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🥣 Starch gelatinisation: roux, slurry and the thickening curve

A gravy stays thin until starch granules drink, swell and fall apart. Roux and slurry are two ways in.

5
lessons
~20 min
to learn
🔬 Science
subject
Adults
level
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What you’ll learn

  1. The granule that pretends to be a crystalTreat thickening as irreversible gelatinisation: semi-crystalline granules of amylose and amylopectin drink, lose their Maltese cross, swell and leak.Flour is a warehouse of granules, only part starch. Amylose is the logjam. Polarised light shows a cross that vanishes when the crystal packing lets go. LibreTexts: the water is taken up in a way you cannot undo.
  2. The thickening curvePlace kitchen thickening on a pasting curve — onset, peak, breakdown, setback — with real temperatures for cornstarch and a general 60°C start.Canadian Baker: start about 60°C, done at a boil. Chin: cornstarch 62–72°C onset, about 95°C full. Roux sauces do not thicken until almost boiling. Cooling brings setback; overcooking brings breakdown.
  3. Roux: fat as a traffic copUse a roux as fat-coated starch that disperses without lumps, and treat darker colour as flavour bought with lost amylose chains.Gritzer: equal starch and fat, then cook. White roux keeps full hold; dark roux's spoon-trail closes. Butter's milk solids can burn in a Cajun-dark roux, which is why oil is common. Gumbo is that bargain in a bowl.
  4. Slurry: the cold doorDisperse purified starch as a cold slurry, add it late, cook it briefly, and contrast its gloss and power with a flour roux.Cornstarch is insoluble in the cold and settles. It thickens about twice as well as flour and looks shiny. Chin: Kingsford, 1840s, laundry then food. Escoffier forecast the swap; Western plates mostly kept the matte sauces.
  5. Why a finished sauce thinsDiagnose a thin sauce as a missed window, lumps, breakdown, or syneresis, and go watch the first shiver tonight.Sugar, fat and acid can block swelling; add acid after cooling. Flour's raw taste fades over about thirty minutes; a slurry is a five-minute clock. Retrogradation thickens leftovers, then syneresis weeps them. Nobody has split last Sunday's thin gravy into never-gelled versus torn.

Questions this course answers

When a starch sauce finally thickens, what has actually happened inside the pot?

Gelatinisation is irreversible swelling: heat plus water lets granules lose crystallinity, swell, and leak amylose. Those long chains jam the liquid. Fat in a roux only keeps granules from clumping first. Gluten is a dough story, not a sauce story.

About what temperature, in °C, does cornstarch begin to gelatinise in water, according to Tim Chin's range?

Chin puts cornstarch onset around 144–162°F, which is 62–72°C, and full gelatinisation around 203°F / 95°C. The Canadian Baker's general start is about 60°C. A shy simmer can sit under the door; a shiver of a boil opens it.

Match each roux colour to the job it is actually doing

Gritzer's four pans of equal flour and oil showed the trail behind a spoon closing as the roux darkened. Long amylose chains break into shorter pieces. Dark is not a better white. It is a different purchase.

Put a cornstarch slurry through the pot in the order the physics actually runs

A slurry is a cold suspension, not a solution — cornstarch settles if you wait. It thickens almost immediately in a hot pot. Chin and the Saucier book both want it added late and not cooked to death.

The Saucier textbook compares purified starches with flour.

Purified starches are almost only starch. Wheat flour is starch plus protein and other cargo. Same weight of cornstarch therefore thickens about twice the liquid, and looks glossier because there is no flour protein to matt the sauce.

In your own words, why can a cornstarch sauce that was thick last night greet you with a watery layer in the morning?

Chin: cooling cornstarch gels thicken and turn slightly opaque as granules reassociate; left cold long enough they can split and weep. That weeping is syneresis. A flour roux or waxy maize holds leftovers better. The gel on the stove is not promised to the fridge.

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