Vol. VII · Starches and Sugars Plate 1 of 11gelatinisation, starch granules, swelling, temperature range, thickening, rice, potato, wheatMH

Starch Gelatinisation.

A starch granule is a tight, dry package. Heat it in water past its gelatinisation temperature and it opens, drinks and swells.

Raw starch granules are partly crystalline and do not dissolve in cold water — which is why flour and cornflour settle to the bottom of a glass. Heated in water, the granules loosen at a particular temperature range, take up water, swell to many times their volume and leak some amylose (V.3) into the liquid. This is gelatinisation, and it is what turns a slurry into a sauce, hard rice into soft, raw potato into floury mash, and dough into crumb.

Each starch gelatinises in its own range: wheat in the fifties to low sixties, potato and tapioca around sixty, maize a little higher, rice the highest, around seventy. Water is essential: a starch with too little water cannot swell fully, which is why rice needs enough water and why a dry crust stays crisp. The workshop heats a starch to the temperature you choose.

Below: starch granules drawn as they swell, and the fraction of five starches gelatinised after ten minutes at each temperature (course kinetics with midpoints from calorimetry).

What you will learn
Workshop
Fraction gelatinised after ten minutes at each temperature, with midpoints from the published DSC ranges (wheat about 58 °C, potato 62, tapioca 64, maize 67, rice 72). The drawing shows the chosen starch's granules at your temperature.

Each starch its own range

At 65 °C, wheat starch is 100% gelatinised in ten minutes but rice starch only 27%: a white sauce thickened with flour is well on its way while rice is still hard. That is why rice is cooked at or near the boil, why maize starch (cornflour) must be brought to a simmer before a sauce reaches full thickness, and why potato starch and tapioca thicken at lower temperatures and are used for delicate sauces and pie fillings that must not boil hard.

Water and time matter as much as temperature. Granules need about their own weight of water or more to swell fully, so a crisp biscuit or the crust of bread stays crisp: its starch never had the water to gelatinise. Sugar competes for water and raises the range, so a sweet sauce thickens at a higher temperature than a savoury one. Acid breaks starch chains and thins a sauce that is cooked too long with lemon or tomato (VII.2).

At the stove

Stir a teaspoon of cornflour into a cup of cold water: it settles. Heat the cup gently in a pan of water with a thermometer in it and watch: around 60 °C nothing much; between about 62 and 72 °C it thickens and clears quite suddenly.

Under a polarising microscope, raw starch granules show a dark "Maltese cross" because of their ordered structure; the cross vanishes as they gelatinise — the textbook way of measuring the process.

Terms

Gelatinisation
Starch granules absorbing water and swelling when heated, losing their order.
Granule
The microscopic package of starch in plant cells.
Gelatinisation range
The temperatures over which a starch swells; characteristic of each starch.
Paste
A thickened mixture of swollen granules and released starch chains.
Heated in water, starch granules swell and thicken — each starch in its own range, and only with enough water.

Sources

  1. Starch gelatinisation ranges measured by DSC: wheat about 52–64 °C, potato 58–66 °C, maize 62–72 °C, rice 68–78 °C, tapioca 59–70 °C (S. Damodaran and K. L. Parkin (eds.), Fennema's Food Chemistry, 5th ed., 2017, ch. 3). The course uses the midpoints.