Effect of isolation procedures on the molecular composition and physical properties of Eucheuma cottonii carrageenan

Abstract

Direct treatment of Eucheuma cottonii with hot potassium hydroxide leads to a solid residue (alkali treated weed) which was found to contain predominantly carrageenan and cellulose. Following dispersion and heating in water, separation into polymer-containing solution and an insoluble particulate phase was readily achieved. Chemical analysis of solubilized polymers showed that the predominant carrageenan present in native weed was a mu/kappa-carrageenan which was readily converted into kappa with hot alkali. The particulate phase from both materials was predominantly cellulose; characterization by X-ray diffraction showed the presence of partial cellulose type I crystallinity. Solid state 13C-NMR spectra showed that this crystalline fraction was ~53% of the total cellulose and was comprised of ~40% type I α and ~60% type I β . The cellulosic part seemed unaffected by the hot alkali treatment. A carrageenan type polysaccharide was found associated with the cellulosic fraction; which underwent the same conversion as the mu/kappa carrageenan on alkali treatment. Further refining of solubilized carrageenan by isopropyl alcohol precipitation, gel pressing or freeze-drying, resulted in only minor differences in chemical composition. Hot solution viscosity and gelation properties in the presence of potassium chloride were measured showing no significant differences in gelling behaviour between alkali treated weed and the different carrageenan isolates but a higher solution viscosity for the alkali treated weed.