Pure starch is a white, tasteless and odorless powder that is insoluble in cold water oralcohol. It consists of two types of molecules: the linear andhelicalamylose and the branchedamylopectin. Depending on the plant, starch generally contains 20 to 25% amylose and 75 to 80% amylopectin by weight.[4]Glycogen, the energy reserve of animals, is a more highly branched version of amylopectin.
In industry, starch is often converted into sugars, for example bymalting. These sugars may befermented to produceethanol in the manufacture ofbeer,whisky andbiofuel. In addition, sugars produced from processed starch are used in many processed foods.
Mixing most starches in warm water produces a paste, such aswheatpaste, which can be used as a thickening, stiffening or gluing agent. The principal non-food, industrial use of starch is as an adhesive in thepapermaking process. A similar paste, clothing orlaundry starch, can be applied to certain textile goods before ironing to stiffen them.
The word "starch" is from aGermanic root with the meanings "strong, stiff, strengthen, stiffen".[5]
Modern GermanStärke (strength, starch) is related and refers to the main historical applications, its uses in textiles:sizingyarn forweaving, and starchinglinen.
TheGreek term for starch, "amylon" (ἄμυλον), which means "not milled", is also related. It provides the rootamyl, which is used as a prefix for several carbon compounds related to or derived from starch (e.g.amyl alcohol,amylose,amylopectin).
Starch grains from therhizomes ofTypha (cattails, bullrushes) asflour have been identified fromgrinding stones in Europe dating back to 30,000 years ago.[6] Starch grains fromsorghum were found on grind stones in caves inNgalue,Mozambique dating up to 100,000 years ago.[7]
Pure extracted wheat starch paste was used inAncient Egypt, possibly to gluepapyrus.[8] The extraction of starch is first described in theNatural History ofPliny the Elder around 77–79CE.[9] Romans used it also incosmetic creams, to powder the hair and tothicken sauces. Persians and Indians used it to make dishes similar to gothumai wheathalva. Rice starch as surface treatment of paper has been used in paper production in China since 700 CE.[10] In the mid eighth century production of paper that wassized with wheat starch started in the Arabic world.[11] Laundry starch was first described in England in the beginning of the 15th century and was essential to make 16th centuryruffed collars.[12]
Potato starch granules incells of the potatoStarch in endosperm in embryonic phase of maize seed
Plants produceglucose fromcarbon dioxide and water byphotosynthesis. The glucose is used to generate the chemical energy required for generalmetabolism as well as a precursor to myriad organic building blocks such asnucleic acids,lipids,proteins, and structural polysaccharides such ascellulose. Most green plants store any extra glucose in the form of starch, which is packed into semicrystalline granules called starch granules oramyloplasts.[13] Toward the end of the growing season, starch accumulates in twigs of trees near the buds.Fruit,seeds,rhizomes, andtubers store starch to prepare for the next growing season. Young plants live on this stored energy in their roots, seeds, and fruits until they can find suitable soil in which to grow.[14] The starch is also consumed at night when photosynthesis is not occurring.
Especially when hydrated, glucose takes up much space and isosmotically 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.[16]
Amylose consists of long chains derived from glucose molecules connected by α-1,4-glycosidic linkage. Amylopectin is highly branched but also derived from glucose interconnected by α-1,6-glycosidic linkages. The same type of linkage is found in the animal reserve polysaccharideglycogen. By contrast, many structural polysaccharides such aschitin, cellulose, andpeptidoglycan are linked byβ-glycosidic bonds, which are more resistant to hydrolysis.[17]
Within plants, starch is stored in semi-crystalline granules. Each plant species has a distinctive starch granular size: rice starch is relatively small (about 2 μm),potato starches have larger granules (up to 100 μm) while wheat and tapioca fall in-between.[18] Unlike other botanical sources of starch, wheat starch has a bimodal size distribution, with both smaller and larger granules ranging from 2 to 55 μm.[18]
Some cultivated plant varieties have pure amylopectin starch without amylose, known aswaxy starches. The most used iswaxy maize, others areglutinous rice andwaxy potato starch. Waxy starches undergo lessretrogradation, 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 aresistant starch (a starch that resists digestion) in food products.
Plants synthesize starch in two types of tissues. The first type is storage tissues, for example, cereal endosperm, and storage roots and stems such as cassava and potato. The second type is green tissue, for example, leaves, where many plant species synthesize transitory starch on a daily basis. In both tissue types, starch is synthesized in a plastids (amyloplasts and chloroplasts).
