Editing Pectin (section)

==Chemistry==
[[File:Galacturonic acid.png|thumb|[[Galacturonic acid]]]]
Pectins, also known as pectic polysaccharides, are rich in galacturonic acid. Several distinct polysaccharides have been identified and characterised within the pectic group. Homogalacturonans are linear chains of α-(1–4)-linked [[D-Galacturonic acid|<small>D</small>-galacturonic acid]].<ref name="Complex Carbohydrate Research Centre">{{cite web  |url=http://www.ccrc.uga.edu/~mao/galact/gala.htm  |title=Galacturonans  |publisher=Complex Carbohydrate Research Centre, University of Georgia, US  |access-date=23 July 2010 |archive-date=15 August 2010 | archive-url= https://web.archive.org/web/20100815065857/http://www.ccrc.uga.edu/~mao/galact/gala.htm |url-status=dead }}</ref> Substituted galacturonans are characterised by the presence of saccharide appendant residues (such as <small>D</small>-[[xylose]] or <small>D</small>-[[apiose]] in the respective cases of xylogalacturonan and [[apiogalacturonan]]) branching from a backbone of <small>D</small>-galacturonic acid residues.<ref name="Complex Carbohydrate Research Centre"/><ref name="Buchanan-2000">{{cite book | vauthors = Buchanan BB, Gruissem W, Jones RL |title        = Biochemistry and Molecular Biology of Plants |publisher = American Society of Plant Biologists |location = Rockville, MD USA |year = 2000 |url          = https://archive.org/details/biochemistrymole00buch |isbn = 978-0-943088-37-2 |access-date = 23 July 2010 |url-status = dead |url-access = registration |archive-url  = https://web.archive.org/web/20200326153824/https://archive.org/details/biochemistrymole00buch |archive-date = 26 March 2020 }}</ref> Rhamnogalacturonan I pectins (RG-I) contain a backbone of the repeating disaccharide: 4)-α-<small>D</small>-galacturonic acid-(1,2)-α-<small>L</small>-[[rhamnose]]-(1. From many of the rhamnose residues, sidechains of various neutral sugars branch off. The neutral sugars are mainly <small>D</small>-[[galactose]], <small>L</small>-[[arabinose]] and <small>D</small>-xylose, with the types and proportions of neutral sugars varying with the origin of pectin.<ref name="Complex Carbohydrate Research Centre"/><ref name="Buchanan-2000"/><ref name="Ccr-RGI">{{cite web | url = http://www.ccrc.uga.edu/~mao/rg1/rg1.htm | title = RG-I | archive-url = https://web.archive.org/web/20091004031015/http://www.ccrc.uga.edu/~mao/rg1/rg1.htm | archive-date=4 October 2009 | work = Ccrc.uga.edu | access-date = 16 July 2012 }}</ref>

Another structural type of pectin is [[rhamnogalacturonan II]] (RG-II), which is a less frequent, complex, highly branched polysaccharide.<ref name="Ccrc-RGII">{{cite web | url = http://www.ccrc.uga.edu/~mao/rg2/intro.htm | title = Rhamnogalacturonan II | work = www.ccrc.uga.edu | archive-url = https://web.archive.org/web/20091003140059/http://www.ccrc.uga.edu/~mao/rg2/intro.htm | archive-date = 3 October 2009 | access-date = 16 July 2012 }}</ref> Rhamnogalacturonan II is classified by some authors within the group of substituted galacturonans since the rhamnogalacturonan II backbone is made exclusively of <small>D</small>-galacturonic acid units.<ref name="Buchanan-2000"/>

The [[molecular weight]] of isolated pectine greatly varies by the source and the method of isolation.<ref>{{cite journal | vauthors = Singaram A, Guruchandran S, Ganesan N | title = Review on functionalized pectin films for active food packaging | journal = Packaging Technology and Science | volume = 37 | issue = 4 | pages = 237–262 | date = 2024 | doi = 10.1002/pts.2793 }}</ref> Values have been reported as low as 28 kDa for apple pomace <ref>{{cite journal | vauthors = Wang X, Chen Q, Lü X | title = Pectin extracted from apple pomace and citrus peel by subcritical water | journal = Food Hydrocoll. | date = 2014 | volume = 38 | pages = 129–137 | doi=10.1016/J.FOODHYD.2013.12.003}}</ref> up to 753 kDa for  sweet potato peels.<ref>{{cite journal | vauthors = Arachchige M, Mu T, Ma M | title = Structural, physicochemical and emulsifying properties of sweet potato pectin treated by high hydrostatic pressure and/or pectinase: a comparative study | journal=J Sci Food Agric | date = 2020 | volume = 100 | issue = 13 | pages = 4911–4920 | doi = 10.1007/s11696-018-0500-0 | pmid = 32483850 }}</ref>

