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Starch chemically a homoglycan is a complete carbohydrate i.e. polysaccharide which consists of a mixture of amylose and amylopectin in a proportion of 1:4. Amylase is a branched chain polymer of 500:200 glucose subunit with only α-1,4 glycosidic linkage. It has a molecular weight from 1000 to 50,000.

Amylopectin has over 1000 glucose nucleotides and its molecular weight is about 1,00,000 to 2,00,000 and has branched structure. It consists of glucose units linked by α-1,4-glycosidic linkage and branching in amylopectin is approximately one per 25 glucose units in the unbranched segments. Amylopectin part is helically coiled structure due to α-1,6 glycosidic linkage one helix consists of 25-35 glucose molecules.

Starch is generally insoluble in water at room temperature because of this starch in nature, it is stored in cells as small granules which can be seen under microscope. Starch on hydrolysis yields lower molecular weight polysaccharide and finally glucose and maltase.


i) Bunsen Burner 

ii) Culture 

iii) Inoculating loop 

iv) Dropper


i) The given starch agar plate was differentiated into two part and labelled each part with the name of organism to be tasted:

ii) Each microorganism was inoculated in the broth part & made a single streak.

iii) The inoculated plate was incubated at 370 c for 24 hrs.

iv) After 24 hrs the surface was flooded with iodine for 30 secs.


S.NSampleMedium usedReagentChange in colourResult
1.E1% Starch agarGram’s IodineBlue-ve starch hydrolysis
2.B1% Starch agarGram’s IodineClear Zone+ve starch hydrolysis


Macromolecules have higher molecular weight. So it can’t penetrate the cell. So it should be hydrolysed into smaller fragments of individual glucose by action of enzymes secreted by moist then enter into the cell. These organisms which produce no enzyme amylase breakdown the starch into maltose 1, 2 and the same monosaccharide like glucose. The smaller fragments enter the bacterial cell which are further metabolised in the presence of endoenzymes. Thus, they don’t give colour. The deep colour of the medium is done due to the action between amylase part of starch & iodine.


Hence, the starch hydrolysis test was performed and given Organisms were differentiated.


i) Iodine should be poured just enough to wet the surface.

ii) Hot loop shouldn’t be used for maculation.


Shah PK, Amatya J, Jahal PR (2007) “Practical Microbiology” Dalta Offset Page 130-132.

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