Question

Hydrolysis of starch. You were asked to let the starch solution cool. Once the solution had cooled and was back to room temperature you would describe your glass as

		all the starch dissolved in the boiling water
		the excess starch stayed suspended in the solution
		there was a white precipitate on the bottom of the glass
		the excess starch floated on the surface of the water

Answer 1

When starch is put into water and heated, it gets dissolved. This is called gelatinisation. It improves the availability of starch for amylose hydrolysis.

This process is used in cooking to make the starch digestible or to thicken bind water in soup, sauce. Etc.

Gelatinized starch, when cooled for a long period thickens and rearrange itself again to a more crystalline structure, this process is called retrogradation.

During cooling, starch molecules gradually aggregate to form a gel, due to molecular associations like amylose-amylose, amylose-amylopectin, and amylopectin-amylopectin.

Amylose molecules due to hydrogen bonding form a stiff gel. and in the presence of amylopectin a strong gel is formed.

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