Starch: Physical and chemical tests / Analysis of crude drugs
Amylose forms a colloidal dispersion in hot water whereas amylopectin is completely insoluble. The structure of amylose consists of long polymer chains of glucose units connected by an alpha acetal linkage. Starch – Amylose shows a very small portion of an amylose chain. All of the monomer units are alpha -D-glucose, and all the alpha acetal links connect C #1 of one glucose and to C #4 of the next glucose. As a result of the bond angles in the αacetal linkage, amylose actually forms a spiral much like a coiled spring. See the graphic below, which shows four views in turning from the side to an end view
Chemical Test for Starch or Iodine
Amylose in starch is responsible for the formation of deep blue colour in the presence of iodine. The iodine molecule slips inside of the amylose coil. Iodine – KI Reagent: Iodine is not very soluble in water; therefore the iodine reagent is made by dissolving iodine in water in the presence of potassium iodide. This makes a linear triiodide ion complex with is soluble and slips into the coil of the starch causing an intense blue-black colour.
From the above morphological characters and chemical tests, the crude drug is identified as starch.
Kokate CK. Practical Pharmacognosy, 4 edition, VallabhPrakashan. Delhi; 1994: 98
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