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Article:Starch
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Glycogen and amylopectin have the same structure, but the former has about one branch point per ten 1,4-alpha bonds, compared to about one branch point per thirty 1,4-alpha bonds in amylopectin.<ref>{{cite book |author=Stryer, Lubert; Berg, Jeremy Mark; Tymoczko, John L. |title=Biochemistry |publisher=W.H. Freeman |location=San Francisco |year=2002 |isbn=0-7167-3051-0 |edition=5th |url=http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=stryer |chapter=Section 11.2.2 |chapterurl=http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=stryer.section.1517#1522}}</ref> Another difference is that glycogen is synthesised from [[Uridine diphosphate glucose|UDP-glucose]] while starch is synthesised from ADP-glucose.
 
Glycogen and amylopectin have the same structure, but the former has about one branch point per ten 1,4-alpha bonds, compared to about one branch point per thirty 1,4-alpha bonds in amylopectin.<ref>{{cite book |author=Stryer, Lubert; Berg, Jeremy Mark; Tymoczko, John L. |title=Biochemistry |publisher=W.H. Freeman |location=San Francisco |year=2002 |isbn=0-7167-3051-0 |edition=5th |url=http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=stryer |chapter=Section 11.2.2 |chapterurl=http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=stryer.section.1517#1522}}</ref> Another difference is that glycogen is synthesised from [[Uridine diphosphate glucose|UDP-glucose]] while starch is synthesised from ADP-glucose.
   
==Properties==
 
===Structure===
 
Starch molecules arrange themselves in the plant in semi-crystalline granules. Each plant species has a unique starch granular size: [[rice starch]] is relatively small (about 2μm) while [[potato starch]]es have larger granules (up to 100μm). Although in absolute mass only about one quarter of the starch granules in plants consist of amylose, there are about 150 times more amylose molecules than amylopectin molecules. Amylose is a much smaller molecule than amylopectin.
 
   
Starch becomes soluble in water when heated. The granules swell and burst, the semi-crystalline structure is lost and the smaller amylose molecules start leaching out of the granule, forming a network that holds water and increasing the mixture's [[viscosity]]. This process is called [[starch gelatinization]]. During cooking the starch becomes a paste and increases further in viscosity. During cooling or prolonged storage of the paste, the semi-crystalline structure partially recovers and the starch paste thickens, expelling water. This is mainly caused by the [[retrogradation (starch)|retrogradation]] of the amylose. This process is responsible for the hardening of bread or [[staling]], and for the water layer on top of a starch gel ([[syneresis (chemistry)|syneresis]]).
 
   
Some cultivated plant varieties have pure amylopectin starch without amylose, known as ''waxy starches''. The most used is [[waxy corn|waxy maize]], others are [[glutinous rice]] and [[waxy potato starch]]. Waxy starches have less retrogradation, resulting in a more stable paste. High amylose starch, [[amylomaize]], is cultivated for the use of its gel strength.
 
   
===Hydrolysis===
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The [[enzyme]]s that break down or [[hydrolysis|hydrolyze]] starch into the constituent sugars are known as [[amylase]]s.
 
 
Alpha-amylases are found in plants and in animals. Human [[saliva]] is rich in amylase, and the [[pancreas]] also secretes the enzyme. Individuals from populations with a high-starch diet tend to have more amylase genes than those with low-starch diets;<ref name="Variations">{{cite article | url = http://www.nature.com/ng/journal/v39/n10/full/ng2123.html | title = Diet and the evolution of human amylase gene copy number variation | journal = Nature Genetics |publisher = Nature Publishing Group|volume=39|pages= 1256&ndash;1260|year=2007|doi=10.1038/ng2123|author = Perry, George H., ''et al''}}</ref> [[chimpanzee]]s have very few amylase genes.<ref name="Variations"/> It is possible that turning to a high-starch diet was a significant event in human evolution.<ref>{{cite article|first=P.Z.|last=Myers|url=http://scienceblogs.com/pharyngula/2008/12/amylase_and_human_evolution.php|title=Amylase and human evolution|date= December 11, 2008}}</ref>
 
 
Beta-amylase cuts starch into [[maltose]] units. This process is important in the digestion of starch and is also used in [[brewing]], where the amylase from the skin of the seed grains is responsible for converting starch to maltose ([[Malting]], [[Mashing]]).
 
 
===Dextrinization===
 
If starch is subjected to dry heat, it breaks down to form [[dextrin]]s, 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.
 
 
===Chemical tests===
 
{{Main|Iodine test}}
 
[[Iodine]] solution is used to test for starch; a dark blue color indicates the presence of starch. The details of this reaction are not yet fully known, but it is thought that the iodine (I<sub>3</sub><sup>−</sup> and I<sub>5</sub><sup>−</sup> ions) fit 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. Waxy starches with little or no amylose present will color red.
 
 
[[File:Stärkemehl 800 fach Polfilter.jpg|thumb|right|250px|Starch, 800x magnified, under polarized light]]
 
 
[[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]] (I<sub>3</sub><sup>−</sup>) ions break up into three iodide ions, disassembling the starch-iodine complex.
 
A 0.3% [[Percentage solution|w/w]] solution is the standard concentration for a starch indicator. It is made by adding 3&nbsp;grams of soluble starch to 1 liter of heated water; the solution is cooled before use (starch-iodine complex becomes unstable at temperatures above 35&nbsp;°C).
 
 
'''Microscopy of starch granules''' - Each species of plant has a unique shape of starch granules in granular size, shape and crystallization 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]]).
 
   
 
==Food==
 
==Food==
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