Starch
Starch or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants for energy storage. Worldwide, it is the most common carbohydrate in human diets, and is contained in large amounts in staple foods such as wheat, potatoes, maize, rice, and cassava.
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 2025% amylose and 7580% amylopectin by weight. 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 malting. These sugars may be fermented 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.
Etymology
The word starch is from a Germanic root with the meanings "strong, stiff, strengthen, stiffen".Modern German Stärke is related and refers to the main historical applications, its uses in textiles: sizing yarn for weaving, and starching linen.
The Greek term for starch, amylon, which means "not milled", is also related. It provides the root amyl, which is used as a prefix for several carbon compounds related to or derived from starch.
History
Starch grains from the rhizomes of Typha as flour have been identified from grinding stones in Europe dating back to 30,000 years ago. Starch grains from sorghum were found on grind stones in caves in Ngalue, Mozambique dating up to 100,000 years ago.Pure extracted wheat starch paste was used in Ancient Egypt, possibly to glue papyrus. The extraction of starch is first described in the Natural History of Pliny the Elder around 77–79 CE. Romans used it also in cosmetic creams, to powder the hair and to thicken 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 sized 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.
Energy store of plants
Plants produce glucose from carbon dioxide and water by photosynthesis. The glucose is used to generate the chemical energy required for general metabolism as well as a precursor to myriad organic building blocks such as nucleic acids, lipids, proteins, and structural polysaccharides such as cellulose. Most green plants store any extra glucose in the form of starch, which is packed into semicrystalline granules called starch granules or amyloplasts. Toward the end of the growing season, starch accumulates in twigs of trees near the buds. Fruit, seeds, rhizomes, and tubers 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. The starch is also consumed at night when photosynthesis is not occurring.Green algae and land-plants store their starch in the plastids, whereas red algae, glaucophytes, cryptomonads, dinoflagellates 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 osmotically 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 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 polysaccharide glycogen. By contrast, many structural polysaccharides such as chitin, cellulose, and peptidoglycan are linked by β-glycosidic bonds, which are more resistant to hydrolysis.
Structure of starch particles
Within plants, starch is stored in semi-crystalline granules. Each plant species has a distinctive starch granular size: rice starch is relatively small, potato starches have larger granules while wheat and tapioca fall in-between. 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.Some cultivated plant varieties have pure amylopectin starch without amylose, known as waxy starches. The most used is waxy maize, 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 in food products.
Biosynthesis
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 plastids.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 synthases 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 glycogen synthesis. 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. 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 introduces α-1,6-glycosidic bonds between the glucose chains, creating the branched amylopectin. The starch debranching enzyme isoamylase removes some of these branches. Several isoforms of these enzymes exist, leading to a highly complex synthesis process.
Degradation
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 require phosphorylation in order to be accessible for degrading enzymes. The enzyme glucan, water dikinase 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 phosphorylates the glucose molecule at the C-3 position. After the second phosphorylation, the first degrading enzyme, beta-amylase 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, 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 is required.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. 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.
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.In the EU 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 glucose syrups. 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.