Question

Starch structure questions

1) Which feature of amylose leads to increased intensity of the blue colour of the amylose iodine complex?

2) What part of the DSC endotherm gives the value of Gelatinisation Temperature?

3) When starch is cooled, what are the two processes that occur?

Answer 1

1. Amylose is one of the main components of starch, along with amylopectin. It is usually a complex helical molecule capable of trapping other ions within its structure. When a solution of Iodine (dissolved in potassium iodide) is added to amylose, the polyiodide gets trapped within the complex. The original colour of the solution is brownish-orange. When trapped within amylose, there is a transfer of electrons , in such a way that amylose donates electrons and the iodide complex accepts them. This complex absorbs light of a different wavelength than polyiodide, which results in the colour turning blue.

2. Starch gelatinization is the process where there is breaking of intermolecular bonds in the presence of heat and water. This allows the H-groups to engage with water molecules, thereby aiding the irreversible dissolution of starch granules in water. The gelatinization temperature can be studied with the help of Differential Scanning Calorimetry (DSC). Most research state the gelatinization onset temperature of cereal starches between 59C and 72C.

3. Cooling of starch leads to starch retrogradation, in which there is realignment of amylose and amylopectin molecules to form more linear molecules. The second process is syneresis where the water is expelled from the liquid leading to formation of a gel.