PHYSICO-CHEMICAL PROPERTIES OF THE GELLING GALACTANS FROM THE RED ALGA CHONDRUS ELATUS

摘要

Red algal galactans are sulfated polysaccharides, which usually have a linear backbone built up of alternating 3-linked β-D-galactopyranose and 4-linked α-galactopyranose residues. The β-galactose residues always belong to D-series, whereas the α-galactose residues are D in carrageenans and L in agars. A substantial part of α-galactose may exist in the form of a 3,6-anhydro derivative. Various hydroxy groups may be substituted with ester sulfate, methyl groups, pyruvic acid acetal, and sometimes with additional monosaccharide residues. The gelling properties characteristic to some algal polysaccharides (e.g. kappa carrageenan) make them valuable as food ingredients. Chondrus elatus Holmes is endemic species to Japan and belongs together with 10 other species to the Chondrus genus which represents a group of thoroughly studied economically important marine seaweeds. Although used as a gelatinous food or food additive known as kanten in Japan, the composition, structure and rheological properties of the polysaccharides from C. elatus have not been described in the literature. In this work the galactans extracted from C. elatus were characterized by ~1H-NMR, ~(13)C-NMR, size-exclusion chromatography, gas chromatography and thermogravimetric methods. By varying the extraction medium (water or phosphate buffer) and temperature, concentration of salt in alcohol used for precipitation of the galactans, separation of polysaccharides on the basis of molecular weight and chemical structure was possible. Coexistence of at least three chemically different polysaccharide fractions in the seaweed was observed, including an abundant cold water soluble fraction with high amounts of lambda and mu carrageenans and hot water soluble fraction containing predominantly kappa carrageenan moieties. Altogether more than six different disaccharide repeating units were determined in the summary polysaccharide from C. elatus, responsible for the characteristic rheological properties of the polysaccharide mixture.