Alginate

Polysaccharide found in brown algae. The different types of alginates are made up of long, linear molecules that are composed of two different monosaccharide groups, β-d-mannuronic acid (M) and α-l-guluronic acid (G). These acid groups can combine linearly as, for example, -M-M-M-M-M-, -G-G-G-G-G-, or -M-G-M-G-M-G. Alginate is the basic form of these substances and the related ammonium and sulfate salts—for example, sodium alginate—are water soluble. Alginates from different species of algae have varying levels of M and G acids. The length of the chains varies and the shortest typically are made up of 500 monosaccharide groups. Alginates form gels in the presence of Ca⁺⁺ (or other divalent ions, including Mg⁺⁺ and Ba⁺⁺) at temperatures that are much lower than those for the formation of gels with pectin. The melting point of alginates is slightly higher than the boiling point of water. Alginates are used as thickeners and stabilizers on account of their ability to bind large volumes of water. They are resistant to acid, which gives them an advantage over other stabilizers. They are useful primarily because of their water solubility, especially in the form of sodium alginate, which in solution takes on an ion form known as a polyelectrolyte. Calcium alginate is not water soluble. When treated with acids, alginates are converted to alginic acid.