Alginic acid

Alginic acid
Names
Other names
Alginic acid, E400
Identifiers
ATC code A02BX13
9005-32-7 Yes
ChemSpider  Yes
EC number 232-680-1
UNII 8C3Z4148WZ 
Properties
(C6H8O6)n
Molar mass 10,000 – 600,000
Appearance white to yellow, fibrous powder
Density 1.601 g/cm3
Acidity (pKa) 1.5–3.5
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references
Macrocystis pyrifera, the largest species of giant kelp

Alginic acid, also called algin or alginate, is an anionic polysaccharide distributed widely in the cell walls of brown algae, where through binding with water it forms a viscous gum. In extracted form it absorbs water quickly; it is capable of absorbing 200–300 times its own weight in water.[1] Its colour ranges from white to yellowish-brown. It is sold in filamentous, granular or powdered forms.

Structure

Alginic acid is a linear copolymer with homopolymeric blocks of (1-4)-linked β-D-mannuronate (M) and its C-5 epimer α-L-guluronate (G) residues, respectively, covalently linked together in different sequences or blocks. The monomers can appear in homopolymeric blocks of consecutive G-residues (G-blocks), consecutive M-residues (M-blocks) or alternating M and G-residues (MG-blocks).

Forms

Alginates are refined from brown seaweeds. A wide variety of brown seaweeds of the phylum Phaeophyceae are harvested throughout the world to be converted into the raw material commonly known as sodium alginate. Sodium alginate has a wide use across a wide variety of industries including food, textile printing and pharmaceutical. Dental impression material utilizes alginate as its means of gelling. Alginate is both food and skin safe.

Seaweeds can be classified into three broad groups based on pigmentation: brown, red and green. These broad groups are the Phaeophyceae, Rhodophyceae and Chlorophyceae, respectively. Brown seaweeds are usually large, and range from the giant kelp Macrocystis pyrifera that is often 20 m long, to thick, leather-like seaweeds from 2–4 m long, to smaller species 30–60 cm long. None of the usual seaweeds for alginate production are cultivated. They cannot be grown by vegetative means, but must go through a reproductive cycle involving an alternation of generations. This makes cultivated brown seaweeds too expensive when compared to the costs of harvesting and transporting wild seaweeds. The only exception is for Laminaria japonica, which is cultivated in China for food but the surplus material is diverted to the alginate industry in China.

Alginates from different species of brown seaweed often have variations in their chemical structure, resulting in different physical properties. For example, some may yield an alginate that gives a strong gel, another a weaker gel; one may readily give a cream/white alginate, another may give that only with difficulty and is best used for technical applications where color does not matter.[2]

Commercial varieties of alginate are extracted from seaweed, including the giant kelp Macrocystis pyrifera, Ascophyllum nodosum, and various types of Laminaria. It is 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.[3]

The chemical compound sodium alginate is the sodium salt of alginic acid. Its empirical formula is NaC6H7O6. Sodium alginate is a gum, extracted from the cell walls of brown algae.

Potassium alginate is a chemical compound that is the potassium salt of alginic acid. It is an extract of seaweed. Its empirical chemical formula is KC6H7O6.

Calcium alginate, made from sodium alginate from which the sodium salt has been removed and replaced with calcium, has the chemical formula C12H14CaO12.

Production

The processes for the manufacture of sodium alginate from brown seaweeds fall into two categories: 1) Calcium alginate method and, 2) Alginic acid method. The chemistry of the processes used to make sodium alginate from brown seaweeds is relatively simple. The difficulties of the processes arise from the physical separations which are required, such as the need to filter slimy residues from viscous solutions or to separate gelatinous precipitates which hold large amounts of liquid within the structure and which resist filtration and centrifugation.[4]

Uses

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. It is also used for waterproofing and fireproofing fabrics, as a gelling agent, and for thickening drinks, ice cream and cosmetics.

Alginate is used in various pharmaceutical preparations such as Gaviscon, Bisodol, and Asilone. Alginate is used extensively as an impression-making material in dentistry, prosthetics, lifecasting and occasionally for creating positives for small-scale casting. It is also used in the food industry, for thickening soups and jellies.

Alginates are allowed and used in organic farming as a fertilizer[5] and in organic food production as a food additive.[6]

Calcium alginate is used in different types of medical products, including burn dressings that promote healing and can be removed with less pain than conventional dressings.

Also, due to alginate's biocompatibility and simple gelation with divalent cations such as Ca2+, it is widely used for cell immobilization and encapsulation.

