So anyone that knows me well will know I am bit of a foodie. I love eating at fabulous restaurants and gobbling down weird and wonderful things from scorpions (yes really) to fermented eggs (ick!). Having worked at a Michelin starred restaurant, I have seen molecular gastronomy first hand but lately I have seen more and more of my lab equipment in the kitchen, most recently I have seen chefs using rotary evaporators (!). I could write about so many different molecular gastronomy toys but the one that has got me most interested is the use of alginate gels in the kitchen. I have seen an increase lately in the use of sodium alginate by chefs, in particular to make artificial caviar or beads that supposedly give a taste explosion in the mouth. I really did not know much about how they were made and was embarrassed slightly when a friend of mine asked how they worked, so here is a brief history of alginates, particularly for use in molecular gastronomy.
In 1881, Stanford (E. C. C. Stanford, Chem. News 245-257, 1883) discovered a colloid (a substance that is microscopically dispersed evenly throughout another substance, much like milk) made from brown algae that he named algin. Stanford continued to investigate this substance and found that alkali salts, such as sodium and potassium alginate, gave viscous, aqueous solutions at very low algin concentrations. The algins were precipitated from solution by the addition of metal ions such as those of calcium and aluminum.1
Alginic acid is a linear polymer based on two monomeric units, β-D-mannuronic acid (M) and α-L-guluronic acid (G). The alginate polymer is formed by these monomers at the C-1 and C-4 positions. An alginate molecule is basically a co-polymer and the proportion of M and G blocks varies depending on the seaweed source with the properties of the alginate source being greatly dependent on the G to M ratio. Most alginate is currently extracted from just three of the 265 reported genera of the marine brown algae (Phaeophyceae). Macrocystis is the major genus used and is harvested off the west coast of the USA while in Europe we use Laminaria and Ascophyllum. These plants are mostly harvested naturally although large-scale cultivation does take place in China.2
Alginates are used in food because they are excellent thickening, stabilising, and gelling agents and because, unlike other gels such as agar, alginate generally forms thermo-stable gels between 0 and 100 °C. Most alginate used in foods is in the form of sodium alginate. In order to form a gel, sodium alginate needs to come into contact with divalent ions such as calcium (Ca2+). As soon as sodium alginate is added to a solution of calcium chloride, a gel forms as the sodium ions are exchanged with calcium ions and the polymer becomes cross-linked. The longer the alginate is in contact with the calcium chloride solution, the more rigid the gel will become, as more cross-links with the calcium ions can be formed. Also, depending on the concentration of calcium ions, the gels are either thermo-reversible (low concentrations) or not (high concentrations).3
Many chefs are now using this process to make alginate beads, which they call spherification. This video shows how the alginate beads are made. For direct spherification, sodium alginate is added to the food that is being spherified and the droplets of food are then dropped into a calcium bath.Alternatively, for reverse spherification, sodium alginate is added to the bath in which calcium rich food is spherified. Spherification was first introduced to the culinary world by the chefs at El Bulli and it is worth reading their story here.
Alginates have many uses other than in the kitchen, one of the most important being in medical applications. Alginates are used in wound dressing materials for the treatment of acute or chronic wounds. Calcium alginate is insoluble in water and can be woven into various textiles and bandages.4 The bandage is removed much more easily than other bandages, such as those made from cellulose, because calcium alginate can be dissolved in a simple salt solution. Alginates are also used in the treatment of cystic fibrosis, wherein bacterial biofilms formed from alginate gels are secreted by P. aeruginosa.5 Alginates are also used widely in the drug delivery applications.6,7
I hope you have learnt something today about alginates and how they can be used for many different applications. I look forward to trying some spherified foods in the future, and when I do, I will update you further.
1 A. B. Steiner and W. H. McNeely, Ind. Eng. Chem., 1951, 43, 2073–2077.
2 P. Gacesa, Carbohyd. Polym., 1988, 8, 161-182.
3 A. S. Waldman, L. Schechinger, G. Govindarajoo, J. S. Nowick, and L. H. Pignolet, J. Chem. Educ., 1998, 75, 1430-1431.
4 C. . Knill, J. . Kennedy, J. Mistry, M. Miraftab, G. Smart, M. . Groocock, and H. . Williams, Carbohyd. Polym., 2004, 55, 65-76.
5 S. N. Pawar and K. J. Edgar, Biomaterials, 2012, 33, 3279-305.
6 S. A. Abukalaf, A. Badwan, A. Abumalooh, and O. Jawan, Drug. Dev. Ind. Pharm., 1985, 11, 239-256.
7 H. H. Tønnesen and J. Karlsen, Drug. Dev. Ind. Pharm., 2002, 28, 621-30.
Picture from molecularrecipes.com