Rheological Properties of Agar-agar Gels -The Effect of Alkaline Concentration in Pretreatment upon Rheological Properties of Hydrogels of Gracilaria and Gelidium-

Abstract

The rheological studies were carried out on the hydrogels obtained from samples from seaweeds Gracilaria blogetti of Chile (Co) as well as Argentina (Ag. b.) and Gelidium amansii of Izu (Ig. a. ) of Japan under various alkali-pretreatment condition. Alkaline concentration during pretreatment varied within the range from 0 to 10% NaOH. The concentrations of gels p-repared were 1.5, 2.0 and 3.0 wt %.
Relaxation curves up to 4 hrs were obtained within the temperature range from 25° C to 55° C. These curves were represented. by a six-element mechanical model consisting of three Maxwell elements in parallel.
The results obtained are as follows; 1) The elastic modulus, E1 for the desulfated Cg, b. sample is nealy zero, but after pretreatment with a solution containing 2% NaOH it increases rapidly with increasing concentration of NaOH. The desulfated Ag. b. sample produces a strong gel in which the magnitude of E1 increases with increasing concentration of NaOH up to 4%, and then decreases slowly.2) The magnitude of E1 increases rapidly, when 3, 6-anhydrogalactose content in the samples is more than about 28%. 3) An apparent activation energy obtained from the temperature dependence of the longest relaxation time ri is nearly constant and about 5 kcal/mol. The effect of increase in alkaline concentration during pretreatment upon the ability of gelation in agar gels is explained in terms of the conformation of agar molecules in water.