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Alginates / PGA / Functionality and Rheology

Viscosity –Through selection of grade and formulation, the flow characteristics of alginates can be controlled; from free-flowing (low viscosity) todrip-free (high viscosity). The aqueous solutions of alginates have shear-thinning characteristics, i.e. the viscosity decreases with increasing shear rate (stirrer speed). This property, illustrated in Figure 6 is also called pseudoplasticity, or non-Newtonian flow. The viscosity of an alginate solution depends on the concentration of alginate and the length of the alginate molecules, i.e. the number of monomer units in the chains. The longer the chains the higher the viscosity at similar concentrations (see Figure 7 ).

Upon dissolving alginates in water, the molecules hydrate and the solution gains viscosity. The dissolved molecules are not completely flexible; rotation around the glycosidic linkages in the G-block regions is somewhat hindered, resulting in a stiffening of the chain. Solutions of stiff macromolecules are highly viscous. Temperature defines the energetic state of any chemical molecule. Hence temperaature influences the response of alginates to shear forces. As a general rule, temperature increases of 1°C lead to a viscosity drop of approximately 2.5%, see Figure 8 .

Acid Conditions –Standard grades of alginate will precipitate or form gels in acid conditions. The pKa values for mannuronic and guluronic acid are 3.38 and 3.65, respectively.  To increase the stability of an alginate to acid, the alginate may be converted to propylene glycol alginate (PGA).  This is achieved by reacting the free carboxylic group of the alginicacid with propylene oxide ( Figure 9 ).