Starch

Starch is a complex carbohydrate, specifically a polysaccharide, that is used by plants as a way to store glucose. After cellulose, it is the most abundant polysaccharide in plant cells. In pure form, starch is insoluable in cold water. Animals, plants, and humans digest starch, converting it to glucose as a source of energy.

While in general starch appears in plants as partially crystalline granules, the exact form, size, and shape varies according to their origin (Sengbusch 2003). For example, starch grains from potato tubers have a diameter of 70 - 100 µm, that of the endosperm of wheat 30 - 45 µm, and that of corn endosperm 12 - 18 µm. In prepared form, starch is usually a white powder, and, depending on the source, may be tasteless and odorless.

Starch is a major source of carbohydrates in human diets, and can be obtained form seeds, fruits, nuts, corn, potatoes, and so forth. In prepared form, starch (in particular cornstarch) is used in cooking for thickening sauces. In industry, it is used in the manufacturing of adhesives, paper, textiles and as a mold in the manufacture of sweets such as wine gums and jelly babies.

Chemistry

Carbohydrates are a class of biological molecules that contain primarily carbon (C) atoms flanked by hydrogen (H) atoms and hydroxyl (OH) groups (H-C-OH). As a polysaccharide, starch is a large polymer composed of a great number of monosaccharides.

Starch is the major polysaccharide in higher plants used for storage of carbohydrate. The othe major polysaccharide used in plant cells is cellulose, which is a structural polysaccharide. the major storage polysaccharide in animals is glycogen.

C_n(H₂O)_m.

As with cellulose and glycogen, start contains the six-carbon sugar glucose as its single repeating unit. However, these three polysaccharaides differ in the type of bond between glucose units and the presence and extent of side branches on the chains.

Starch is a polysaccharide of glucose with α-1,4 glycosidic linkages.

Glycogen is a highly branched polysaccharide of glucose with α-glycosidic linkages. α-1,6 glycosidic linkages produce branching at carbon 6.

Cellulose is an unbranched polysaccharide of glucose with β-1,4 glycosidic linkages that are chemically very stable.

Starch occurs as both unbranched amylose and branched amylopectin. Like glycogen, amylopectin has α-1,6 branches, but these occur less frequently along the helical backbone (once every 12 to 25 glucose units), producing longer side chains (lengths of 20 to 25 glucose units). Starch readily binds water, and when that water is removed, polysaccharide chains aggregate, forming hydrogen bonds. This bonding is what causes bread to become hard and stale. The addition of water and gentle heat softens the bread by separating the polysaccharide chains. Since branching limits the number of hydrogen bonds that can form between molecules, solid deposits of the highly-branched glycogen are more compact than those of starch. Starch deposits are generally about 10-30 percent amylose and 70-90 percent amylopectin.

While mammals cannot use cellulose, such as grass, as a food, they can digest starch (such as in potatoes).

Starches as food

As an additive for food processing, arrowroot and tapioca are commonly used as well. Commonly used starches around the world are: arracacha, buckwheat, banana, barley, cassava, kudzu, oca, sago, sorghum, sweet potato, taro and yams. Edible beans, such as favas, lentils and peas, are also rich in starch.

When a starch is pre-cooked, it can then be used to thicken cold foods. This is referred to as a pregelatinized starch. Otherwise starch requires heat to thicken, or "gelatinize." The actual temperature depends on the type of starch.

A modified food starch undergoes one or more chemical modifications that allow it to function properly under high heat and/or shear frequently encountered during food processing. Food starches are typically used as thickeners and stabilizers in foods such as puddings, custards, soups, sauces, gravies, pie fillings, and salad dressings, but have many other uses.

Non-food applications

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. Starch was widely used in Europe in the 16th and 17th centuries to stiffen the wide collars and ruffs of fine linen which surrounded the necks of the well-to-do. 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. Chefs have traditionally worn starched uniforms because the starch acts as a fire retardant. Then, after each laundering, the starch would be reapplied.

Starch glues are widely used in the bonding of paper, wood and cotton.

Use as a mould

Gummed sweets such as jelly babies and wine gums are not manufactured using a mould in the conventional sense. A tray is filled with starch and levelled. A positive mould is then pressed into the starch leaving an impression of 100 or so jelly babies. The mix is then poured into the impressions and then put into a stove to set. This method greatly reduces the number of moulds that must be manufactured.

Tests

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 0.3% 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.

Under the microscope, starch grains show a distinctive Maltese cross effect (also known as 'extinction cross' and birefringence) under polarized light.

whoever wrote the above is an idiot starch solution doesnt test for iodine iodine tests for starch also when iodine forms ions they arent iodide- thats what u call iodine when it is attached to something else like calcium iodide iodine ions are just that iodine ions

Starch derivatives

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 because fructose tastes sweeter than glucose, and less sweetener may be used.

References

ISBN links support NWE through referral fees

Sengbusch, P. V. 2003. The Structure of a Plant Cell. (accessed December 6, 2006).

External links

• Jones, Orlando, "US2000 Improvement in the manufacture of starch". (Class: 127/68; 48/119; 127/69). Middlesex, England, USPTO.

• Detailed description and pictures of starch molecular structure: http://www.lsbu.ac.uk/water/hysta.html

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