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Starch or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by . This polysaccharide is produced by most green for energy storage. Worldwide, it is the most common carbohydrate in human diets, and is contained in large amounts in such as wheat, , maize (corn), rice, and cassava (manioc).
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 helix amylose and the branched amylopectin. Depending on the plant, starch generally contains 20 to 25% amylose and 75 to 80% amylopectin by weight. (2025). 9780471444510, Wiley. ISBN 9780471444510 Glycogen, the energy reserve of animals, is a more highly branched version of amylopectin.
In industry, starch is often converted into sugars, for example by . These sugars may be fermentation to produce ethanol in the manufacture of beer, whisky and biofuel. In addition, sugars produced from processed starch are used in many processed foods.
Mixing most starches in warm water produces a paste, such as wheatpaste, which can be used as a thickening, stiffening or gluing agent. The principal non-food, industrial use of starch is as an adhesive in the papermaking process. A similar paste, clothing or laundry starch, can be applied to certain textile goods before ironing to stiffen them.
Modern German Stärke (strength, starch) is related and refers to the main historical applications, its uses in textiles: sizing yarn for weaving, and starching linen.
The Greek language term for starch, amylon (ἄμυλον), which means "not milled", is also related. It provides the root , which is used as a prefix for several carbon compounds related to or derived from starch (e.g. amyl alcohol, amylose, amylopectin).
Pure extracted wheat starch paste was used in Ancient Egypt, possibly to glue papyrus.Pliny the Elder, The Natural History (Pliny), Book XIII, Chapter 26, The paste used in preparation of paper The extraction of starch is first described in the Natural History of Pliny the Elder around 77–79 Common Era.Pliny the Elder, The Natural History (Pliny), Book XIII, Chapter 17, [2] Romans used it also in Cosmetics creams, to powder the hair and to Thickening agent sauces. Persians and Indians used it to make dishes similar to gothumai wheat halva. Rice starch as surface treatment of paper has been used in paper production in China since 700 CE. In the mid eighth century production of paper that was with wheat starch started in the Arabic world. Laundry starch was first described in England in the beginning of the 15th century and was essential to make 16th century ruffed collars.
Green algae and land-plants store their starch in the , whereas red algae, , , and the parasitic apicomplexa store a similar type of polysaccharide called floridean starch in their cytosol or periplast.
Especially when hydrated, glucose takes up much space and is osmosis active. Starch, on the other hand, being insoluble and therefore osmotically inactive, can be stored much more compactly. The semicrystalline granules generally consist of concentric layers of amylose and amylopectin which can be made bioavailable upon cellular demand in the plant.
Amylose consists of long chains derived from glucose molecules connected by α-1,4-glycosidic bond. Amylopectin is highly branched but also derived from glucose interconnected by α-1,6-glycosidic bond The same type of linkage is found in the animal reserve polysaccharide glycogen. By contrast, many structural polysaccharides such as chitin, cellulose, and peptidoglycan are linked by glycosidic bond, which are more resistant to hydrolysis.
Some cultivated plant varieties have pure amylopectin starch without amylose, known as waxy starches. The most used is waxy corn, others are glutinous rice and waxy potato starch. Waxy starches undergo less retrogradation, resulting in a more stable paste. A maize cultivar with a relatively high proportion of amylose starch, amylomaize, is cultivated for the use of its gel strength and for use as a resistant starch (a starch that resists digestion) in food products.
The biochemical pathway involves conversion of glucose 1-phosphate to ADP-glucose using the enzyme glucose-1-phosphate adenylyltransferase. This step requires energy in the form of ATP. A number of starch synthase available in plastids then adds the ADP-glucose via α-1,4-glycosidic bond to a growing chain of glucose residues, liberating ADP. The ADP-glucose is almost certainly added to the non-reducing end of the amylose polymer, as the UDP-glucose is added to the non-reducing end of glycogen during Glycogenesis.Nelson, D. (2013) Lehninger Principles of Biochemistry, 6th ed., W.H. Freeman and Company (p. 819) The small glucan chain, further agglomerate to form initials of starch granules.
The biosynthesis and expansion of granules represent a complex molecular event that can be subdivided into four major steps, namely, granule initiation, coalescence of small granules, phase transition, and expansion. Several proteins have been characterized for their involvement in each of these processes. For instance, a chloroplast membrane-associated protein, MFP1, determines the sites of granule initiation. Another protein named PTST2 binds to small glucan chains and agglomerates to recruit starch synthase 4 (SS4). Three other proteins, namely, PTST3, SS5, and MRC, are also known to be involved in the process of starch granule initiation. Furthermore, two proteins named ESV and LESV play a role in the aqueous-to-crystalline phase transition of glucan chains. Several catalytically active starch synthases, such as SS1, SS2, SS3, and GBSS, are critical for starch granule biosynthesis and play a catalytic role at each step of granule biogenesis and expansion.
