Starch gelatinization is a process of breaking down of intermolecular bonds of starch molecules in the presence of water and heat, allowing the hydrogen bonding sites (the hydroxyl hydrogen and oxygen) to engage more water. This irreversibly dissolves the starch granule in water. Water acts as a plasticizer.

Three main processes happen to the starch granule: granule swelling, crystallite and double-helical melting, and amylose leaching.

The gelatinization temperature of starch depends upon plant type and the amount of water present, pH, types and concentration of salt, sugar, fat and protein in the recipe, as well as starch derivatisation technology are used. Some types of unmodified native starches start swelling at 55 °C (131 °F), other types at 85 °C (185 °F). The gelatinization temperature of modified starch depends on, for example, the degree of cross-linking, acid treatment, or acetylation.

Gel temperature can also be modified by genetic manipulation of starch synthase genes. Gelatinization temperature also depends on the amount of damaged starch granules; these will swell faster. Damaged starch can be produced, for example, during the wheat milling process, or when drying the starch cake in a starch plant. There is an inverse correlation between gelatinization temperature and glycemic index. High amylose starches require more energy to break up bonds to gelatinize into starch molecules.

Gelatinization improves the availability of starch for amylase hydrolysis. So gelatinization of starch is used constantly in cooking to make the starch digestible or to thicken/bind water in roux, sauce, or soup.

Gelatinized starch, when cooled for a long enough period (hours or days), will thicken (or gel) and rearrange itself again to a more crystalline structure; this process is called retrogradation. During cooling, starch molecules gradually aggregate to form a gel. The following molecular associations can occur: amylose-amylose, amylose-amylopectin, and amylopectin-amylopectin. A mild association amongst chains come together with water still embedded in the molecule network.

Due to strong associations of hydrogen bonding, longer amylose molecules (and starch which has a higher amylose content) will form a stiff gel. Amylopectin molecules with longer branched structure, which makes them more similar to amylose, increases the tendency to form strong gels. High amylopectin starches will have a stable gel, but will be softer than high amylose gels.

Retrogradation restricts the availability for amylase hydrolysis to occur, which reduces the digestibility of the starch.