Gelatinization and retrogradation are two important physical properties in food processing and storage. Learning about them not only understands starch deeply, but also explains the changes in starchy food in everyday life.
What happens in starch gelatinization?
Although starch resembles instant coffee powder and powdered milk, it does not dissolve quickly when poured into water. Instead, it sinks to container bottom and absorb very little water initially. When heated to a threshold temperature, they swell violently to be a thick paste. This phenomenon is gelatinization that stems from their complex granular structure. Their crystallinity and molecular organization are lost. A hydrophilic colloid is formed as starch disperses in water.
The three stages
The first stage is a process for reversible water absorption. Water preferentially enters the amorphous regions and bond to hydroxyl groups. It is difficult to enter the semi crystalline region where amylopectin is arranged tightly and neatly. A little of water is absorbed and expansion is slightly, so the semi-crystal has not been destroyed. If starch is dried, you will find it returns to its original state.
When it reaches to gelatinization temperature, water molecules not only enter amorphous region but also penetrate the semi-crystalline region. They have enough kinetic energy to break down hydrogen bonds and unwind double helices of amylopectin. Starch granules absorb a great deal of water irreversibly and suddenly burst to 60-100 times their original volume. At this stage, the orderly structure has transitioned to a disordered state, which cannot revert to its original state even upon drying.
The third stage involves the complete collapse of granules. If heating continues, the attractive force no longer restricts thermal motion, and molecules gradually detaches from granules. Finally, a viscous and semi-transparent paste is formed in water. Sometimes, prolonged heat treatment will degrade some starch molecules.
Starch retrogradation: amylose vs amylopectin
It is essentially a transition from a disordered to a relatively ordered state. When starch is heated in water and cooled, the disrupted molecules gradually reassociate into an ordered structure due to hydrogen bond and decreased thermal motion. These structures are similar to native starch, but not identical. They only have a double helix structure at the entangled regions, but do not have a hilum and radial distribution like native starch granules.
Amylose automatically stick together to form a network that provide hydrogel with elasticity and strength against deformation. It promotes the initial stage of starch retrogradation.
Although side chains in amylopectin sometimes hold together by hydrogen bonds, it's difficult for them to form a stretchy web, just as you would have a hard time to build a web via branched sticks. Therefore, their retrogradation of is slower.