In addition to above proteins, starch branching enzymes (BEs) introduces α-1,6-glycosidic bonds between the glucose chains, creating the branched amylopectin. The starch debranching enzyme (DBE) isoamylase removes some of these branches. Several of these enzymes exist, leading to a highly complex synthesis process.
The products of starch degradation are predominantly maltose and smaller amounts of glucose. These molecules are exported from the plastid to the cytosol, maltose via the maltose transporter and glucose by the plastidic glucose translocator (pGlcT). These two sugars are used for sucrose synthesis. Sucrose can then be used in the oxidative pentose phosphate pathway in the mitochondria, to generate ATP at night.
In the European Union the starch industry produced about 11 million tonnes in 2011, with around 40% being used for industrial applications and 60% for food uses, most of the latter as .International Starch Institute Denmark, Starch production volume In 2017 EU production was 11 million ton of which 9,4 million ton was consumed in the EU and of which 54% were starch sweeteners.
The US produced about 27.5 million tons of starch in 2017, of which about 8.2 million tons was high fructose syrup, 6.2 million tons was glucose syrups, and 2.5 million tons were starch products. The rest of the starch was used for producing ethanol (1.6 billion gallons).
Crude starch is processed on an industrial scale to maltodextrin and glucose syrups and fructose syrups. These massive conversions are mediated by a variety of enzymes, which break down the starch to varying extents. Here breakdown involves hydrolysis, i.e. cleavage of bonds between sugar subunits by the addition of water. Some sugars are isomerized. The processes have been described as occurring in two phases: liquefaction and saccharification. The liquefaction converts starch into . Amylase is a key enzyme for producing dextrin. The saccharification converts dextrin into maltoses and glucose. Diverse enzymes are used in this second phase, including pullanase and other amylases.
Before processed foods, people consumed large amounts of uncooked and unprocessed starch-containing plants, which contained high amounts of resistant starch. Microbes within the large intestine ferment or consume the starch, producing short-chain fatty acids, which are used as energy, and support the maintenance and growth of the microbes. Upon cooking, starch is transformed from an insoluble, difficult-to-digest granule into readily accessible glucose chains with very different nutritional and functional properties.
In current diets, highly processed foods are more easily digested and release more glucose in the small intestine—less starch reaches the large intestine and more energy is absorbed by the body. It is thought that this shift in energy delivery (as a result of eating more processed foods) may be one of the contributing factors to the development of metabolic disorders of modern life, including obesity and diabetes.
The amylose/amylopectin ratio, molecular weight and molecular fine structure influences the physicochemical properties as well as energy release of different types of starches. In addition, cooking and food processing significantly impacts starch digestibility and energy release. Starch has been classified as rapidly digestible starch, slowly digestible starch and resistant starch, depending upon its digestion profile. Raw starch granules resist digestion by human enzymes and do not break down into glucose in the small intestine - they reach the large intestine instead and function as prebiotic dietary fiber. When starch granules are fully gelatinized and cooked, the starch becomes easily digestible and releases glucose quickly within the small intestine. When starchy foods are cooked and cooled, some of the glucose chains re-crystallize and become resistant to digestion again. Slowly digestible starch can be found in raw cereals, where digestion is slow but relatively complete within the small intestine. Widely used prepared foods containing starch are bread, , , , pasta, porridge and tortilla.
During cooking with high heat, sugars released from starch can react with amino acids via the Maillard reaction, forming advanced glycation end-products (AGEs), contributing aromas, flavors and texture to foods. One example of a dietary AGE is acrylamide. Recent evidence suggests that the intestinal fermentation of dietary AGEs may be associated with insulin resistance, atherosclerosis, diabetes and other inflammatory diseases. This may be due to the impact of AGEs on intestinal permeability.
Starch gelatinization during cake baking can be impaired by sugar competing for water, preventing gelatinization and improving texture.
These starch sugars are by far the most common starch based food ingredient and are used as sweeteners in many drinks and foods. They include:
INS 1400, 1401, 1402, 1403 and 1405 are in the EU food ingredients without an E-number. Typical modified starches for technical applications are cationic starches, hydroxyethyl starch, carboxymethylated starches and thiolated starches.
Gummed sweets such as and are not manufactured using a mold in the conventional sense. A tray is filled with native starch and leveled. A positive mold is then pressed into the starch leaving an impression of 1,000 or so jelly beans. The jelly mix is then poured into the impressions and put onto a stove to set. This method greatly reduces the number of molds that must be manufactured.
In the dry end of the papermaking process, the paper web is rewetted with a starch based solution. The process is called 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 presses). Together with surface sizing agents 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 coatings as one of the binders for the coating formulations which include a mixture of pigments, binders and thickeners. Coated paper has improved smoothness, hardness, whiteness and gloss and thus improves printing characteristics.
