Rheological Characteristics of Alginate Isolated from Commercially Cultured Nemacystus decipiens (Itomozuku)

Abstract

The non-Newtonian behavior and storage modulus of an alginate isolated from Nemacystus decipiens which had been harvested in Uruma City (Okinawa, Japan) in aqueous solutions were investigated. Gelation occurred at a concentration of 0.5% of the alginate on addition of a mixed solution of NaCl (0.1%), KCl (0.1%) and CaCl₂ (0.1%) at room temperature (25°C). The flow curves, at 35°C, of the alginate showed plastic behavior at various concentrations. The storage modulus of the alginate increased with an increase in concentration and decreased gradually with an increase in temperature up to 50°C, which was estimated to be a transition temperature; then it, on the contrary, increased gradually with a further increase in the temperature and showed the maximum value at a temperature of 60, 70 and 65°C at 0.3, 0.5 and 0.7%, respectively. The storage modulus of the alginate showed a large value even in the presence of urea (4.0 M). The storage modulus of the alginate showed a very large value at the pH range of 6.0-7.2, but very small at pH 4.0. However, a comparable large storage modulus was observed in the alkaline range (pH 9.0-11.0). Intra- and intermolecular Ca²⁺-bridges may play a dominant role in the gel formation of alginate molecules in the presence of Ca²⁺ ions with ionic bonding and electrostatic forces of attraction. The hydrophobic interaction may take place on the L-guluronic acid-rich homopolymeric blocks of the alginate molecules in the high temperature range.