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STARCHES

Starch is a carbohydrate consisting of a large number of glucose units joined together by

glycosidic bonds. This polysaccharide is produced by all green plants as an energy

store. It is the most important carbohydrate in the human diet and is contained in suchstaple foods as

Maize (Corn) ------------- Zea mays

Rice ------------------------ Oryza sativa

Wheat ------------------------ Triticum aestivum

Potatoes ----------------------- Solanum tuberosum

Starch constitutes principal form of carbohydrate reserve in the green plant andis to be found especially in sedds and underground organs.

The green parts of plant exposed to sunlight contain small granules of

transitional starch which arise from photosynthesis. During the hours of

darkness these are removed to the storage organs. Starch occurs in the form of

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Amylose reacts with iodine to form a deep blue complex; amylopectin gives a

blue-violet or purple color.

Most of the starches have a similar ratio of amylose to amylopecin, averaging

about 25% of the amylose to 75% of Amylopectin.

Hydrolysis:

Enzymatic Hydrolysis:

The enzymes that break down or hydrolyze starch into the constituent sugars areknown as amylases.

Alpha-amylases are found in plants and in animals. Human saliva is rich

in amylase, and the pancreas also secretes the enzyme.

Beta-amylase cuts starch into maltose units.

Acidic Hydrolysis:

Hydrolysis of starch by mineral acid ultimately produces glucose in

nearly quantitative yields. The course of hydrolysis may be

conveniently followed by the iodine reaction, which changes

successively from blue black to purple to red to no reaction.

Biosynthesis Of Starch

Biosynthesis of Amylose fraction of starch is affected by enzymes known as

transglycosylases. The reaction involve lengthening of priming chains of

identical composition by addition of single glucose residues.

In certain microorganisms, glucose-1-phosphate is the glucose donor, and the

enzyme that catalyzes the transfer is phosphorylase.

Various sugar nucleotides, such as UDP-Glucose and ADP-Glucose, function as

glycosyl donor in higher plants.

The following equations illustrating this reaction show UDP-glucose as the

source of the glucose residues:

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(Glucose)n + UDP-Glucose (Glucose)n+1 + UDP(Glucose)n+1 + UDP-Glucose (Glucose)n+2 + UDP, etc.

Biosynthesis of Amylopectin, the branched component of starch is affected by

the action of transglycosylase designated Q-enzyme.

This enzyme affects splitting of a monosaccharide chain containing at least 40

glucose units into two fragments.

Preparation of Starch

From Maize ,

From Rice,

From Potato,

From Wheat

Please See Text Book of Pharmacognosy by M. ALI (Section on Preparation ofStarch)

Uses:

Dusting Powder Emolient Antidote for Iodine poisoning Tablet Disintegrant, Binder, Diluent and Bulk former Sterilized maize starch is used as a lubricant for surgeon gloves; it is maize

starch subjected to physical and chemical treatments so that it does notgelatinize on exposure to moisture of steam sterilization. Unlike talc, it iscompletely absorbed by Body Tissues.

Please Also See Text Book of Pharmacognosy by M. ALI (Section on Uses ofStarch)

Tests for Identification:

Molischs test & Fehling Solution Test Positive.Deep blue color wit Iodine thatdisappears on heating and reappears on cooling.