Funorans from Gloiopeltis species. Part II. Rheology and thermal properties

• Funoran forms strong and turbid gels in the presence of Ba2+ ions.
• Funoran does not show gelling ability in the absence of added cations.
• Anions do not significantly impede the gel formation process of funoran.
• Funoran gels can be prepared at room temperature without heating.
• The helical pitch of funoran is 1.8 nm, which is slightly smaller than for agarose.

The rheology and thermal properties of funorans from the main species of the red algal genus Gloiopeltis – G. furcata, G. tenax and G. complanata, isolated by the successive cold and hot water extraction processes were comparatively investigated with special attention given to the effect of co-solutes, thermal history and galactan concentration. The galactan samples were characterized by dynamic rheometry, differential scanning calorimetry, optical density studies and X-ray analysis. Funoran gel formation is a slow process depending highly on the presence of cationic substances. The ability of metal ions to enhance funoran gelation follows the order of Li+ < Na+ < NH4+ < K+ < Rb+ < Cs+ for monovalent and the sequence of Cu2+ < Mg2+, Mn2+ < Zn2+ < Ca2+ < Sr2+ << Ba2+ for divalent cations. The Ba2+ ions exhibit a specific effect on the gel forming process of funoran. The ability of funoran to form very strong gels was observed for the first time, especially in the presence of Ba2+ ions. Anionic substances exhibit a weak inhibitive effect on the gelation of funoran, with the strongest effect observed for SCN− and the weakest for Cl−. The galactans from G. furcata and G. tenax show similar rheological properties, latter giving solutions with somewhat higher viscosities; the presence of idealized agarose diads in the composition of G. complanata funorans notably enhances their gelling ability. The helical pitch of funoran (1.8 nm), determined by X-ray diffraction analysis, is slightly smaller than that of agarose.

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