Vol. V · The Molecules of Food Plate 3 of 11starch, amylose, amylopectin, granules, sticky rice, waxy starch, gelM

Starch.

Two molecules made of the same glucose: one straight, one branched like a bush. Their ratio is the difference between basmati and sticky rice.

Plants store energy as starch: thousands of glucose units chained together and packed into microscopic granules. The chains come in two shapes. Amylose is long and mostly straight; amylopectin is enormous and branched like a bush. Cooked in water, the granules swell and release some of their chains (VII.1); what happens next depends on the mix.

Straight amylose chains line up with each other as they cool, forming firm gels and, over time, crystalline regions — the firmness of cold rice and the staling of bread (VII.3). Branched amylopectin cannot line up easily; it stays soft and sticky. So a starch with more amylose cooks to separate, fluffy grains and sets firm; one with almost none — glutinous (sticky) rice, waxy maize — cooks to a clinging, chewy mass that stays soft. The workshop compares starches.

Below: the amylose share of common starches (approximate published ranges, midpoints) and a teaching index of how firmly each sets and how sticky it cooks.

What you will learn
Workshop
Amylose as a share of starch (approximate; varieties vary): glutinous rice and waxy maize near 0 %, short-grain rice about 15 %, long-grain rice about 23 %, wheat about 25 %, potato about 21 %, maize about 27 %, high-amylose maize 55 % or more. The firmness and stickiness indices are teaching models proportional to amylose and amylopectin.

Straight and branched

Long-grain rice such as basmati has about 23 % amylose, so it cooks to separate grains that firm up when cold (firmness index 0.92); glutinous rice, with almost no amylose, has a firmness index of 0.04 and a stickiness index of 1.29 — it is what holds 粽子, 汤圆 and mochi together and stays soft for hours. Japanese short-grain rice sits between, sticky enough for sushi and chopsticks.

Cooks choose starches by these properties. Potato and wheat starch thicken gravies that set a little as they cool; waxy maize starch is used in fillings that must stay soft and clear when frozen; high-amylose starches make firm noodles and resistant starch. And every amylose-rich food stales: cold rice hardens, bread firms, and reheating undoes it for a while (VII.3).

At the stove

Cook a cup of long-grain rice and a cup of glutinous rice side by side (glutinous rice is best soaked and steamed). Compare them hot, then again after an hour in the fridge: the long-grain rice firms; the glutinous rice stays soft and sticky.

Glutinous rice contains no gluten; the name describes its glue-like stickiness. Its near-absence of amylose comes from a single mutation in the gene for granule-bound starch synthase, selected by early farmers in East and Southeast Asia.

Terms

Amylose
Long, mostly straight starch chains that line up into firm gels on cooling.
Amylopectin
Huge, branched starch molecules that stay soft and sticky.
Starch granule
The microscopic package in which plants store starch.
Waxy starch
Starch with almost no amylose, from glutinous rice or waxy maize.
Starch is straight amylose and branched amylopectin: more amylose cooks separate and sets firm; almost none cooks sticky and stays soft.

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.
  2. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.