Physico-chemical properties of the gelling galactans from the red alga Chondrus elatus

M Saluri1, D Fujita2 and R Tuvikene1

1Institute of Mathematics and Natural Sciences, Tallinn University, Narva mnt 29, 10120, Tallinn, Estonia
2Department of Marine Biosciences, Tokyo University of Marine Science and Technology, 
4-5-7 Konan, Minato-ku, 108-8477 Tokyo, Japan

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 hydroxyl groups may be substituted with ester sulfate, methyl groups, pyruvic acid acetal and sometimes with additional monosaccharide residues1. The gelling properties characteristic to some algal polysaccharides (e.g. kappa carrageenan) make them valuable food ingredients.

Chondrus elatus Holmes is endemic species to Japan and belongs together with 10 other species to the Chondrus genus2 which represents a group of thoroughly studied economically important marine seaweeds. Although used as a gelatinous food or food additive known as kanten in Japan3, 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, 13C-NMR, FT-IR and FT-Raman spectroscopy, size-exclusion chromatography, rheological and optical analysis 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 6 different disaccharide repeating units were determined in the summary polysaccharide from C. elatus, responsible for the characteristic rheological properties of the polysaccharide mixture.

References:
1 Usov, A.I. (1998). Structural analysis of red seaweed galactans of agar and carrageenan groups. Food Hydrocolloids, 12, 301-308.
2 Freshwater, D.W., Frederico, S., Butler, B.S., Hommersand, M.H., Chase, M.W. (1994). A gene phylogeny of the red algae (Rhodophyta) based on plastid rbcL. Proceedings of the National Academy of Sciences, 91, 7281-7285.
3 Johnston, H.W. (1966). The Biological and Economic Importance of Algae, Part 2. Tuatara: Journal of the Biological Society, 14, 30-62.