Alginic acid

{{Short description|Polysaccharide found in brown algae}} {{Chembox | Verifiedfields = changed | verifiedrevid = 477314688 | ImageFile = Alginsäure.svg | ImageSize = 250px | IUPACName = | OtherNames = Alginic acid; E400; [D-ManA(β1→4)L-GulA(α1→4)]n |Section1={{Chembox Identifiers | CASNo_Ref = {{cascite|correct|CAS}} | CASNo = 9005-32-7 | EINECS = 232-680-1 | UNII_Ref = {{fdacite|changed|FDA}} | UNII = 8C3Z4148WZ | PubChem = | SMILES = | ChemSpiderID_Ref = {{chemspidercite|changed|chemspider}} | ChemSpiderID = None }} |Section2={{Chembox Properties | Formula = (C6H8O6)n | MolarMass = 10,000 – 600,000 | Appearance = White to yellow, fibrous powder | Density = 1.601{{nbsp}}g/cm3 | MeltingPt = | BoilingPt = | Solubility = | pKa = 1.5–3.5 }} |Section6={{Chembox Pharmacology | ATCCode_prefix = A02 | ATCCode_suffix = BX13 }} |Section7={{Chembox Hazards | MainHazards = | FlashPt = | AutoignitionPt = }} }} [[File:Giantkelp2 300.jpg|thumb|''Macrocystis pyrifera'', the largest species of giant kelp]] '''Alginic acid''', also called '''algin''', is a naturally occurring, edible polysaccharide found in brown algae. It is hydrophilic and forms a viscous gum when hydrated. When the alginic acid binds with sodium and calcium ions, the resulting salts are known as '''alginates'''. Its colour ranges from white to yellowish-brown. It is sold in filamentous, granular, or powdered forms.

It is a significant component of the biofilms produced by the bacterium ''Pseudomonas aeruginosa'', a major pathogen found in the lungs of some people who have cystic fibrosis.<ref name="Davies-2002">{{cite journal |last1=Davies |first1=Jane C. |title=Pseudomonas aeruginosa in cystic fibrosis: pathogenesis and persistence |journal=Paediatric Respiratory Reviews |date=June 2002 |volume=3 |issue=2 |pages=128–134 |doi=10.1016/s1526-0550(02)00003-3 |pmid=12297059 }}</ref> The biofilm and ''P. aeruginosa'' have a high resistance to antibiotics,<ref name="Boyd-1995">{{cite journal |last1=Boyd |first1=A |last2=Chakrabarty |first2=A M |title=Pseudomonas aeruginosa biofilms: role of the alginate exopolysaccharide |journal=Journal of Industrial Microbiology |date=September 1995 |volume=15 |issue=3 |pages=162–168 |doi=10.1007/BF01569821 |pmid=8519473 |doi-access=free }}</ref> but are susceptible to inhibition by macrophages.<ref name="Leid-2005">{{cite journal |last1=Leid |first1=Jeff G. |last2=Willson |first2=Carey J. |last3=Shirtliff |first3=Mark E. |last4=Hassett |first4=Daniel J. |last5=Parsek |first5=Matthew R. |last6=Jeffers |first6=Alyssa K. |title=The Exopolysaccharide Alginate Protects Pseudomonas aeruginosa Biofilm Bacteria from IFN-γ-Mediated Macrophage Killing |journal=The Journal of Immunology |date=December 2005 |volume=175 |issue=11 |pages=7512–7518 |doi=10.4049/jimmunol.175.11.7512 |pmid=16301659 |bibcode=2005JImm..175.7512L |doi-access=free }}</ref>