Alginic acid (alginato) is also used in culinary arts, most notably in the Spherification techniques of Ferran Adrià.[7]

Due to its ability to absorb water quickly, alginate can be changed through a lyophilization process to a new structure that has the ability to expand. It is used in the weight loss industry as an appetite suppressant.

In March, 2010 researchers at Newcastle University announced that dietary alginates can reduce human fat uptake by more than 75%.[8]

Sodium alginate

As a flavorless gum, it is used by the foods industry to increase viscosity and as an emulsifier. It is also used in indigestion tablets and the preparation of dental impressions.

A major application for sodium alginate is in reactive dye printing, as thickener for reactive dyestuffs (such as the Procion cotton-reactive dyes) in textile screen-printing and carpet jet-printing. Alginates do not react with these dyes and wash out easily, unlike starch-based thickeners.

Sodium alginate is a good chelator for pulling radioactive substances from the body, such as iodine-131 and strontium-90, that have taken the place of their non-radioactive counterparts.[9][10] It is also used in immobilizing enzymes by inclusion.

As a food additive, sodium alginate is used especially in the production of gel-like foods. For example, bakers' "Chellies" are often gelled alginate "jam." Also, the pimento stuffing in prepared cocktail olives is usually injected as a slurry at the same time that the stone is ejected; the slurry is subsequently set by immersing the olive in a solution of a calcium salt, which causes rapid gelation by electrostatic cross-linking. A similar process can be used to make "chunks" of everything from cat food through "reformed" ham or fish to "fruit" pieces for pies. It has the E-number 401.

Nowadays, it is also used in the biological experiments for the immobilization of cells to obtain important products like alcohols, organic acids, etc.

In recent years, sodium alginate has been used in molecular gastronomy. Ferran Adrià pioneered the technique, and it has since been used by chefs such as Grant Achatz and Heston Blumenthal. Sodium alginate is combined with calcium lactate or similar compounds to create spheres of liquid surrounded by a thin jelly membrane.

Potassium alginate

Potassium alginate is widely used in foods as a stabilizer, thickener, and emulsifier.

Its use as a pharmaceutical excipient is currently limited to experimental hydrogel systems. The viscosity, adhesiveness, elasticity, stiffness, and cohesiveness of potassium alginate hydrogels have been determined and compared with values from a range of other hydrogel-forming materials. The effect of calcium ions on the rheological properties of procyanidin hydrogels containing potassium alginate and intended for oral administration has also been investigated.

See also

References

  1. ↑ Raymond C. Rowe, Paul J. Sheskey, Marian E. Quinn, ed. (2009), "Adipic Acid", Handbook of Pharmaceutical Excipients (Rowe, Handbook of Pharmaceutical Excipients) (Sixth ed.), Pharmaceutical Pr, pp. 11–12, ISBN 0-85369-792-2
  2. ↑ FAO FISHERIES TECHNICAL PAPER 441, Dennis J. McHugh, School of Chemistry, University College, University of New South Wales and Australian Defence Force Academy Canberra Australia
  3. ↑ Remminghorst and Rehm (2009). "Microbial Production of Alginate: Biosynthesis and Applications". Microbial Production of Biopolymers and Polymer Precursors. Caister Academic Press. ISBN 978-1-904455-36-3.
  4. ↑ FAO Fisheries Technical Paper, 2003
  5. ↑ INTRODUCTION TO COMMERCIAL SEAWEEDS, U.N. Food and Agriculture Organization, Technical Paper 441, 2003.
  6. ↑ Organic food and additives, Food Additives and Ingredients Association, accessed April 24, 2015.
  7. ↑ "Lo Mejor de la Gastronomia". StarChefs.com. Archived from the original on 1 December 2007. Retrieved 2007-11-14.
  8. ↑ "Seaweed to tackle rising tide of obesity". Newcastle University. Archived from the original on 25 March 2010. Retrieved 2010-03-22.
  9. ↑ Sutton, A., Harrison, G. E., Carr, T. E., and Barltrop, D. Reduction in the absorption of dietary strontium in children by an alginate derivative. Br.J.Radiol. 44[523], 567. 1971
  10. ↑ Sutton, A., Harrison, B. E., Carr, T. E., and Barltrop, D. Reduction in the absorption of dietary strontium in children by an alginate derivative. Int.J.Radiat.Biol.Relat Stud.Phys.Chem.Med. 19[1], 79–85. 1971

External links