Gelatinization and retrogradation
Gelatinization is the process in which starch becomes soluble, binds water and forms a gel. This process makes the starch more easily digestible. The use of starch as a thickening agent is based on this process.
Starch is swelling up by heating and continues to swell up absorbing water and showing more viscosity and clarity along with increase of temperature. At a certain point the maximum viscosity has been reached. By further heating the starch molecules will move further apart and the viscosity is decreasing. Viscosity is gradually increasing again when the solution is cooled down and continuous cooling makes the solution cloudy. Leaving to stand the solution it will form a gel. The strength of the gel is determined by the type and concentration of starch in the product.
When a starch gel is left to stand for some time, the amylose molecules will lose water and bind together. A similar process occurs when starch rich products, such as potatoes, will be stored for a long time. This process of recrystallisation of starch is called retrogradation