Gelatinization and Retrogradation

Starch is a polysaccharide which means containing large number of glucose units bind by glycosidic bond. Starch is mostly found in plant tissue in tubers and endosperm of seeds. Starch consists of two types of glucose polymer which are amylose and amylopectin. While amylose is a linear glucan with α[1,4] glycosidic linkages, amylopectin is highly branched glucan with α[1,4] and α[1,6] linkages. They occur together in granules but amylase may be separated from solutions because it is less soluble in organic solvents. Most starches contain 20-25% amylose.

Undamaged starch granules are not soluble in cold water because of the hydrogen bonds binding the chains together which also have collective strength. However, as the temperature increases to initial gelatinization temperature water begins to be absorbed. The initial gelatinization temperatures are different for different starches but generally lie in the range 55-70°C. As water is absorbed the granules swell and these swollen granules begin to collide on each other, the viscosity of solution rises dramatically. The amylose molecules are leached out of the swollen granules and contribute to the viscosity what is best described as a paste. If heating is done with stirring viscosity begins to fall again as the integrity of the granules is destroyed by the help of the physical effects of mixing. When the paste is allowed to cool the viscosity rises again because the hydrogen bond attractions between amylose and amylopectin are re-established and it gives a gel like consistency to the solution. 

When starch solutions or pastes are allowed to stand for a few hours, they begin to show changes in their rheological properties. Dilute solutions become less viscous while in concentrated solutions viscosity increase and the solution expels water. These changes are explained due to the phenomenon called retrogradation. (Coultate, 2002) Retrogradation enhances drastically at temperatures between -8 and +8°C. Expelling water from the polymer network process is called syneresis. Retrogradation is directly related to aging of bread.