Reduction is simmering a liquid to evaporate water, concentrating everything that remains — flavour compounds, sugars, proteins, dissolved solids, gelatin. It is the simplest sauce technique in existence: make a liquid taste like more of itself by making less of it. A cup of thin, watery pan drippings becomes two tablespoons of intensely flavoured, glossy, spoon-coating sauce. Nothing was added. Water was removed. The alchemy is in the subtraction.
Quality hierarchy: 1) Season at the END — this is NON-NEGOTIABLE. Salt, dissolved minerals, acids, sugars — everything concentrates as water leaves. A sauce that tastes perfectly seasoned at one cup will be inedibly salty at half a cup. Season only when the reduction has reached its final consistency. Taste, adjust, serve. 2) Simmer, don't boil — a gentle simmer gives controlled, even evaporation. A hard boil causes three problems: it emulsifies proteins and fat unpredictably (cloudy sauce), it can reduce too fast and overshoot (burnt, bitter), and it spatters. The surface should show lazy, intermittent bubbles — not a churning roll. 3) The visual test — tilt the pan and let the sauce pool. Dip a spoon and hold it horizontal. At nappé stage — the consistency you want for most sauces — the sauce coats the back of the spoon evenly and when you draw a line through it with your finger, the line holds without the sauce running back together for at least 3 seconds. That's your target. 4) The flavour arc — as a liquid reduces, its flavour changes character, not just intensity. Wine loses raw alcohol bite and gains sweetness and depth. Stock gains body as gelatin concentrates. Tomato sauce loses acidity and gains sweetness. Taste throughout the reduction — there's a moment where the flavour peaks before it begins to taste over-concentrated and bitter. That moment is when you stop. 5) Pan width matters — a wide pan reduces faster (more surface area for evaporation). A narrow pot reduces slowly. Choose based on urgency: wide sauté pan for a 3-minute pan sauce, narrow saucepan for a stock reduction you want to control carefully.
The pan sauce technique in under 4 minutes: after searing a steak, pour off excess fat but leave the fond (brown bits). High heat. Add 60ml wine, scrape the fond, boil hard for 60 seconds to cook off alcohol. Add 120ml stock, reduce by half — about 2 minutes of steady simmering. Off heat, swirl in a tablespoon of cold butter (this emulsifies into the sauce giving it gloss and body). Season now. Pour over the rested steak. You've just made a sauce that took zero skill to learn, uses no recipe, and will taste better than anything in a bottle. For stock reductions: strain out all solids first. Reduce on medium in a wide pan, skimming any foam. Demi-glace (stock reduced to one-tenth its original volume) should coat the back of a spoon and set like jelly when cold. A single tablespoon stirred into any braise, stew, or pan sauce adds the kind of depth that people describe as 'restaurant quality' without knowing why.
Seasoning before reducing — the most expensive mistake in sauce-making. You salt at one cup, reduce to half a cup, and the sauce is twice as salty as you intended. Reducing too aggressively — a hard boil can take a sauce past the sweet spot in 30 seconds. You can always reduce more; you cannot un-reduce. Over-shooting — if you reduce too far, the sugars and proteins begin to caramelise on the bottom of the pan, adding burnt bitterness. Add unseasoned stock (not water) to rescue an over-reduced sauce. Reducing wine without cooking off the alcohol first — raw alcohol has a harsh, volatile taste that concentrates if you just simmer. Bring wine to a vigorous boil for 60–90 seconds first to drive off the ethanol, then reduce the heat and simmer to concentrate the flavour. Not tasting throughout — a reduction crosses multiple flavour stages. If you're not tasting every minute during the final stages, you'll miss the peak.