Alginates are salts of alginic acid, a polyuronide made up of a sequence of two hexuronic acid residues: 

• (beta) - D - mannuronic acid unit and
• (alpha) - L - guluronic acid

Guluronic acid block

Mannuronic acid block

Three types of blocks exist: 

• Homogeneous mannuronic acid (M-M-M-M )
• Homogeneous guluronic acid (G-G-G-G)
• Alternating acids (M-G-M-G)

The two sugars are not distributed at random along the chain, but sometimes form blocks of up to twenty units. The proportion of these blocks depends on the species of seaweed and whether the stipe ("stalk") or blade ("leaf") of the seaweed is used. The number and length of the blocks is very important as these factors determine the physical properties of alginates.

In solution, alginates can be used as thickening or gelling agents. These properties depend on the presence or absence of calcium ions.

Thickening

When a soluble alginate is dissolved, the acid groups are entirely ionized and a viscous solution is obtained. With high viscosity and shear-thinning properties, its rheology is typical of solutions of flexible coil macromolecules. These two properties are proportional to the concentration and the molecular weight. As the temperature rises, the viscosity decreases. This is reversible.

Calcium-induced gelation of alginates 

The guluronic segments of alginate chains resemble a buckled ribbon. In the presence of calcium ions, segments that are homogeneous can associate to form aggregates similar to an "eggbox". Within these junction zones the chains are in a regular pleated structure that is stabilized by calcium ions, each neutralizing a negative charge on two different chains. As a result, alginates richer in these blocks form stronger gels. When the calcium-induced crosslinks are strong and numerous, an alginate gel will be thermally irreversible.