Starch, reassemble

Another term in the NRIB report that I looked into was retrogradation (the kanji used, 老化, rōka, also mean age-related deterioration or ageing in a negative sense). Rice grown in cool weather (short amylopectin side-chains) is resistant to it, rice grown in hot weather (long amylopectin side-chains) is liable to it, but what is it? (Apparently the NRIB has equipment with the wonderful name of a Phytotron that they use to grow different rice varieties under different conditions to investigate all this.)

Starch in plants is usually stored in groups of molecules called granules. Cooking (steaming for sake making) causes the starch to absorb water and swell as the previously clustered starch molecules are pushed apart by water molecules – a process known as gelatinisation.

As the gelatinised starch cools, water escapes and the starch molecules reassemble – but in a more orderly crystalline structure. This is retrogradation, which is also responsible for the change in texture of bread as it goes stale.

It also explains why the NRIB graph specifies that the rice was steamed and then left in the open air for a day, which would give this reassembly time to happen.

The new structure taken on by the cooked-then-cooled starch molecules is harder for enzymes to digest, hence the rice turns out to be less soluble in the main ferment. So joining the dots, temperature during earing affects the type of starch molecules that form in the rice, which then affects the structure of the starch in the rice after steaming, and makes it more or less easy for enzymes to digest.