Calcium alginate

Calcium alginate is a water-insoluble, gelatinous, cream-colored substance formed by adding aqueous calcium chloride to aqueous sodium alginate. It is used for enzyme entrapment and creating artificial seeds in plant tissue culture. Alginate refers to salts, derivatives, or alginic acid itself, found in brown algae cell walls as calcium, magnesium, and sodium salts.

To extract alginate, seaweed pieces are stirred with hot sodium carbonate solution, forming a thick slurry of sodium alginate. The undissolved residue, mainly cellulose, is removed by diluting the slurry with water. Filtration is challenging due to viscosity, so filter aids like diatomaceous earth or air bubbles are used to trap residue particles. Air is forced into the diluted extract, causing fine residue to float and be skimmed off before filtration.

Calcium alginate is prepared by adding calcium chloride to sodium alginate solution, precipitating insoluble calcium alginate. This can be redissolved in sodium carbonate solutions to produce alginates with specific sodium-to-calcium ratios, altering physical and chemical properties.

The structure of calcium alginate hydrogels is explained by the "Egg-Box" model, where hydroxyl groups of guluronate units form cavities for cations. This model, proposed in the 1970s, remains a key explanatory tool despite its simplicity.

Calcium alginate is used in plant tissue culture for artificial seeds, enzyme immobilization, as an edible substance, in wound dressings as a hemostatic, in controlled-release drug delivery systems, and in molecular gastronomy for spherification.