Alginate was discovered by the British chemical scientist E. C. C. Stanford in 1881, and he patented an extraction process for it in the same year.<ref name="Bojorges-2023" /> The alginate was extracted, in the original patent, by first soaking the algae in water or diluted acid. The resulting insoluble alginic acid is then treated with an alkaline solution such as sodium carbonate, creating sodium alginate, before finally precipitating and purifying the alginate from solution.<ref>{{cite book |last1=Pereira |first1=Leonel |last2=Cotas |first2=João |title=Alginates - Recent Uses of This Natural Polymer |chapter=Introductory Chapter: Alginates - A General Overview |date=2020 |doi=10.5772/intechopen.88381 |isbn=978-1-78985-641-5 }}</ref><ref>{{Cite journal |last1=Abka-Khajouei |first1=Roya |last2=Tounsi |first2=Latifa |last3=Shahabi |first3=Nasim |last4=Patel |first4=Anil Kumar |last5=Abdelkafi |first5=Slim |last6=Michaud |first6=Philippe |date=2022-05-29 |title=Structures, Properties and Applications of Alginates |journal=Marine Drugs |volume=20 |issue=6 |page=364 |doi=10.3390/md20060364 |doi-access=free |issn=1660-3397 |pmc=9225620 |pmid=35736167 |bibcode=2022MarDr..20..364A }}</ref>

==Structure== Alginic acid is a linear copolymer with homopolymeric blocks of (1→4)-linked β-D-mannuronate (M) and α-L-guluronate (G) residues, respectively, covalently linked together in different sequences or blocks. The monomers may appear in homopolymeric blocks of consecutive G-residues (G-blocks), consecutive M-residues (M-blocks) or alternating M and G-residues (MG-blocks). α-L-guluronate is the C-5 epimer of β-D-mannuronate.<ref>{{Cite journal |last1=Spannenkrebs |first1=Jan Benedict |last2=Petersen |first2=Agness Beenfeldt |last3=Aachmann |first3=Finn Lillelund |last4=Kabisch |first4=Johannes |date=3 April 2025 |title=Immobilization of alginate C-5 epimerases using Bacillus subtilis spore display |journal=Applied and Environmental Microbiology |volume=91 |issue=4 |pages=e00298-25 |doi=10.1128/aem.00298-25 |pmid=40178254 |pmc=12016494 |bibcode=2025ApEnM..91E.298S }}</ref>

==Forms== Alginates are refined from brown seaweeds. Throughout the world, many of the Phaeophyceae class brown seaweeds are harvested to be processed and converted into sodium alginate. Sodium alginate is used in many industries including food, animal food, fertilisers, textile printing, and pharmaceuticals. Dental impression material uses alginate as its means of gelling. Food grade alginate is an approved ingredient in processed and manufactured foods.<ref>{{cite web|title=Alginates|url=https://www.ams.usda.gov/sites/default/files/media/Alginates%20TR%202015.pdf|publisher=Agricultural Marketing Service, US Department of Agriculture|access-date=1 March 2018|date=5 February 2015}}</ref>

Brown seaweeds range in size from the giant kelp ''Macrocystis pyrifera'' which can be 20–40 meters long, to thick, leather-like seaweeds from 2–4 m long, to smaller species 30–60 cm long. Most brown seaweed used for alginates are gathered from the wild, with the exception of ''Laminaria japonica'', which is cultivated in China for food and its surplus material is diverted to the alginate industry in China.<ref name=":0" />

Alginates from different species of brown seaweed vary in their chemical structure, resulting in different physical properties of alginates. Some species yield an alginate that gives a strong gel, another a weaker gel, some may produce a cream or white alginate, while others are difficult to gel and are best used for technical applications where color does not matter.<ref>{{cite book |last1=McHugh |first1=Dennis J. |title=A Guide to the Seaweed Industry |date=2003 |publisher=Food and Agriculture Organization of the United Nations |isbn=978-92-5-104958-7 |url=https://openknowledge.fao.org/handle/20.500.14283/y4765e }}{{pn|date=November 2025}}</ref>

Commercial grade alginate is extracted from giant kelp ''Macrocystis pyrifera'', ''Ascophyllum nodosum'', and types of ''Laminaria''. Alginates are also produced by two bacterial genera ''Pseudomonas'' and ''Azotobacter'', which played a major role in the unravelling of its biosynthesis pathway. Bacterial alginates are useful for the production of micro- or nanostructures suitable for medical applications.<ref name="Remminghorst-2009">{{cite book |author=Remminghorst and Rehm|year=2009|chapter=Microbial Production of Alginate: Biosynthesis and Applications|title=Microbial Production of Biopolymers and Polymer Precursors|publisher=Caister Academic Press|isbn = 978-1-904455-36-3}}</ref>

Sodium alginate (NaC6H7O6) is the sodium salt of alginic acid. Sodium alginate is a gum.

