Starch is the main contributor in our diet. It exist exclusively in plants, stored in the seeds, roots, and fibres as food reserve. The primary sources of starch are cereals, corn, potatoes, and rice.
Starch occurs normally as small globules enclosed in cellular tissue. These are soaked in water and crushed. The pulp thus obtained is carried by a stream of water on to a sieve. The starch is separated from the liquid moving through the sieve by decantation, and dried.
Starch is actually a mixture of two structurally differently polysaccharides, Amylose (20%) and amylopectin (80%). When starch is heated with hot water, it can be separated into these components. The part that is soluble in water is amylase and remaining fraction is amylopectine. Both amylase and amylopectin are composed of D-glucose unites. The amylase molecules is made up of D-glucose units jointed by α-glycosidic linkages between C-1 of one glucose unit and C-4 of the next glucose unit. The number of D-glucose units in amylase ranges from 60-300.
Amylopectin has a branched-chain structure. It is composed of chains 25 to 30 D-glucose units joined by α-glycosidic linkages between C-1 of one glucose unit and C-4 of the next glucose unit. These chains are in turn connected to each other by 1,6-linkages. The number of D-glucose units in amylopectin ranges from 300 to 6000.
Starch is a colourless amorphous. Powder having no definite melting point. It gives colloidal solution in water.
- Hydrolysis. The hydrolysis of starch with hot dilute acids yields a syrupy combination of glucose. Maltose, and high-molecular weight saccharides. The combination is called Dextrin and marketed as Corn syrup. The digestive enzymes in humans convert starch ultimately to glucose which is the source of energy. In the presence of the enzyme amylase ( diastase) starch is converted to maltose, an important step in the manufacture of alcohol from starch.
- Iodine Test. It is amylase which gives a blue colour when treated with iodine. It is believed that amylase forms a helical structure and iodine is lodged in it.Starch does not reduce Tollens reagent and Fehling’s solution. It does not form an osazone with phenylhydrazine.