Starch Gelatinization - Retrogradation

Retrogradation

Cooked, unmodified starch, when cooled for a long enough period, 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. Molecular associations occur: Amylose-Amylose ; Amylose-Amylopectin; Amylopectin-Amylopectin. A mild association amongst chains come together with water still embedded in the molecule. Due to the tightly packed organization of small granule starches, retrogradation occurs much slower compared to larger starch granules. High amylose starches require more energy to break up bonds to gelatinize into starch molecules, leading to a rigid and stiff gel. A mild association amongst chains come together with water still embedded in the molecule.

Due to strong associations of hydrogen bonding, longer amylose molecules will form a stiff gel. Amylopectin molecules with longer branched structure, increases the tendency to form strong gels. Granule size do not directly impact starch performance, but it is one of the main factors affecting starch gelatinzation and retrogradation. 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.