The encyclopaedia of gravy
Flour thickens gravy because its starch granules absorb water and swell when heated, a process called gelatinisation — which also explains lumps, raw-flour taste and why gravy thickens as it cools.
What starch is¶
Wheat flour is roughly three-quarters starch, stored in the grain as microscopic granules. Each granule is a densely packed bundle of two glucose polymers: amylose, which is long and mostly straight, and amylopectin, which is branched.
Cold water cannot get into these granules in any quantity. They sit in the liquid as suspended particles and thicken nothing.
Gelatinisation¶
Heat changes that. From around 60 °C, the ordered structure inside the granule begins to break down and water floods in. Each granule swells to many times its original volume.
Three things follow:
- The granules take up space. Swollen granules crowd the liquid and impede flow — the liquid becomes viscous.
- Amylose leaks out. Long amylose chains escape into the surrounding liquid and tangle with each other, forming a loose network that holds water.
- At around 90–95 °C the process completes. Peak thickness is reached. Beyond this, continued heat and agitation start to break granules apart and the mixture can thin again.
This is why an under-heated gravy is thin, a properly simmered one is right, and one boiled hard for half an hour can go thin and sticky at once.
Why lumps form¶
If dry flour meets hot water, the granules on the outside of a clump gelatinise instantly and form a sealed, gelled skin. The dry flour inside is now waterproofed and will never hydrate.
Preventing that means separating the granules before heat arrives — by coating them in fat (a roux) or dispersing them in cold liquid (a slurry). See avoiding lumps in gravy.
Why raw flour tastes of raw flour¶
Un-gelatinised starch is perceived as chalky and pasty, and flour also contains enzymes and proteins that need heat to be deactivated and cooked. Two minutes in the roux plus five to ten minutes at a simmer removes it. See making a roux.
Why a dark roux thickens less¶
Prolonged dry heat breaks the long starch polymers into shorter fragments through dextrinisation. Shorter chains tangle less and hold less water, so a chocolate-brown roux has perhaps half the thickening power of a white one — while tasting considerably better. Use more of it. See making a roux.
Why gravy thickens as it cools¶
As the temperature drops, the amylose chains that leaked out during cooking begin to re-associate and align with each other — a process called retrogradation. The network tightens and the gravy stiffens. It is the same process that makes day-old bread stale and leftover gravy set to a spoonable block.
Reheating reverses most of it, which is why cold gravy loosens when warmed.
Cornflour vs wheat flour¶
Cornflour is essentially pure starch, with none of the protein, bran or fat that dilutes wheat flour. That is why it thickens roughly twice as strongly by weight, and why it gives a clearer result — there is no protein to cloud it. It is also more fragile: its network breaks down faster under prolonged boiling. See thickening with flour vs cornflour.