In nature, around 80 percent of [[carboxyl group]]s of galacturonic acid are [[ester]]ified with [[methanol]]. This proportion is decreased to a varying degree during pectin extraction. Pectins are classified as high- versus low-methoxy pectins (short HM-pectins versus LM-pectins), with more or less than half of all the galacturonic acid esterified.<ref name="Liang-2012">{{cite journal | vauthors = Liang RH, Chen J, Liu W, Liu CM, Yu W, Yuan M, Zhou XQ | title = Extraction, characterization and spontaneous gel-forming property of pectin from creeping fig (Ficus pumila Linn.) seeds | journal = Carbohydrate Polymers | volume = 87 | issue = 1 | pages = 76–83 | date = January 2012 | pmid = 34663033 | doi = 10.1016/j.carbpol.2011.07.013 }}</ref> The ratio of esterified to non-esterified galacturonic acid determines the behaviour of pectin in food applications – HM-pectins can form a gel under acidic conditions in the presence of high sugar concentrations, while LM-pectins form gels by interaction with divalent cations, particularly [[Calcium in biology|Ca<sup>2+</sup>]], according to the idealized 'egg box' model, in which ionic bridges are formed between calcium ions and the ionised carboxyl groups of the galacturonic acid.<ref name="Durand-1990">{{cite journal | vauthors = Durand D, Bertrand C, Clark AH, Lips A | title = Calcium-induced gelation of low methoxy pectin solutions--thermodynamic and rheological considerations | journal = International Journal of Biological Macromolecules | volume = 12 | issue = 1 | pages = 14–18 | date = February 1990 | pmid = 2083236 | doi = 10.1016/0141-8130(90)90076-M }}</ref><ref name="Migliori-2010">{{cite journal | vauthors = Migliori M, Gabriele D, Checchetti A, Battipede B | year = 2010| title = Compatibility analysis of pectin at different esterification degree from intrinsic viscosity data of diluted ternary solutions | journal = Reactive and Functional Polymers | volume = 70 | issue = 10| pages = 863–867 | doi = 10.1016/j.reactfunctpolym.2010.07.011 | bibcode = 2010RFPol..70..863M}}</ref><ref name="Liang-2012" />

In high-methoxy pectins at soluble solids content above 60% and a pH value between 2.8 and 3.6, [[hydrogen bonds]] and [[hydrophobic interactions]] bind the individual pectin chains together. These bonds form as water is bound by sugar and forces pectin strands to stick together. These form a three-dimensional molecular net that creates the macromolecular gel. The gelling-mechanism is called a low-water-activity gel or sugar-acid-pectin gel.{{Citation needed|date=July 2018}}

While low-methoxy pectins need calcium to form a gel, they can do so at lower soluble solids and higher pH than high-methoxy pectins. Normally low-methoxy pectins form gels with a range of pH from 2.6 to 7.0 and with a soluble solids content between 10 and 70%.{{Citation needed|date=July 2018}}

The non-esterified galacturonic acid units can be either free acids (carboxyl groups) or salts with sodium, potassium, or calcium. The salts of partially esterified pectins are called pectinates, if the degree of esterification is below 5 percent the salts are called pectates, the insoluble acid form, pectic acid.{{Citation needed|date=July 2018}}

Some plants, such as [[sugar beet]], potatoes and pears, contain pectins with acetylated galacturonic acid in addition to methyl esters. Acetylation prevents gel-formation but increases the stabilising and emulsifying effects of pectin.{{Citation needed|date=July 2018}}

Amidated pectin is a modified form of pectin. Here, some of the galacturonic acid is converted with [[ammonia]] to [[carboxylic acid]] [[amide]]. These pectins are more tolerant of varying calcium concentrations that occur in use.<ref name="Belitz-2004">{{cite book | vauthors = Belitz HD, Grosch W, Schieberle P | title = Food Chemistry | publisher = Springer | location = Berlin | date = April 2004 }}</ref>

Thiolated pectin exhibits substantially improved gelling properties since this [[thiomer]] is able to crosslink via [[disulfide]] bond formation. These high gelling properties are advantageous for various pharmaceutical applications and applications in food industry.<ref name="Majzoob-2006">{{cite journal | vauthors = Majzoob S, Atyabi F, Dorkoosh F, Kafedjiiski K, Loretz B, Bernkop-Schnürch A | title = Pectin-cysteine conjugate: synthesis and in-vitro evaluation of its potential for drug delivery | journal = The Journal of Pharmacy and Pharmacology | volume = 58 | issue = 12 | pages = 1601–1610 | date = December 2006 | pmid = 17331323 | doi = 10.1211/jpp.58.12.0006 | s2cid = 24127477 | doi-access = free }}</ref><ref name="Perera-2010">{{cite journal | vauthors = Perera G, Hombach J, Bernkop-Schnürch A | title = Hydrophobic thiolation of pectin with 4-aminothiophenol: synthesis and in vitro characterization | journal = AAPS PharmSciTech | volume = 11 | issue = 1 | pages = 174–180 | date = March 2010 | pmid = 20101485 | pmc = 2850493 | doi = 10.1208/s12249-009-9370-7 | s2cid = 25025639 }}</ref><ref name="Chen-2023">{{cite journal | vauthors = Chen J, Cui Y, Zhang S, Ma Y, Yang F | title = Compound treatment of thiolated citrus high-methoxyl pectin and sodium phosphate dibasic anhydrous improved gluten network structure | journal = Food Chemistry | volume = 404 | issue = Pt B | pages = 134770 | date = March 2023 | pmid = 36332584 | doi = 10.1016/j.foodchem.2022.134770 | s2cid = 253214393 }}</ref>

To prepare a pectin-gel, the ingredients are heated, dissolving the pectin. Upon cooling below gelling temperature, a gel starts to form. If gel formation is too strong, [[syneresis (chemistry)|syneresis]] or a granular texture are the result, while weak gelling leads to excessively soft gels.{{Citation needed|date=July 2018}}

Amidated pectins behave like low-ester pectins but need less calcium and are more tolerant of excess calcium. Also, gels from amidated pectin are thermoreversible; they can be heated and after cooling solidify again, whereas conventional pectin-gels will afterwards remain liquid.{{Citation needed|date=July 2018}}

High-ester pectins set at higher temperatures than low-ester pectins. However, gelling reactions with calcium increase as the degree of esterification falls. Similarly, lower pH-values or higher soluble solids (normally sugars) increase gelling speeds. Suitable pectins can therefore be selected for jams and jellies, or for higher-sugar confectionery jellies.{{Citation needed|date=July 2018}}