Rando Tuvikene

Gel-forming structures and stages of red algal galactans of different sulfation levels

Sol-gel transition processes of algal galactans were studied using cryofixation method in combination with freeze-drying and scanning electron microscopy (SEM) techniques. The structures formed in successive stages of gelling process upon cooling were rapidly frozen at defined temperature points and viewed by SEM. It was established that in the case of both types of gelling galactans investigated, a fine honeycomb-like network exists for a wide range of solution temperatures. The formation and structure of this network depends on the structural type, gelling stage, and concentration of the galactan in solution. The honeycomb suprastructures exist also in carrageenan and agarose sols (at temperatures considerably exceeding the gelling temperatures). An additional helical network formed showed different behaviour in the case of carrageenan and agar-type polysaccharides. In the gel-formation process, tightening of the network takes place in both types of galactan gels; the honeycomb structures persist in carrageenan (furcellaran) but not in agarose gels.

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.