Boiling a liquid uncovered so water leaves as steam, concentrating everything that stays behind.
Technique at a glance
A compact visual model of the lesson. Use the full mechanism and success criteria below when you practice.
Concept
Boiling a liquid uncovered so water leaves as steam, concentrating everything that stays behind.
What changes
Water evaporates and flavour compounds, sugars, salts and dissolved proteins mostly do not, so removing water raises the concentration of everything else.
What good looks like
A spoon drawn across the base leaves a track that fills in slowly rather than instantly.
Practice counts when you cook
Reading this brief does not change your School state. Finish a real practice dish in Cook mode and log the cook; that cook is the evidence that advances the skill.
What to focus on
Mechanism
Water evaporates and flavour compounds, sugars, salts and dissolved proteins mostly do not, so removing water raises the concentration of everything else. Body increases too: gelatin, starch and emulsified fat become proportionally denser and the liquid starts to coat rather than run. Two consequences follow that people routinely get wrong. First, evaporation happens at the surface, so the rate depends on pan width, not on volume or heat — a wide pan reduces several times faster than a tall one at the same temperature, and a lid stops the process almost entirely by returning condensate. Second, salt concentrates along with everything else, which is why a sauce seasoned to taste before reducing is usually too salty afterwards.
Success criteria
A spoon drawn across the base leaves a track that fills in slowly rather than instantly.
The back of a spoon holds a coating that a finger can part cleanly — the classic nappe.
Bubbles change character, from thin and fast to larger, slower and more viscous-looking.
The colour deepens noticeably, and in a fatty sauce the fat separates into pools at the edge, which is a sign it is nearly there.
Failure guards
It is taking forever and the flavour has gone flat and stewed.
Reducing in a narrow deep pan, or with a lid on. The liquid is being cooked for a long time without much leaving, which dulls fresh aromatics without concentrating anything.
Far too salty.
Seasoned before reducing. Salt does not evaporate. Season at the end, always.
Scorched, bitter taste from the bottom of the pan.
Reduced too far or too fast with sugars or dairy present. Once the liquid is thin the base catches almost immediately.