The biochemical pathway involves conversion ofglucose 1-phosphate toADP-glucose using the enzymeglucose-1-phosphate adenylyltransferase. This step requires energy in the form ofATP. A number ofstarch synthases available in plastids then adds the ADP-glucose via α-1,4-glycosidic bond to a growing chain of glucose residues, liberatingADP. 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 duringglycogen synthesis.[19] 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,[20] 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.[21] Another protein named PTST2 binds to small glucan chains and agglomerates to recruit starch synthase 4 (SS4).[22] Three other proteins, namely, PTST3, SS5, and MRC, are also known to be involved in the process of starch granule initiation.[23][24][25] Furthermore, two proteins named ESV and LESV play a role in the aqueous-to-crystalline phase transition of glucan chains.[26] 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.[27]
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. Severalisoforms of these enzymes exist, leading to a highly complex synthesis process.[28]
The starch that is synthesized in plant leaves during the day is transitory: it serves as an energy source at night. Enzymes catalyze release of glucose from the granules. The insoluble, highly branched starch chains requirephosphorylation in order to be accessible for degrading enzymes. The enzymeglucan, water dikinase (GWD) installs a phosphate at the C-6 position of glucose, close to the chain's 1,6-alpha branching bonds. A second enzyme,phosphoglucan, water dikinase (PWD) phosphorylates the glucose molecule at the C-3 position. After the second phosphorylation, the first degrading enzyme,beta-amylase (BAM) attacks the glucose chain at its non-reducing end.Maltose is the main product released. If the glucose chain consists of three or fewer molecules, BAM cannot release maltose. A second enzyme,disproportionating enzyme-1 (DPE1), combines two maltotriose molecules. From this chain, a glucose molecule is released. Now, BAM can release another maltose molecule from the remaining chain. This cycle repeats until starch is fully degraded. If BAM comes close to the phosphorylated branching point of the glucose chain, it can no longer release maltose. In order for the phosphorylated chain to be degraded, the enzyme isoamylase (ISA) is required.[29]
The products of starch degradation are predominantly maltose[30] and smaller amounts of glucose. These molecules are exported from the plastid to the cytosol, maltose via the maltose transporter and glucose by theplastidic glucose translocator (pGlcT).[31] 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.[29]
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.[32]
In theEU thestarch industry produced about 11 million tonnes in 2011, with around 40% being used for industrial applications and 60% for food uses,[33] most of the latter asglucose syrups.[34] 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.[35]
TheUS produced about 27.5 million tons of starch in 2017, of which about 8.2 million tons washigh fructose syrup, 6.2 million tons was glucose syrups, and 2.5 million tons were starch products.[clarification needed] The rest of the starch was used for producingethanol (1.6 billion gallons).[36][37]
The starch industry extracts and refines starches from crops by wet grinding, washing, sieving and drying. Today, the main commercial refined starches arecornstarch,tapioca, arrowroot,[38] and wheat, rice, andpotato starches. 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.
Crude starch is processed on an industrial scale tomaltodextrin 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 intodextrins.Amylase is a key enzyme for producing dextrin. The saccharification converts dextrin into maltoses and glucose. Diverse enzymes are used in this second phase, includingpullanase and other amylases.[39]
Corn starch, 800x magnified, under polarized light, showing characteristicextinction crossRice starch under transmitted light microscopy. A characteristic of rice starch is that granules have an angular outline and tend to clump.
If starch is subjected to dry heat, it breaks down to formdextrins, 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.[40]
Before processed foods, people consumed large amounts of uncooked and unprocessed starch-containing plants, which contained high amounts ofresistant starch. Microbes within the large intestine ferment or consume the starch, producingshort-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.[42]
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.[43]
The amylose/amylopectin ratio, molecular weight and molecular fine structure influences the physicochemical properties as well as energy release of different types of starches.[44] 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.[45] 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 asprebioticdietary fiber.[46] 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.[47] Widely used prepared foods containing starch arebread,pancakes,cereals,noodles,pasta,porridge andtortilla.
