Recipe 03 · Polysaccharide · gelatinisation
The classic school-lab bioplastic and a real industrial one — thermoplastic starch is the base of compostable bags and cutlery. Heat bursts the starch granules, vinegar tidies the chains, glycerol keeps them supple, and a thin pour dries into a translucent film.
Method
Put the cornstarch, water, glycerol and vinegar in a small pan and stir until there are no lumps and it looks like thin milk.
Why: Starch granules only wet properly in cold water. Add them to hot water and the outsides gelatinise instantly into lumps with dry cores.Over low heat, stir constantly. After 2–4 minutes the mixture thickens suddenly and turns from milky white to translucent.
Why: Between about 62 and 72 °C the granules swell, amylose leaks out, and the crystalline order melts — gelatinisation. The sudden thickening is the freed chains entangling.As soon as it is translucent and the texture of thick glue, take it off the heat. If it bubbles, it has gone too far.
Why: Overcooking and boiling break chains and can brown the sugars; the film ends up weak and yellow.Stir in colouring now, while it is hot and mobile.
Why: Once the paste cools it becomes too thick to mix evenly.Pour onto aluminium foil, baking paper or a silicone mat and spread with the back of a spoon to 2–3 mm. Pop any bubbles.
Why: Thin dries evenly; thick cracks and can take a week.Leave in a warm, airy place for 1–2 days until translucent and no longer cool to the touch. Peel from the foil.
Why: Water leaves; amylose chains realign and hydrogen-bond into a continuous film. Slow drying gives fewer cracks.Under the microscope
A starch granule is a plant's battery: glucose polymerised into two kinds of chain and packed into layered spheres 5–25 µm across. Amylose is long and straight (a few thousand glucose units); amylopectin is huge and branched like a tree. The branches line up into crystalline shells — the “growth rings” you can see under a microscope — which is why a granule is hard, insoluble and scatters light white.
Heat the granules in water and, past a threshold around 62–72 °C for corn, water forces its way in, the crystalline shells melt, the granule swells to many times its size and amylose leaks out. This is gelatinisation. The escaped chains entangle and the suspension turns into a thick, translucent paste — the moment your spoon suddenly drags.
Vinegar speeds the granules' breakdown and gently hydrolyses the chains, snipping them a little shorter. Shorter chains flow better, so the paste is smoother and the film is clearer with fewer lumps. Acetic acid also discourages the mould that would otherwise enjoy a damp starch film.
Glycerol is essential here. Amylose loves to recrystallise as it dries — a process called retrogradation — and a pure starch film is brittle within hours. Glycerol wedges between the chains and keeps the film flexible; it will still stiffen slowly over weeks, which you can watch happen.
Troubleshooting
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