starch

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starch,

white, odorless, tasteless, carbohydrate powder. It plays a vital role in the biochemistry of both plants and animals and has important commercial uses. In green plants starch is produced by photosynthesisphotosynthesis
, process in which green plants, algae, and cyanobacteria utilize the energy of sunlight to manufacture carbohydrates from carbon dioxide and water in the presence of chlorophyll. Some of the plants that lack chlorophyll, e.g.
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; it is one of the chief forms in which plants store food. It is stored most abundantly in tubers (e.g., the white potato), roots (e.g., the sweet potato), seeds, and fruits; it appears in the form of grains that differ in size, shape, and markings in various plants. The plant source can usually be identified by microscopic examination of the starch grains. Starch obtained by animals from plants is stored in the animal body in the form of glycogenglycogen
, starchlike polysaccharide (see carbohydrate) that is found in the liver and muscles of humans and the higher animals and in the cells of the lower animals. Chemically it is a highly branched condensation polymer of glucose; it is readily hydrolyzed to glucose.
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. Digestive processes in both plants and animals convert starch to glucoseglucose,
 dextrose,
or grape sugar,
monosaccharide sugar with the empirical formula C6H12O6 . This carbohydrate occurs in the sap of most plants and in the juice of grapes and other fruits.
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, a source of energy. Starch is one of the major nutrients in the human diet. Its presence in foods and other substances can be detected by the blue-black color produced when iodine solution is added to a sample of the material to be tested. By treatment with hot water, starch granules have been shown to consist of at least two components, known as amylopectin and amylose. Amylopectin is a branched glucose polymer; amylose is a linear glucose polymer. Commercially starch is prepared chiefly from corn and potatoes. Starch is widely used for sizing paper and textiles, for stiffening laundered fabrics, in the manufacture of food products, and in making dextrindextrin,
any one of a number of carbohydrates having the same general formula as starch but a smaller and less complex molecule. They are polysaccharides and are produced as intermediate products in the hydrolysis of starch by heat, by acids, and by enzymes.
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. In addition to its other uses, cornstarchcornstarch,
material made by pulverizing the ground, dried residue of corn grains after preparatory soaking and the removal of the embryo and the outer covering. It is used as laundry starch, in sizing paper, in making adhesives, and in cooking.
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 is a source of corn syrup, of which large quantities are used in making table syrup, preserves, ice cream, and other confections. Corn sugar (glucose) is also derived from cornstarch. See also arrowrootarrowroot,
any plant of the genus Maranta, usually large perennial herbs, of the family Marantaceae, found chiefly in warm, swampy forest habitats of the Americas and sometimes cultivated for their ornamental leaves.
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.

Starch

 

the main storage carbohydrate of plants. Starch is formed in cellular organisms in chloroplasts and amyloplasts and is accumulated primarily in seeds, bulbs, and tubers but also in leaves and stems. It is stored away in cells in the form of granules that contain small amounts of proteins and lipids. The starch granules in various plant types differ in size and shape— the largest granules are in the potato (average diameter, approximately 33 μ) and the smallest are in rice (approximately 15 μ)—and have a laminar structure. The origin of starch granules can be determined by examination of their shape under a microscope.

Starch is a mixture of two polysaccharides, linear amyloses and branched amylopectins, which both have the general formula (C6H10O5)n. As a rule, the amylose content in starch is 10-30 percent and the amylopectin content is 70–90 percent. Starch polysaccharides are constructed of glucose residues bound in amylose and in linear amylopectin chains by cross α-l,4-glucoside bonds and at branch points by α-l,6-glucoside bonds. (On the average, approximately 1,000 glucose residues are bound in amyloses, and the individual linear segments of amylopectin molecules consist of 20–30 such units.)

The characteristic blue color of starch produced by an iodine solution (iodine reaction) is used for its identification. Polysaccharides with a reduced degree of polymerization, known as dextrins, are produced in the partial acid hydrolysis of starch. Complete hydrolysis leads to glucose. Enzymatic decomposition of starch may be carried out in various ways. In the presence of inorganic phosphate, plant phosphorylase breaks the α-1,4 bonds with the formation of glucoso-1-phosphate, thus converting starch from its storage form to a metabolically active form.

The enzymes a- and β-amylases, widespread in nature, also break only α-1,4 bonds: β-amylase effects cleavage to maltoses and dextrins, and α-amylase is capable of bypassing the branch points and completely breaking down starch into low-molecular-weight products (for instance, maltose and glucose).

The cleavage of α-1,6 bonds with the formation of free glucose is catalyzed by amylo-l,6-glucosidase. Glucoamylase, an enzyme found in molds, cleaves starch down to glucose. The final products of the enzymatic cleavage of starch are glucose and glucoso-1-phosphate; they are very important substrates for both energetic exchange and biosynthetic processes. The biosynthesis of unbranched starch chains is carried on using glucosyl-trans-ferases that catalyze the transfer of the glucose residue from nucleoside diphosphate glucose to the growing carbohydrate chain. The “branching” Q enzyme transfers the end glucose residue from the major chain to a side chain with the formation of the α-1,6 bond in amylopectins. The starting substrate in starch biosynthesis in plants may be saccharose.

Starch is a basic component of the most important food products. For example, starch makes up 75–80 percent of flour and 25 percent of potatoes. It is easily digested in the gastrointestinal tract and has a high caloric content: 16.75 kilojoules per g (approximately 4 kilocalories per g). Starch and its products are used in the production of paper, textiles, and glues, as well as in foundry casting and other industrial fields.

Starch is added to powders, salves, and pastes. It is used in 1-percent solutions as an indicator of iodine. Starch is also used as a coating (sizing and boiled starch). Capsules are prepared from a mixture of starch (or wheat flour) and starch paste.

REFERENCES

Khimiia i tekhnologiia krakhmala, 2nd ed. Moscow, 1956. (Translated from English.)
Khimiia uglevodov. Moscow, 1967.
Stepanenko, B. N. Uglevody: Uspekhi ν izuchenii stroeniia i metabolizma. Moscow, 1968.

D. M. BELEN’KII

starch

[stärch]
(biochemistry)
Any one of a group of carbohydrates or polysaccharides, of the general composition (C6H10O5) n, occurring as organized or structural granules of varying size and markings in many plant cells; it hydrolyzes to several forms of dextrin and glucose; its chemical structure is not completely known, but the granules consist of concentric shells containing at least two fractions: an inner portion called amylose, and an outer portion called amylopectin.

starch

1. a polysaccharide composed of glucose units that occurs widely in plant tissues in the form of storage granules, consisting of amylose and amylopectin
2. a starch obtained from potatoes and some grain: it is fine white powder that forms a translucent viscous solution on boiling with water and is used to stiffen fabric and in many industrial processes
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