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Starch is a complex carbohydrate which is insoluble in water, it is used by plants as a way to store excess glucose. Starch (in particular cornstarch) is used in cooking for thickening sauces. In industry, it is used in the manufacture of adhesives, paper, and textiles. It is a white powder, and is tasteless and odorless.
Biochemically, starch is a combination of two polymeric carbohydrates (polysaccharides) called amylose and amylopectin. Amylose is constituted by glucose monomer units joined to one another head-to-tail forming alpha-1,4 linkages. Amylopectin differs from amylose in that branching occurs, with an alpha-1,6 linkage every 24-30 glucose monomer units. The overall structure of amylopectin is not that of a linear polysaccharide chain since two glucose units frequently form a branch point, so the result is the coiled molecule most suitable for storage in starch grains. Both amylopectin and amylose are polymers of glucose, and a typical starch polymer chain consists of around 2500 glucose molecules in their varied forms of polymerisation. In general, starches have the formula (C6H10O5)n, where "n" denotes the total number of glucose monomer units.
Structurally, the starch forms clusters of linked linear polymers, where the alpha-1,4 linked chains form columns of glucose units which branch regularly at the alpha-1,6 links. The relative content of amylose and amylopectin varies between species, and between different cultivars of the same species. For example, high-amylose corn (maize) has starch consisting of about 85% amylose, which is the linear constituent of starch, while waxy corn starch is more than 99% amylopectin, or branched starch. The primary function of starch in plants, is to act as an energy storage molecule for the organism. In plants simple sugars are linked into starch molecules by specialized cellular organs called amyloplasts.
Starches are insoluble in water. They can be digested by hydrolysis, catalyzed by enzymes called amylases, which can break the glycosidic bonds between the 'alpha-glucose' components of the starch polysaccharide. Humans and other animals have amylases, so they can digest starch. Digestion of starches consists of the process of the cleavage of the starch molecules back into their constituent simple sugar units by the action of the amylases. The resulting sugars are then processed by further enzymes (such as maltase) in the body, in the same manner as other sugars in the diet.
Starches as food
Starch is often found in the fruit, seeds, rhizomes or tubers of plants. The four major resources for starch production and consumption in the USA are corn, potatoes, rice, and wheat. Pasta is an important dietary source of starch which is commonly prepared from wheat, rice or beans. Bread is another important source of starch and is commonly prepared from wheat.
As an additive for food processing, arrowroot, guar gum, locust bean, and tapioca are commonly used as well. Commonly used starches around the world are: arracacha, buckwheat, banana, barley, cassava, konjac, kudzu, oca, sago, sorghum, sweet potato, taro and yams. Edible beans, such as favas, lentils and peas, are also rich in starch.
When starch is used in cooking it is normally prepared with ingredients such as lemon, tomato, vinegar, hot pepper, onion or garlic to change its characteristic 'starchiness.' An example of this would be the use of ketchup or vinegar in the presentation of french fries or chips.
When a starch is pre-cooked it can then be used to thicken chilled foods. This is referred to on packaging as modified food starch. Agar, carrageenan, gelatins and pectins are used in very much the same way.
Clothing starch or laundry starch is a liquid that is prepared by mixing a vegetable starch in water (earlier preparations also had to be boiled), and is used in the laundering of clothes. During the 19th century and early 20th century, it was stylish to stiffen the collars and sleeves of men's shirts and the ruffles of girls' petticoats by applying starch to them as the clean clothes were being ironed.
Aside from the smooth, crisp edges it gave to clothing, it served a practical purpose as well. Dirt and sweat from a person's neck and wrists would stick to the starch rather than fibers of the clothing, and would easily wash away along with the starch. Then, after each laundering, the starch would be reapplied.
Starch solution is used to test for elemental iodine. A blue/black color indicates the presence of iodine in starch solution. The details of this reaction are not yet fully known, but it is thought that the iodine (I3- and I5- ions) fits inside the coils of amylose, the charge transfers between the iodine and the starch, and the energy level spacings in the resulting complex correspond to the absorption spectrum in the visible light region. A 0.4% w/w solution is the standard concentration for a dilute starch indicator solution. It is made by adding 4 grams of soluble starch to 1 litre of heated water; the solution is cooled before use (starch-iodine complex becomes unstable at temperatures above 35°C). This complex is often used in redox titrations: in presence of an oxidizing agent the solution turns blue, in presence of reducing agent blue color disappears because I5- ions break up into iodine and iodide.
Animal starch is the common name of glycogen. It is not the same as ordinary starch.
Starch can be hydrolyzed into simpler carbohydrates by acids, various enzymes, or a combination of the two. The extent of conversion is typically quantified by dextrose equivalent (DE), which is roughly the fraction of the glycoside bonds in starch that have been broken. Food products made in this way include
- Maltodextrin, a lightly hydrolyzed (DE 10–20) starch product used as a bland-tasting filler and thickener.
- Various corn syrups (DE 30–70), viscous solutions used as sweeteners and thickeners in many kinds of processed foods.
- Dextrose (DE 100), commercial glucose, prepared by the complete hydrolysis of starch.
- High fructose syrup, made by treating dextrose solutions to the enzyme glucose isomerase, until a substantial fraction of the glucose has been converted to fructose. In the United States, high fructose corn syrup is the principal sweetener used in sweetened beverages.
- Jones, Orlando, "US2000 Improvement in the manufacture of starch". (Class: 127/68; 48/119; 127/69). Middlesex, England, USPTO.