Starch is used in the manufacture of various adhesives or glues for book-binding, wallpaper adhesives, paper sack production, tube winding, Gum tape, envelope adhesives, school glues and bottle labeling. Starch derivatives, such as yellow dextrins, can be modified by addition of some chemicals to form a hard glue for paper work; some of those forms use borax or soda ash, which are mixed with the starch solution at to create a very good adhesive. Sodium silicate can be added to reinforce these formula.
A related 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.
Starch industry
In addition to starchy plants consumed directly, 66 million tonnes of starch were processed industrially in 2008. By 2011, production had increased to 73 million tons.
Industrial processing
The starch industry extracts and refines starches from crops by wet grinding, washing, sieving and drying. Today, the main commercial refined starches are cornstarch, tapioca, arrowroot, and wheat, rice, and . To a lesser extent, sources of refined starch are sweet potato, sago and mung bean. To this day, starch is extracted from more than 50 types of plants.
Dextrinization
If starch is subjected to dry heat, it breaks down to form , also called "pyrodextrins" in this context. This break down process is known as dextrinization. (Pyro)dextrins are mainly yellow to brown in color and dextrinization is partially responsible for the browning of toasted bread.
Food
Starch is the most common carbohydrate in the human diet and is contained in many . The major sources of starch intake worldwide are the (rice, wheat, and maize) and the ( and cassava).Anne-Charlotte Eliasson (2004). Starch in food: Structure, function and applications. Woodhead Publishing. . Many other starchy foods are grown, some only in specific climates, including , arrowroot, arracacha, , barley, breadfruit, buckwheat, canna, colocasia, cuckoo-pint, katakuri, kudzu, malanga, millet, , oxalis tuberosa, polynesian arrowroot, sago, sorghum, , rye, taro, , Water caltrop, and yams, and many kinds of , such as vicia faba, , , , and .
Starch sugars
Starch can be hydrolyzed into simpler carbohydrates by , various , or a combination of the two. The resulting fragments are known as . The extent of conversion is typically quantified by dextrose equivalent (DE), which is roughly the fraction of the in starch that have been broken.
Modified starches
The modified food starches are E number according to European Food Safety Authority and INS coded Food Additives according to the Codex Alimentarius: Modified Starches . CODEX ALIMENTARIUS published in FNP 52 Add 9 (2001)
Use as food additive
As an additive for 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, and to make noodles and pastas. They function as thickeners, extenders, emulsion stabilizers and are exceptional binders in processed meats.
Resistant starch
Resistant starch is starch that escapes digestion in the small intestine of healthy individuals. High-amylose starch from wheat or corn has a higher gelatinization temperature than other types of starch, and retains its resistant starch content through baking, mild Food extrusion and other food processing techniques. It is used as an insoluble dietary fiber in processed foods such as bread, pasta, cookies, crackers, pretzels and other low moisture foods. It is also utilized as a dietary supplement for its health benefits. Published studies have shown that resistant starch helps to improve insulin sensitivity, reduces pro-inflammatory biomarkers interleukin 6 and tumor necrosis factor alpha and improves markers of colonic function.
It has been suggested that resistant starch contributes to the health benefits of intact whole grains.
Synthetic starch
A cell-free Biocatalysis process has been demonstrated to synthesize starch from CO2 and hydrogen. The chemical pathway of 11 core reactions was drafted by computational pathway design and converts CO2 to starch at a rate that is ~8.5-fold higher than starch synthesis Corn starch.
Non-food applications
Papermaking
Papermaking is the largest non-food application for starches globally, consuming many 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 agents help to give the necessary strength properties to the paper web formed in the papermaking process (wet strength), and to provide strength to the final paper sheet (dry strength).
Adhesives
Corrugated board adhesives are the next largest application of non-food starches globally. Starch are mostly based on unmodified native starches, plus some additive such as borax and caustic soda. Part of the starch is gelatinized to carry the slurry of uncooked starches and prevent sedimentation. This opaque glue is called a SteinHall adhesive. 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 adhesive for corrugated board production.
Other
Chemical tests
A solution of Polyiodide (I3−) (formed by mixing iodine and potassium iodide) can be used to test for starch. The colorless solution turns dark blue in the presence of starch. The strength of the resulting blue color depends on the amount of amylose present. Waxy starches with little or no amylose present will color red. Benedict's test and Fehling's test is also done to indicate the presence of starch.
Safety
In the US, the Occupational Safety and Health Administration (OSHA) has set the legal limit (Permissible exposure limit) for starch exposure in the workplace as 15 mg/m3 total exposure and 5 mg/m3 respiratory exposure over an eight-hour workday. The National Institute for Occupational Safety and Health (NIOSH) has set a Recommended exposure limit (REL) of 10 mg/m3 total exposure and 5 mg/m3 respiratory exposure over an eight-hour workday.
See also
External links
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