Réduire (to reduce) is the most fundamental concentration technique in French cooking — simmering or boiling a liquid to evaporate water, progressively intensifying flavour, body, and colour with every minute the liquid remains on the heat. Reduction is the mechanism by which a thin, watery braising liquid becomes a glossy, concentrated sauce; by which a litre of stock becomes a tablespoon of glace de viande; by which a full bottle of wine becomes a few tablespoons of intense, syrupy essence. Understanding reduction is understanding the architecture of French sauce-making. The physics are straightforward: as water evaporates from a simmering liquid, the concentration of dissolved solids (sugars, proteins, gelatin, minerals, flavour compounds) increases proportionally. A stock reduced by half has twice the concentration of flavour per millilitre. A stock reduced to one-tenth (a glace) is ten times as concentrated. The rate of reduction depends on surface area (a wide pan reduces faster than a narrow one), heat intensity, and volume. Classical reductions serve different purposes at different stages: réduction à sec (reduce to near-dry — used for shallots in wine for béarnaise), réduction de moitié (reduce by half — standard for pan sauces), réduction au tiers (reduce to one-third — for concentrated sauce bases), and glace (reduce to one-tenth or more — for meat glaze). Wine reductions require special attention: as wine reduces, its acidity and tannins concentrate alongside the fruit. A harsh wine becomes harsher; a good wine becomes better. The first reduction (by half) concentrates raw alcohol and harsh tannins; the second stage (to a third) cooks off the remaining alcohol and softens tannins; the final stage creates a wine syrup of extraordinary depth. This is why classical recipes specify adding wine to the pan and reducing before adding stock — the raw wine compounds must be cooked out before the gentler stock is introduced.
Evaporating water concentrates all dissolved solids proportionally. Wide pan = faster reduction (more surface area). Wine reduced in stages: half (alcohol off), third (tannins soften), syrup (pure essence). Réduction à sec, de moitié, au tiers — different levels for different purposes. Always reduce wine before adding stock — raw wine compounds must cook out. Gelatin-rich stocks become glossier and more viscous as they reduce.
Reduce in stages, tasting at each stage — the sauce tells you when it's ready. A sauce that's over-reduced can be rescued by adding a splash of stock or water and reducing again. The mark of a great jus is the 'nappé test': dip a spoon, run your finger across the back — if the line holds cleanly without running, the sauce is perfectly reduced. Glace de viande (meat glaze) keeps indefinitely refrigerated and is the most powerful flavour enhancer in the kitchen. For wine reductions, add a split shallot and a few peppercorns during reduction for complexity — strain before proceeding.
Reducing a stock that lacks gelatin — it becomes salty water, not a glossy sauce. Over-reducing, which produces a burnt, bitter concentrate. Reducing with the lid on, which traps steam and prevents evaporation. Not tasting during reduction — saltiness concentrates along with flavour. Adding stock before wine has reduced sufficiently — the raw wine taste persists in the finished sauce.
Le Guide Culinaire — Auguste Escoffier