in the French kitchen involves reduction at some stage, yet the technique is often treated as passive when it demands active management. The rate of evaporation depends on surface area (wider pan = faster reduction), heat intensity, and humidity. A reduction that takes 15 minutes in a wide sauté pan takes 45 in a tall saucepot. The saucier controls concentration by choosing the vessel as much as the flame. During reduction, the Maillard reaction continues at the liquid's edges where thin films form on the pan walls — scraping these back into the sauce periodically captures flavour that would otherwise carbonise and be lost. Reduction concentrates everything: flavour, salt, acid, sweetness, and bitterness. A sauce that is perfectly seasoned at full volume will be over-salted at half volume. The cardinal rule: season after reduction, never before. The consistency changes follow a predictable arc: thin liquid → syrupy → nappante (coats a spoon) → glacé (coats and sets). Each stage has its use: thin for consommé, syrupy for jus, nappante for finished sauces, glacé for glaze. The skill of the saucier is knowing when to stop. Over-reduction is more dangerous than under-reduction — a sauce reduced past its ideal point is irretrievably too intense, too salty, and too thick.
Wide pan = faster evaporation. Season AFTER reduction, not before — concentration amplifies salt and acid. Scrape pan edges to capture Maillard flavour. Stages: thin → syrupy → nappante → glacé. Know when to stop — over-reduction is irreversible.
Test reduction progress by dragging a spoon across the bottom of the pan — the rate at which the liquid fills the track tells you the viscosity. At the nappante stage, the track fills in 3-4 seconds; at glacé, the track holds indefinitely. For the cleanest reduction, use a pan with straight (not sloped) sides — the vertical walls minimise the surface area where thin films burn. A squeeze of lemon added after reduction corrects any bitterness that develops from over-concentration of Maillard compounds.
Seasoning before reducing — the sauce becomes inedibly salty. Using a tall, narrow pot — the reduced surface area makes the process painfully slow. Walking away during the final stages — the transition from nappante to burnt happens in minutes. Reducing cream too far — it breaks into grainy butterfat.
Escoffier, Le Guide Culinaire; McGee, On Food and Cooking; The Professional Chef (CIA)