Starch or amylum ( chemical formula (C6H10O5)n) is a polysaccharide carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. Starch is produced by all green plants as an energy store and is a major food source for humans.
Pure starch is a white, tasteless and odorless powder that is insoluble in cold water or alcohol. It consists of two types of molecules: the linear and helical amylose and the branched amylopectin. Depending on the plant, starch generally contains 20 to 25% amylose and 75 to 80% amylopectin. Glycogen, the glucose store of animals, is a more branched version of amylopectin.
The word "starch" is derived from Middle English sterchen, meaning to stiffen, which is appropriate since starch can be used as a thickening or glueing agent when dissolved in water and heated, giving wheatpaste.
Starch as food
Starch is the most important carbohydrate in the human diet. The major sources of starch intake worldwide are rice, wheat, maize (corn), potatoes and cassava. Widely used prepared foods containing starch are bread, pancakes, cereals, noodles, pasta and tortilla.
Depending on the local climate other starch sources are used, such as arrowroot, arracacha, buckwheat, barley, oat, millet, rey, banana, malanga, kudzu, oca, sago, sorghum, sweet potato, canna, taro and yams. Chesnuts and edible beans, such as favas, lentils, mung bean and peas, are also rich in starch.
The starch industry extracts and refines starches from seeds, roots and tubers, by wet grinding, sieving and drying. Today, the main commercial refined starches are cornstarch, tapioca, wheat and potato starch. To a much lesser extent, sources include sweet potato, sago and mung bean. Historically, Florida arrowroot was also commercialized. Still starch is extracted from more than 50 types of plants.
Alpha-amylases are the enzymes that break down starch into the constituent sugars. They are found in plants and in animals. Human saliva is rich in amylase, and the pancreas also excretes the enzyme. Individuals from populations with a high-starch diet tend to have more amylase genes than those with low-starch diets; chimpanzees have very few amylase genes. It is possible that turning to a high-starch diet was a significant event in human evolution.[6]
Resistant starch is starch that escapes digestion in the small intestine of healthy individuals.
Starch sugars
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 equivalency (DE), which is roughly the fraction of the glycosidic 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 glucose syrup / 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 with 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 has better handling characteristics, such as microbiological stability, and more consistent sweetness/flavor. High fructose corn syrup has the same sweetness as sugar.
Industrial applications
Papermaking is the largest non-food application for starches globally, consuming millions of metric tons annually. In a typical sheet of copy paper for instance, the starch content may be as high as 8%. Both chemically modified and unmodified starches are used in papermaking. In the wet part of the papermaking process, generally called the “wet-end”, the starches used are cationic and have a positive charge bound to the starch polymer. These starch derivatives associate with the anionic or negatively charged paper fibers / cellulose and inorganic fillers. Cationic starches together with other retention and internal sizing agent help to give the necessary strength properties to the paper web to be formed in the papermaking process (wet strength), and to provide strength to the final paper sheet (dry strength).
In the dry end of the papermaking process the paper web is rewetted with a starch based solution. The process is called surface sizing. Starches used have been chemically, or enzymatically depolymerized at the paper mill or by the starch industry (oxidized starch). The size - starch solutions are applied to the paper web by means of various mechanical presses (size press). Together with surface sizing agent the surface starches impart additional strength to the paper web and additionally provide water hold out or “size” for superior printing properties. Starch is also used in paper coating as one of the binders for the coating formulation a mixture of pigments, binders and thickeners. Coated paper has improved smoothness, hardness, whiteness and gloss and thus improves printing characteristics.
Corrugated board adhesives are the next largest application of non-food starches globally. Starch glues are mostly based on unmodified native starches plus some additive as borax and caustic soda. Part of the starch is gelatinized to carrier slurry of uncooked starches and prevent sedimentation. This opaque glue is called a SteinHall adhesives. The glue is applied on tips of the fluting. The fluted paper is pressed to paper called liner. This is then dried under high heat, which causes the rest of the uncooked starch in glue to swell/gelatinize. This gelatinizing makes the glue a fast and strong for corrugated board production.
Another large non-food starch application is in the construction industry where starch is used in the gypsum wall board manufacturing process. Chemically modified or unmodified starches are added to the stucco containing primarily gypsum. Top and bottom heavyweight sheets of paper are applied to the formulation and the process is allowed to heat and cure to form the eventual rigid wall board. The starches act as a glue for the cured gypsum rock with the paper covering and also provide rigidity to the board.
Adhesives - Starch is used in the manufacture of various glues for book-binding, wallpaper adhesives, paper sack production, tube winding, gummed paper, envelop adhesives, school glues, bottle labeling.
Starch derivatives as yellow dextrins can be modified by addition of some chemical forms to be a hard glue for paper work, some of those forms are Borax, Soda Ash, which mixed with the starch solution at 50-70 °C to gain a very good adhesive, Sodium Silicate can be added to reinforce this formula.
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 practical purposes as well. Dirt and sweat from a person's neck and wrists would stick to the starch rather than to the fibers of the clothing, and would easily wash away along with the starch. After each laundering, the starch would be reapplied. Today the product is sold in aerosol cans for home use.
Starch is also used to make some packing peanuts, and some dropped ceiling tiles.
Textile chemicals - To reduce breaking of yarns during weaving, the warp yarns are sized. Starch is one of the main agents used for cotton sizing. Starch is also used as printing thickener.
Printing industry - in the printing industry food grade starch is used in the manufacture of anti-set-off spray powder used to separate printed sheets of paper to avoid wet ink being set off.
Bioplastics - starch is used to produce various bioplastics, as polylactic acid, synthetic polymers that are biodegradable.
Body powder - Powdered corn starch is used as a substitute for talcum powder in many health and beauty products.
Oil exploration - starch is used to adjust the viscosity of drilling fluid which is used to lubricate the drill head in (mineral) oil extraction.
Bio-ethanol - Glucose from starch can be further fermented to ethanol.
Polyols - maltitol, erythritol, sorbitol, mannitol
Hydrogen production - Starch can be used to produce hydrogen, using enzymes.
Tests
Iodine solution is used to test for starch; a blue color indicates the presence of starch. 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. The strength of the resulting blue color depends on the amount of amylose present.
Starch indicator solution consisting of water, starch and iodine is often used in redox titrations: in the presence of an oxidizing agent the solution turns blue, in the presence of reducing agent the blue color disappears because triiodide (I3−) ions break up into three iodide ions, disassembling the starch-iodine complex. A 0.3% w/w solution is the standard concentration for a starch indicator. 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).
Microscopy of starch granules - Each species of plant has a unique shape of starch granules in granular size, shape and crystallisation pattern. Under the microscope, starch grains stained with iodine illuminated from behind with polarized light show a distinctive Maltese cross effect (also known as extinction cross and birefringence).
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