Abstract
Alginic acid has a physiological function to promote the excretion of sodium ion. As the chemical structure is known to affect the function of alginic acid as a dietary fibre and also the affinity for ions to a great extent, it is necessary to clarify the effect of the chemical properties on the sodium ion binding capacity in order to establish the extraction conditions and origins of alginic acid having higher binding capacity. In the present investigation, alginic acid (H-Alg) and potassium alginate (K-Alg) were extracted from Laminaria japonica Areschoung, Ecklonia cava Kjellman and Sargassum hemiphyllum under different acid hydrolysis conditions, and the relationship between the sodium ion binding capacity in vitro and the chemical properties such as the composition and sequence of uronic acid and the molecular weight were examined. The ratio of D-mannuronic acid to L-guluronic acid (M/G ratio) and the molecular weight of K-Alg prepared in this study were in the range of 0.3 to 1.2 and 1.5×104 to more than 15×104 depending on the species of brown algae and the extraction conditions, respectively. In spite of such a large variation of the chemical properties, no significant difference was observed in the sodium ion binding capacity, suggesting that the sodium ion binding capacity does not depend on those chemical properties. It was noteworthy that K-Alg from S. hemiphyllum had a much lower M/G ratio (0.3-0.4) and a higher percentage of homopolymeric blocks of L-guluronic acid than that from the others, while the yield and the sodium ion binding capacity were equivalent. As the homopolymeric blocks of L-guluronic acid were less susceptible to enter- obacteria than the homopolymeric blocks of D-mannuronic acid, S. hemiphyllum may be a promising bioresourse for the extraction of alginate that can have higher capacity to excrete sodium ion.
