Starch

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 photosynthesis; 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 glycogen. Digestive processes in both plants and animals convert starch to glucose, 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 dextrin. In addition to its other uses, cornstarch 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 arrowroot.

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starch

Any of several white, granular organic compounds produced by all green plants. They are polysaccharides with the general chemical formula (C₆H₁₀O₅)_n, where n may range from 100 to several thousand; the constituent monosaccharides are glucose units made in photosynthesis. The glucose chains are unbranched in amylose and branched in amylopectin, which occur mixed in starches. Starch consumed by animals is broken down into glucose by enzymes during digestion. Commercial starch is made mainly from corn, though wheat, tapioca, rice, and potato starch are also used. Starch has many uses in foods and the food industry, as well as in the paper, textile, and personal-care products industries and in adhesives, explosives, and oil-well drilling fluids and as a mold-release agent. Animal starch is another name for glycogen. See also carbohydrate.

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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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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.

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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