Potassium alginate (KC6H7O6) is the potassium salt of alginic acid.

Calcium alginate (CaC12H14O12) is the calcium salt of alginic acid. It is made by replacing the sodium ion in sodium alginate with a calcium ion (ion exchange).

==Production== The manufacturing process used to extract sodium alginates from brown seaweed fall into two categories: 1) calcium alginate method where the brown seaweed is first treated with calcium chloride to form a calcium alginate intermediate before washing with hydrochloric acid, and, 2) alginic acid method where there is no calcium alginate intermediate and the brown seaweed is treated only with the hydrochloric acid to extract sodium alginate.<ref name=":0" />

Chemically the process is simple, but difficulties arise from the physical separations required between the slimy residues from viscous solutions and the separation of gelatinous precipitates that hold large amounts of liquid within their structure, so they resist filtration and centrifugation.<ref name=":0">{{Cite book |title=Production and utilization of products from commercial seaweeds |date=1987 |publisher=Food and Agriculture Organization of the United Nations |isbn=978-92-5-102612-0 |editor-last=MacHugh |editor-first=Dennis J. |series=FAO fisheries technical paper |location=Rome |chapter=CHAPTER 2 - PRODUCTION, PROPERTIES AND USES OF ALGINATES |editor-last2=McHugh |editor-first2=Dennis J.}}</ref> The conventional process involves large amounts of reagents and solvents, as well as time-consuming steps.<ref name="Bojorges-2023"/> Simpler and newer techniques, such as microwave-assisted extraction, ultrasound, high pressure, pressurized fluid extraction, and enzyme-assisted extraction, are the subject of research.<ref name="Bojorges-2023"/>

The most common, conventional extraction process involves six steps: pre-treatment of the algal biomass, acid treatment, alkaline extraction, precipitation, bleaching, and drying.<ref name="Bojorges-2023">{{cite journal |last1=Bojorges |first1=Hylenne |last2=López-Rubio |first2=Amparo |last3=Martínez-Abad |first3=Antonio |last4=Fabra |first4=María José |title=Overview of alginate extraction processes: Impact on alginate molecular structure and techno-functional properties |journal=Trends in Food Science & Technology |date=October 2023 |volume=140 |article-number=104142 |doi=10.1016/j.tifs.2023.104142 |hdl=10261/336757 |hdl-access=free }}</ref> Pre-treatments mainly aim at either breaking the cell wall to help extract the alginate, or removing other compounds and contaminants from the algae.<ref name="Bojorges-2023"/> Drying is of the first kind, also helping to prevent bacterial growth; algae which is dried is also usually powdered to expose more surface area.<ref name="Bojorges-2023"/> Common treatments to remove contaminants include treatments with ethanol and formaldehyde, the latter of which is very common; ethanol solutions help remove compounds bonded to the alginate, and formaldehyde solutions help prevent enzymatic or microbial reactions.<ref name="Bojorges-2023"/>

The algae is then treated with an acidic solution to help disrupt cell walls, which converts the alginate salts into insoluble alginic acid; a subsequently applied alkaline solution (pH 9-10), usually sodium carbonate, converts it back into water-soluble sodium alginate, which is then precipitated.<ref name="Bojorges-2023"/> It is also possible to extract the alginate directly with an alkaline treatment, but this is less common.<ref name="Bojorges-2023"/>

Alginic acid is usually precipitated, through different techniques, with either an alcohol (usually ethanol), calcium chloride, or hydrochloric acid.<ref name="Bojorges-2023"/> After the alginin is precipitated into a fine paste, it is dried, ground to the desired grain size, and finally purified through a variety of techniques.<ref name="Bojorges-2023"/> Commercial alginate for biomedical and pharmaceutical use is extracted and purified through more rigorous techniques, but these are trade secrets.<ref name="Bojorges-2023"/>

===Derivatives=== Various alginate-based materials can be produced, including porous scaffold material, alginate hydrogel, nonwoven fabric, and alginate membranes.<ref name="Zhang-2022"/> Techniques used to produce these include ion cross-linking, microfluidic spinning, freeze drying, wet spinning, and immersive centrifugal jet spinning.<ref name="Zhang-2022"/>

Calcium salts added to a sodium alginate solution to induce ionic cross-linking, which produces the hydrogel. Freeze-drying the hydrogel to eliminate water produces the porous scaffold material.<ref name="Zhang-2022"/>

