Starch
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Potato starch

Starch , common name applied to a white, granular or powdery, odourless, tasteless, complex carbohydrate, (C6H10O5)x, abundant in the seeds of cereal plants and in bulbs and tubers. Molecules of starch are made of hundreds or thousands of atoms, corresponding to values of x, as given in the formula above, that range from about 50 to many thousands. Native starch denotes untreated starch.

amylose.gif (8424 bytes)AMYLOSE AND AMYLOPECTIN Normal native starches consist of a mixture of 15-30 per cent. amylose and 70-85 per cent. amylopectin. Amylose approximately between 40 000 and 340 000, the chains containing 250 to 2000 anhydroglucose units. Amylopectin is considered to be composed of anhydroglucose chains with many branch points; the molecular weight may reach as high as 80 000 000 (Re. WHO). Amylose is an unbranched chain which is coiled in the shape of a helix. If iodine is added to a solution containing amylose molecules, the iodine inserts itself into the helix making it rigid. This changes the color of the starch mixture to blue or purple depending on the length of the amylose molecule. Amylopectin is a branching molecule which does not form a helical coil. Thus the iodine is not able to bind to the starch molecule. Amylose contributes to the gelling property of starch whereas amylopectin contributes high viscosity. This classic statement, however, may not be entirely valid. Both properties are used in the preparation of foods.

In amylose the C6H10O5 groups are arranged in a continuous but curled chain somewhat like a coil of rope; in the second kind, amylopectin, considerable side-branching of the molecule occurs.

Starch is manufactured by green plants during the process of photosynthesis. It forms part of the cell walls in plants, constitutes part of rigid plant fibres, and serves as a kind of energy storage for plants, because its oxidation to carbon dioxide and water releases energy. The granules of starch present in any plant have size, shape, and markings characteristic of the species of plant in which the starch is made.

Starch is almost insoluble in cold water and in alcohol, but with boiling water it gives a colloidal suspension that may form a jelly on cooling. Hot water changes starch slowly into smaller molecules. This reaction, an example of hydrolysis, is catalyzed by acids and by some enzymes giving still simpler molecules, the ultimate products being maltose, C12H22O11, a disaccharide, and glucose,  C6H12O6, a monosaccharide.

The digestion of starch in the human body takes the following course: the hydrolysis begins in the mouth under the action of salivary ptyalin, but is completed in the small intestine. The body does not immediately use all the glucose absorbed from the digestion of starch, but converts much of it to glycogen, which is stored in the liver. (Glycogen, called animal starch, has a structure nearly identical with that of amylopectin.) As the body requires glucose, hydrolysis of glycogen releases it into the bloodstream. Glycogen provides an energy reserve for animals in the same way that ordinary starch does for plants.

Amylopectin (Fig. 2):

amylopectin.gif (6000 bytes)

Amylose Content and Granule Size of Various Starches

Starch Source % Amylose Granule Size Range (m) Average size m
Waxy Rice 0 2 - 15 6
High Amylose Corn 70 4 - 20 10
Corn 28 5 - 25 14
Cassava 17 3 - 30 14
Waxy Sorghum 0 - -
Wheat 26 3 - 35 7 and 20
Sweet Potato 18 4 - 40 19
Arrowroot 21 9 - 40 23
Sago 26 15 - 50 33
Potato 20 10 - 100 36