Thermal stability of red algal galactans: Effect of molecular structure and counterions.

Thermal degradation of κ-, ι-, λ-carrageenans, furcellaran, funoran and agarose samples in dry and sol states was investigated. The polysaccharides subjected to heat treatment were characterized by 1H NMR, 13C NMR and FTIR spectroscopy, size exclusion chromatography and static rheometry methods. The microstructure of galactan gels was studied using a cryofixation method in combination with freeze-drying and SEM techniques. Thermal stability at high temperatures decreases in the order of agarose>furcellaran>funoran>κ-carrageenan>λ-carrageenan>ι-carrageenan for dry preparations. The respective sequence for sol state is ι-carrageenan>λ-carrageenan>κ-carrageenan>furcellaran>funoran>agarose. The presence of methoxy groups stabilizes algal polysaccharides whereas divalent cations as counterions increase the susceptibility towards thermal degradation. In dry state the thermal treatment leads to significant desulfation of the galactan before the complete depolymerization occurs. Depending on the sulfation degree and the presence of 3,6-anhydrogalactose residues in the galactan, a notable amount of 5-hydroxymethylfurfural (yield 0.7-21.8%) is formed during the decomposition in sol state.