Wet spinning consists of extruding an alginate solution from a spinneret into a calcium salt solution to induce ionic cross-linking (forming the gel), and then drawing the fibers out of the bath with draft rollers. Microfluidic spinning, a simpler and more eco-friendly implementation of the process, involves introducing calcium salt flows flowing alongside and touching a central "core" flow of alginate. These flows form a "sheath". The fiber then emerges from the core flow. This technique can be used to produce shaped and grooved fibers.<ref name="Zhang-2022"/>

thumb|Preparation of the alginate nonwoven fabric by the acupuncture technique Alginate fiber, which is used in fabric, is usually produced through either microfluidic spinning, wet spinning, or electrospinning to obtain thinner fibers.<ref name="Zhang-2022"/> Those fibers are used to produce alginate nonwoven fabric by carding and needle punching. The resulting felts are used in wound dressings, facial masks, and tissue scaffolds due to its hygroscopic and water retention ability.<ref name="Zhang-2022" />

==Uses== As of 2022, alginate had become one of the most preferred materials as an abundant natural biopolymer.<ref name="Zhang-2022">{{cite journal |last1=Zhang |first1=Xiaolin |last2=Wang |first2=Xinran |last3=Fan |first3=Wei |last4=Liu |first4=Yi |last5=Wang |first5=Qi |last6=Weng |first6=Lin |title=Fabrication, Property and Application of Calcium Alginate Fiber: A Review |journal=Polymers |date=8 August 2022 |volume=14 |issue=15 |page=3227 |doi=10.3390/polym14153227 |doi-access=free |pmc=9371111 |pmid=35956740}}</ref> It is particularly useful as a biomaterial because of its nontoxicity, hygroscopicity, and biocompatibility, and can imitate local bioenvironments; its degradation product can be easily cleared by the kidneys.<ref name="Zhang-2022" />

Alginate absorbs water quickly, which makes it useful as an additive in dehydrated products such as slimming aids, and in the manufacture of paper and textiles.<ref>{{cite journal |last1=Wang |first1=Honglei |last2=Qiu |first2=Qiqing |last3=Li |first3=Bingfeng |last4=Hu |first4=Yu |last5=Xu |first5=Ling |last6=Qiu |first6=Jieqiong |title=Alginate-based flame-retardant coatings for sustainable fire protection: A review |journal=International Journal of Biological Macromolecules |date=May 2025 |volume=308 |issue=Pt 3 |article-number=142448 |doi=10.1016/j.ijbiomac.2025.142448 |pmid=40147670 }}</ref>

Alginate is also used for waterproofing and fireproofing fabrics, in the food industry as a thickening agent for drinks, ice cream, cosmetics, as a gelling agent for jellies, known by the code E401 and sausage casing.<ref>{{cite web |title=What is Sodium Alginate (E401) in food? Properties, Uses, Safety |author= |work=FOODADDITIVES |date=14 May 2020 |url= https://foodadditives.net/thickeners/sodium-alginate/}}</ref><ref>{{cite book |title=Bioactive Seaweeds for Food Applications |date=2018 |doi=10.1016/C2016-0-04566-7 |isbn=978-0-12-813312-5 }}</ref> Sodium alginate is mixed with soybean protein to make meat analogue.<ref>{{cite book |last1=Arasaki |first1=Seibin |title=Low Calorie, High Nutrition Vegetables from the Sea |last2=Arasaki |first2=Teruko |publisher=Japan Publications, Inc. |date=January 1983 |isbn=0-87040-475-X |edition=1st |location=Tokyo, Japan |page=35}}</ref>

Alginate is used as an ingredient in various pharmaceutical preparations, such as Gaviscon, in which it combines with bicarbonate to inhibit gastroesophageal reflux.<ref>{{cite journal |last1=Kapoor |first1=Devesh U. |last2=Pareek |first2=Anil |last3=Sharma |first3=Swapnil |last4=Prajapati |first4=Bhupendra G. |last5=Thanawuth |first5=Kasitpong |last6=Sriamornsak |first6=Pornsak |title=Alginate gels: Chemistry, gelation mechanisms, and therapeutic applications with a focus on GERD treatment |journal=International Journal of Pharmaceutics |date=April 2025 |volume=675 |article-number=125570 |doi=10.1016/j.ijpharm.2025.125570 |pmid=40199431 }}</ref>

