Granule swelling

Starches are quite hydrophilic and the granules swell in water to several times their volume. In the preparation of starches for coating, the granules are heated at 93°C to ensure complete breakdown. The starch is then mixed with the pigment while it is fluid. Normal practice is to coat the paper at an elevated temperature to help control viscosity. 

Hydroxyethyl group introduction at low DS results in distinct modification of physical properties. Among these are decreased gelatinization temperature range (126), increased granule swelling rate (127), and decreased abiUty of starch pastes to gel and retrograde. 

When starch is heated in excess water, the crystalline structure is disrupted (due to breakage of hydrogen bonds) and water molecules become linked by hydrogen bonding to the exposed hydroxyl groups of amylose and amylopectin. This causes an increase in granule swelling... 

Monitor the gelatinization microscopically. Gelatinized granules swell and lose the blue-black staining characteristics with iodine in potassium iodide. 

The appearance of different kinds of starch under the microscope is characteristic. Starch is insoluble in cold water, but when heated the granules swell up and burst their contents (granulose) dissolve and the cell walls remain in suspension, giving an opalescent solution. 

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