Starch — the storage carbohydrate in grains, legumes, and root vegetables — transforms when heated in the presence of water through a process called gelatinisation: the starch granules absorb water, swell, and eventually rupture, releasing their amylose and amylopectin chains into the surrounding liquid. This transformation is what thickens sauces, gives bread its structure, produces the texture of perfectly cooked pasta and rice, and allows starch-based gels to form. Every sauce thickened with flour, cornstarch, or arrowroot is exploiting starch gelatinisation.
**The chemistry:** - Starch granules are composed of two polysaccharides: amylose (linear chains, approximately 20–30% of most starches) and amylopectin (highly branched chains, 70–80%) - Below gelatinisation temperature: granules are intact, starch is white and opaque, does not thicken - Above gelatinisation temperature (varies by starch source): granules absorb water and swell up to 1,000 times their original size; amylose chains leach out - Cooling: amylose chains form a network (retrogradation) that creates the gel structure **Gelatinisation temperatures by starch type:** - Wheat flour: 58–65°C - Cornstarch: 62–72°C - Potato starch: 58–66°C (but very sensitive — over-mixing post-gelatinisation destroys gel) - Waxy corn (amylopectin-only): 63–72°C (more stable gel, does not retrograde as readily) - Arrowroot: 58–65°C (clearer gel than cornstarch, breaks down if held too long) - Tapioca: 58–70°C **The critical distinction between starches:** - *Flour-thickened sauces* (roux-based): the gluten in the flour creates structural support; the sauce must reach a full boil to fully gelatinise the starch and cook out the raw flour taste - *Cornstarch-thickened sauces*: gelatinise at lower temperatures; produce a clearer gel; break down when boiled too long (the starch chains rupture from mechanical action) - *Potato starch*: produces the most stable, clearest gel for Japanese cooking; also the most sensitive to over-mixing (produces gluey, elastic texture if the gel is disturbed post-set) **Retrogradation:** When starch gels cool, amylose chains crystallise and the gel becomes firmer — this is retrogradation. In bread, this is staling. In cooked rice, this explains why leftover rice is drier and firmer than fresh. Retrogradation can be reversed by reheating.
Modernist Cuisine Vol. 2