Sodium alginate is used as an impression-making material in dentistry, prosthetics, lifecasting, and for creating positives for small-scale casting.<ref>{{Cite journal |last1=Cervino |first1=Gabriele |last2=Fiorillo |first2=Luca |last3=Herford |first3=Alan Scott |last4=Laino |first4=Luigi |last5=Troiano |first5=Giuseppe |last6=Ameroso |first6=Giulia |last7=Crimi |first7=Salvatore |last8=Matarese |first8=Marco |last9=D'Amico |first9=Cesare |last10=Siniscalchi |first10=Enrico Nastro |last11=Cicciu |first11=Marco |date=28 December 2018 |title=Alginate Materials and Dental Impression Technique: A Current State of the Art and Application to Dental Practice |journal= Marine Drugs|volume=17 |issue=1 |page=18 |doi=10.3390/md17010018 |pmid=30597945 |pmc=6356954 |doi-access=free }}</ref>

Sodium alginate is used in reactive dye printing and as a thickener for reactive dyes in textile screen-printing.<ref>{{Cite journal |last1=Akter |first1=Nahida |last2=Akter |first2=Nasrin |last3=Pervin |first3=Mahfuza |last4=Repon |first4=Md. Reazuddin |date=2023-03-01 |title=The influence of mixed thickeners on printing over lyocell knitted fabric |journal=Heliyon |volume=9 |issue=3 |article-number=e14175 |doi=10.1016/j.heliyon.2023.e14175 |bibcode=2023Heliy...914175A |doi-access=free |pmid=36923884 |pmc=10009539 |issn=2405-8440 }}</ref> Alginates do not react with these dyes and wash out easily, unlike starch-based thickeners. It also serves as a material for micro-encapsulation.<ref>{{cite journal |last1=Aizpurua-Olaizola |first1=Oier |last2=Navarro |first2=Patricia |last3=Vallejo |first3=Asier |last4=Olivares |first4=Maitane |last5=Etxebarria |first5=Nestor |last6=Usobiaga |first6=Aresatz |title=Microencapsulation and storage stability of polyphenols from Vitis vinifera grape wastes |journal=Food Chemistry |date=January 2016 |volume=190 |pages=614–621 |doi=10.1016/j.foodchem.2015.05.117 |pmid=26213018 }}</ref>

Calcium alginate is used in different types of medical products, including skin wound dressings, in the form of hydrogels, to promote healing, since alginate can increase the fluid uptake capacity of dressings, reducing the amount of times the dressing needs to be changed.<ref>{{cite journal |last1=Lansdown |first1=Alan B. G. |title=Calcium: a potential central regulator in wound healing in the skin |journal=Wound Repair and Regeneration |date=September 2002 |volume=10 |issue=5 |pages=271–285 |doi=10.1046/j.1524-475x.2002.10502.x |pmid=12406163 }}</ref><ref>{{cite journal |last1=Stubbe |first1=Birgit |last2=Mignon |first2=Arn |last3=Declercq |first3=Heidi |last4=Van Vlierberghe |first4=Sandra |last5=Dubruel |first5=Peter |title=Development of Gelatin-Alginate Hydrogels for Burn Wound Treatment |journal=Macromolecular Bioscience |date=August 2019 |volume=19 |issue=8 |article-number=1900123 |doi=10.1002/mabi.201900123 |pmid=31237746 }}</ref>

==Alginate hydrogels== In research on bone reconstruction, alginate composites have favorable properties encouraging regeneration, such as improved porosity, cell proliferation, and mechanical strength.<ref>{{cite journal|pmid=25020082|year=2015|last1=Venkatesan|first1=J|title=Alginate composites for bone tissue engineering: A review|journal=International Journal of Biological Macromolecules|volume=72|pages=269–81|last2=Bhatnagar|first2=I|last3=Manivasagan|first3=P|last4=Kang|first4=K. H.|last5=Kim|first5=S. K.|doi=10.1016/j.ijbiomac.2014.07.008}}</ref> Alginate hydrogel is a common biomaterial for bio-fabrication of scaffolds and tissue regeneration.<ref>{{cite journal |last1=Rastogi |first1=Prasansha |last2=Kandasubramanian |first2=Balasubramanian |title=Review of alginate-based hydrogel bioprinting for application in tissue engineering |journal=Biofabrication |date=10 September 2019 |volume=11 |issue=4 |page=042001 |doi=10.1088/1758-5090/ab331e |pmid=31315105 |bibcode=2019BioFa..11d2001R }}</ref>

