What Is a Reduction? Boiling Away Water to Leave Everything Else

Water boils off and carries some volatile aroma compounds with it, while everything non-volatile stays behind in a smaller volume. Salt concentrates, which is why a sauce seasoned before reduction becomes inedible and why salting is done at the end. Sugars concentrate and eventually begin to caramelise, which adds flavour and colour and can burn. Acids concentrate, so a wine reduction becomes sharper and needs balancing. Gelatine from stock concentrates, which is what produces body and the glossy coating quality of a properly reduced sauce, and it is why a stock-based reduction thickens while a wine-based one does not. Meanwhile the volatile aromatics that make fresh wine or herbs smell as they do are driven off, which is why a long-reduced wine sauce tastes of wine's structure rather than its perfume, and why delicate herbs are added after reduction rather than during it. Understanding which of those you want determines how far to take it.

Classical practice recognises degrees of reduction with different uses:

• •A light reduction, removing perhaps a quarter of the volume, which concentrates flavour without changing texture much
• •Reduction by half, the commonest instruction, which noticeably thickens a stock-based liquid
• •Demi-glace, a brown stock reduced substantially and combined with sauce base, which is the foundation of a large family of classical sauces
• •Glace de viande, a meat stock reduced until it sets solid when cold, which is used in small quantities to add depth to anything
• •Gastrique, sugar and vinegar reduced together, used to balance rich dishes
• •Balsamic and other vinegar reductions, where sugar content is high enough that the result becomes syrupy and can crystallise if taken too far

Several variables determine how the process goes and most are about surface area and heat. A wide pan evaporates far faster than a tall narrow one, because evaporation happens at the surface, so the choice of pan changes the time by a large factor. A vigorous boil evaporates faster and agitates more, which can emulsify fat into the liquid and cloud it, so a steady simmer is preferred for clarity. Skimming as reduction proceeds removes fat and scum that would otherwise concentrate along with everything else. Tasting throughout is essential and is the only reliable guide, since the endpoint is a matter of flavour and texture rather than of volume. Finishing off the heat with butter whisked in, called mounting, adds richness and gloss and will split if boiled afterwards. And reduction is not reversible, so adding liquid back to an over-reduced sauce dilutes the flavour along with the salt rather than restoring the original.

Concentration is the obvious purpose and several others matter as much in practice. Reduction cooks off alcohol, which is why wine is generally reduced substantially before other liquids are added, since raw alcohol tastes harsh in a finished sauce and its evaporation also removes the sharpness that would otherwise dominate. It cooks out the raw flavour of tomato paste, vinegar and stock, which mellows them. It develops new flavours, since prolonged heating produces browning reactions at the surface of the pan and along the edges of the liquid, contributing to the depth of a long-reduced sauce in a way that simply having less water would not. It also has a practical storage function, since a reduced stock takes a fraction of the freezer space of the original and can be diluted later, which is why glace is worth making. And it clarifies, since skimming during a long reduction removes fat and impurities progressively and produces a cleaner result than a short one.

Restaurant sauce-making moved substantially from flour-thickened classical sauces to reduction-based ones during the late twentieth century, and the reasons are specific. Reduction produces body without adding a starch flavour, which leaves the base ingredient tasting more distinctly of itself. It produces a glossier appearance and a cleaner mouthfeel. It concentrates rather than dilutes, so the flavour intensity rises as the texture develops, which a flour-thickened sauce does not do. It also requires far more stock, far more time and far more expensive ingredients per portion, which is why it suits restaurants with a stock pot running continuously and suits home cooking less well. The trade-off is real in both directions, and the sensible domestic position is to use reduction where a small quantity of intense sauce is wanted and starch where a larger volume of coating sauce is needed, which is a practical division rather than a hierarchy of sophistication.

Water leaves and everything non-volatile stays, so flavour, salt, sugar, acid and gelatine all concentrate together, which is why salting comes at the end and why stock thickens where wine does not. Aromatic volatiles are driven off, so delicate herbs go in afterwards. Pan width changes the time by a large factor. Over-reduction cannot be undone by adding liquid, since that dilutes the flavour too.