Property of Alginate
Only alginic acid and its polyvalent metal salts are insoluble, except magnesium alginate. That is to say, alginic acid with alkali metal (Na+, K+, etc.), ammonium and quaternary ammonium compounds are water-soluble.
Alginate is soluble in alcohols and ketones but not soluble in hard water and milk because both of them contain Ca2+. If sodium alginate needs to be added in such solution, a chelant agent such as EDTA of sodium hexametaphosphate can be used to sequester Ca2+. Propylene glycol alginate (80–85% esterified) is less affected by calcium ions and can be used in milk. It is acid resisted, remaining soluble when pH downs to about pH 2.
The viscosity of alginate increases with the concentration of alginate used and decreases with increasing temperature. Such phenomenon is more apparent for high viscosity alginates. The viscosity increases and flow properties decreases if increasing the concentration of polyvalent metal ion.
Sodium alginate solution decreases in apparent viscosity with increasing shear rate. Low molecular weight alginate exhibits the Newtonian behavior. At low levels of calcium ion, the effect of calcium increasing viscosity is particularly apparent in the case of apparent in the case of alginates with higher content of D-mannuronic acid.
Like many natural polysaccharides, dried alginates are not stable to heat, oxygen, metallic ions, etc. Stored in such circumstances, alginates will be degraded naturally. The high-viscosity alginate is more rapidly degraded than the medium-or low- viscosity ones. The order of stability in storage is: sodium alginate > ammonium alginate > alginic acid.
Different kind of alginate salt gives different stability, so does the different grade product. The industrial grade alginate solution is more easily degraded by microbe in the air, because such products contain much algal particles and nitrogenous matter which offer plenty of nutrition for microbe. The pure sodium alginate solution can be kept at room temperature for several months without obvious change in viscosity. When temperature increases, all alginate solutions will depolymerize. Alginate solutions are stable in the pH range 5.5 – 10 at room temperature for a long time, but will form the gel below pH 5.5. Propylene glycol alginate solution is relatively stable at room temperature at pH 3–4, but it will lose the viscosity rapidly below pH 2 and above pH 6.
Calcium ion makes the ion exchange action take place rapidly and give an anti-reversibility gel. The gel formed by alginate with high guluronic acid is crispness but much rigider, while gel formed by alginate with high mannuronic acid performs better flexibility but poor rigidity. Gel with different rigidity could be produced with the control of guluronic acid/mannuronic acid ratio.
The insoluble alginate salt behaves like typical ion exchange resin. The affinities of divalent metal ions are dependent on the relative amounts of D-mannuronic acid and L-guluronic acid units in the alginate. The affinity of alginates for divalent ions decreases in the following order:
a. For the alginate
rich in M from Laminaria digitata:
Pb > Cu > Cd > Ba > Sr > Ca > Co, Ni, Zn, Mn > Mg,
b. For the alginate rich in G from Laminaria hyperborea:
Pb > Cu > Ba > Sr > Cd > Ca > Co, Ni, Zn, Mn > Mg.
The concentration of divalent cations required to bring about gel formation and precipitation for sodium alginate from two type seaweeds is the same, and increases in the order:
<Pb < Cu < Sr < Cd < Ca < Zn < Ni < Co < Mn, Fe < Mg.
Aside from the interaction of metal ions with carboxyl groups of alginate, the hydroxyl groups on the polymer also play some role in ion binding.