Covalent bonding of thiol groups to alginate improves in-situ gelling and mucoadhesive properties; the thiolated polymer (thiomer) forms disulfide bonds within its polymeric network and with cysteine-rich subdomains of the mucus layer.<ref>{{cite journal |last1=Leichner |first1=Christina |last2=Jelkmann |first2=Max |last3=Bernkop-Schnürch |first3=Andreas |title=Thiolated polymers: Bioinspired polymers utilizing one of the most important bridging structures in nature |journal=Advanced Drug Delivery Reviews |date=November 2019 |volume=151-152 |pages=191–221 |doi=10.1016/j.addr.2019.04.007 |pmid=31028759 }}</ref> Thiolated alginates are used as in situ gelling hydrogels,<ref>{{cite journal |last1=Xu |first1=Guanzhe |last2=Cheng |first2=Liang |last3=Zhang |first3=Qintong |last4=Sun |first4=Yunlong |last5=Chen |first5=Changlin |last6=Xu |first6=Heng |last7=Chai |first7=Yimin |last8=Lang |first8=Meidong |title=In situ thiolated alginate hydrogel: Instant formation and its application in hemostasis |journal=Journal of Biomaterials Applications |date=November 2016 |volume=31 |issue=5 |pages=721–729 |doi=10.1177/0885328216661557 |pmid=27485953 }}</ref> and are under preliminary research as possible mucoadhesive drug delivery systems.<ref>{{cite journal |last1=Kassem |first1=Abeer Ahmed |last2=Issa |first2=Doaa Ahmed Elsayed |last3=Kotry |first3=Gehan Sherif |last4=Farid |first4=Ragwa Mohamed |title=Thiolated alginate-based multiple layer mucoadhesive films of metformin forintra-pocket local delivery: in vitro characterization and clinical assessment |journal=Drug Development and Industrial Pharmacy |date=2 January 2017 |volume=43 |issue=1 |pages=120–131 |doi=10.1080/03639045.2016.1224895 |pmid=27589817 }}</ref> Alginate hydrogels may be used for drug delivery, exhibiting responses to pH changes, temperature changes, redox, and the presence of enzymes.<ref>{{cite journal |last1=Abasalizadeh |first1=Farhad |last2=Moghaddam |first2=Sevil |last3=Alizadeh |first3=Effat |last4=Fazljou |first4=Mohammad |last5=Torbati |first5=Mohammadali |last6=Akbarzadeh |first6=Abolfazl |date=13 March 2020 |title=Alginate-based hydrogels as drug delivery vehicles in cancer treatment and their applications in wound dressing and 3D bioprinting |journal=Journal of Biological Engineering |volume=14 |issue=8 |page=8 |doi=10.1186/s13036-020-0227-7|doi-access=free |pmid=32190110 |pmc=7069202 }}</ref>

==See also== * Hyaluronic acid: a polysaccharide in animals. * Agar

==References== {{Reflist}}

==External links== * [http://www.cybercolloids.net/library/alghistory/history-alginate-chemistry-seaweeds Alginate seaweed sources] {{Webarchive|url=https://web.archive.org/web/20130917233613/http://www.cybercolloids.net/library/alghistory/history-alginate-chemistry-seaweeds |date=2013-09-17 }} * [http://www.cybercolloids.net/library/alginate/introduction-alginate-production Alginate properties] {{Webarchive|url=https://web.archive.org/web/20130917065320/http://www.cybercolloids.net/library/alginate/introduction-alginate-production |date=2013-09-17 }}

{{DEFAULTSORT:Alginic Acid}} Category:Dietary fiber Category:Polysaccharides Category:Natural gums Category:Edible thickening agents Category:Copolymers Category:Dental materials Category:Excipients Category:Algal food ingredients Category:Brown algae Category:Food stabilizers Category:E-number additives