During cooking with high heat, sugars released from starch can react with amino acids via theMaillard reaction, formingadvanced glycation end-products (AGEs), contributing aromas, flavors and texture to foods.[48] One example of a dietary AGE isacrylamide. Recent evidence suggests that the intestinal fermentation of dietary AGEs may be associated withinsulin resistance,atherosclerosis,diabetes and other inflammatory diseases.[49][50] This may be due to the impact of AGEs on intestinal permeability.[51]
Starch gelatinization during cakebaking can be impaired by sugar competing forwater, preventing gelatinization and improving texture.
Karo corn syrup advert 1917Niagara corn starch advert1880s
Starch can behydrolyzed into simpler carbohydrates byacids, variousenzymes, or a combination of the two. The resulting fragments are known asdextrins. The extent of conversion is typically quantified bydextrose equivalent (DE), which is roughly the fraction of theglycosidic bonds in starch that have been broken.
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:
Maltodextrin, a lightly hydrolyzed (DE 10–20) starch product used as a bland-tasting filler and thickener.
Variousglucose syrups (DE 30–70), also calledcorn syrups in the US, 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.
Highfructose syrup, made by treating dextrose solutions with the enzymeglucose isomerase, until a substantial fraction of the glucose has been converted to fructose. In the U.S.high-fructose corn syrup is significantly cheaper than sugar, and is the principal sweetener used in processed foods and beverages.[52] Fructose also has better microbiological stability. One kind of high fructose corn syrup, HFCS-55, is sweeter thansucrose because it is made with more fructose, while the sweetness of HFCS-42 is on par with sucrose.[53][54]
As an additive forfood 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.
Gummed sweets such asjelly beans andwine gums 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.
Resistant starch is starch that escapes digestion in the small intestine of healthy individuals. High-amylose starch from wheat or corn has a highergelatinization temperature than other types of starch, and retains its resistant starch content throughbaking, mildextrusion and other food processing techniques. It is used as an insolubledietary 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,[58][59] reduces pro-inflammatory biomarkersinterleukin 6 andtumor necrosis factor alpha[60][61] and improves markers of colonic function.[62]It has been suggested that resistant starch contributes to the health benefits of intact whole grains.[63]
A cell-freechemoenzymatic process has been demonstrated to synthesize starch from CO2 and hydrogen. The chemical pathway of 11 core reactions was drafted bycomputational pathway design and converts CO2 to starch at a rate that is ~8.5-fold higher than starch synthesisin maize.[64][65]
Papermaking is the largest non-food application for starches globally, consuming many millions of metric tons annually.[33] 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 internalsizing 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).
In the dry end of the papermaking process, the paper web is rewetted with a starch based solution. The process is calledsurface 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.
Corrugated board adhesives are the next largest application of non-food starches globally. Starchglues are mostly based on unmodified native starches, plus some additive such asborax andcaustic 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.
Starch is used in the manufacture of variousadhesives or glues[66] for book-binding,wallpaper adhesives,paper sack production, tube winding,gummed paper, 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 orsoda ash, which are mixed with the starch solution at 50–70 °C (122–158 °F) 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 gypsumwall board manufacturing process. Chemically modified or unmodified starches are added to the stucco containing primarilygypsum. 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.
Clothing orlaundry starch is used in thelaundering of clothes. It was widely used in Europe in the 16th and 17th centuries.
Textile chemicals from starch:warpsizing agents are used to reduce breaking ofyarns duringweaving. Starch is mainly used to sizecotton based yarns. Modified starch is also used astextile printing thickener.
In oil exploration, starch is used to adjust the viscosity ofdrilling fluid, which is used to lubricate the drill head and suspend the grinding residue in petroleum extraction.
For body powder, powdered corn starch is used as a substitute fortalcum powder, and similarly in other health and beauty products.
Starch is used to produce variousbioplastics, synthetic polymers that are biodegradable. An example ispolylactic acid based on glucose from starch.
Glucose from starch can be further fermented tobiofuelcorn ethanol using the so-calledwet milling process. Today mostbioethanol production plants use the dry milling process to ferment corn or other feedstock directly to ethanol.[68]
In the pharmaceutical industry, starch is also used as anexcipient, astablet disintegrant, and as binder. Synthetic amylose made from cellulose has a well-controlled degree of polymerization. Therefore, it can be used as a potential drug deliver carrier.[69]
Granules of wheat starch, stained with iodine, photographed through a light microscope
A solution oftriiodide (I3−) (formed by mixingiodine andpotassium iodide) can be used to test for starch. The colorless solution turns dark blue in the presence of starch.[70] 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.
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