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

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. (C) 2014 Elsevier Ltd. All rights reserved.