 Starch Gelatinization

When we boil rice for example, we observe the grains swelling and becoming much more soft. This manifests itself in the process of starch gelatinization.

At high temperatures, the intermolecular bonds of the starch molecules (for example the double helices formed by amylopectin) are broken down (new water-starch hydrogen bonds are formed) and thus the structure of the starch granules is altered. First the amorphous regions are disrupted and then the granule's whole structure gets effected. The granules lose their integrity and burst. Amylose (and a smaller amount of amylopectin) molecules leave the granule and contribute to the increased viscosity of the liquid.

When the temperature drops, recrystallization occurs. This is referred to as starch retrogradation and is responsible for bread staling.