As previously mentioned, alginates contain various proportions of mannuronate and guluronate monomers.
To form a gel, alginates must contain a sufficient level of guluronate monomers in a block to react with calcium. The reactivity with calcium and the consequent gelling capacity is a direct function of the average length of the G-blocks. The alginate containing the highest level of GG fractions (i.e. the stem grade alginate from Laminaria hyperborea ) possess the highest ability to form strong gels ( See Table 1 ). FMC BioPolymer is the leading supplier of alginates for high gel strength applications.
Regions of guluronate monomers in one alginate molecule can be linked to a similar region in another alginate molecule by means of calcium ions or other multivalent cations (see Figure 10 ).
The divalent calcium cation, Ca 2+ , fits into the guluronate block structure like eggs in an egg box (Figure 11 ). This binds the alginate polymers together by forming junction zones, resulting in gelation of the solution.
An alginate gel may be considered part solid and part solution. After gelation, the water molecules are physically entrappedby the alginate matrix, but are still free to migrate. The water holding capacity of the gel is due to capillary forces.