Reduction Concentrates The Salt As Well As The Flavour

Reducing a sauce is subtraction, not addition. Water leaves as steam and everything else stays, which is why the order of seasoning matters more here than anywhere else.

Only water evaporates

Salt is not volatile and does not leave with the steam. A litre of stock reduced by half contains the same salt in half the liquid, so it is twice as salty.

Sugars, acids and dissolved proteins behave the same way, which is what produces the concentrated flavour that reduction is done for.

The one exception is aromatic compounds that are volatile, which do leave, and that is why a long reduction tastes deep but loses the top notes of fresh herbs and wine.

Season at the end, taste along the way

A stock that tasted correctly seasoned before reducing will be inedible after. Anything intended for heavy reduction should start under-seasoned or unseasoned.

This is why professional stocks contain no salt at all. They are a base for sauces of unknown final volume, and salting them removes that flexibility.

Tasting during reduction is unreliable in the other direction too, because the sauce is hot and heat suppresses the perception of both salt and sweetness.

Thickening and reducing are not the same

A reduced sauce is thicker because there is less water relative to the dissolved and suspended material, particularly gelatine, which sets into a glossy coating.

Starch thickening adds body without concentrating flavour, so a floury or cornflour-thickened sauce can be thick and thin-tasting at once.

Which is why the two are often combined: reduce for flavour until the taste is right, then thicken separately if the texture still needs help.

Surface area sets the speed

Evaporation happens at the surface, so a wide pan reduces far faster than a tall one holding the same volume, at the same heat.

A lid stops reduction almost completely, since the steam condenses and returns. Simmering with a lid on is a way to cook without concentrating.

Hard boiling reduces quickly but agitates the liquid, which can emulsify fat into it and cloud a stock that was intended to stay clear.

Correcting a reduction that has gone too far

Adding water back dilutes salt but also dilutes everything else, so the sauce becomes weak rather than correctly balanced.

Adding unsalted stock or an unsalted starchy liquid is better, since it restores volume while contributing some of the body that was concentrated.

A bulk ingredient can also absorb the excess: potato, rice, a starchy pulse or extra vegetables take up salt from the liquid as they cook in it.