Determination of D-Mannuronic to L-Guluronic Acids Ratio in Acid Hydrolysis of Alginate under Improved Conditions

Abstract

Decomposition of alginate under the hydrolysis procedure with sulfuric acid was evaluated by determining the recovery of monouronates produced and its depolymerization extent. The first three hour hydrolysis with 80% sulfuric acid at 30°C, followed by the two hour hydrolysis with 2 N sulfuric acid at 100°C, resulted in complete hydrolysis with the highest recovery in the conditions examined. Under these conditions, the recovery of authentic uronates was 80.9% for D-man-nuronic acid and 62.8% for L-guluronic acid. Prolonged treatment resulted in a lower recovery of monouronates, and showed indications of an increase in the degradation of monouronates. The degradation products showing reducing power, were detectable with the phenol-sulfuric acid method but undetectable with the carbazole-sulfuric acid method. D-Mannuronic acid to L-guluronic acid ratios (M/G ratio) of nine species of alginates determined by the improved procedure were compared to those determined by the method of Haug and Larsen. The values of M/G ratio obtained by both methods were nearly equal in six species and were slightly different among three specíes. The modified method, however, showed a 9∼17% higher recovery of monouronates than the method of Haug and Larsen. In addition, the time for the completion of the analysis process was shortened by at least 1/5.