Physical and Chemical Characterization of Agar Polysaccharides Extracted from the Thai and Japanese Species of Gracilaria

Abstract

Agar polysaccharides extracted from two Thai species of Gracilaria (G. fisheri and G. edulis) and one Japanese species (Gracilaria sp.) were investigated by physical and chemical analysis, and 1 H, 13 C NMR and FT-IR spectroscopy. Agar with partial 6-O-methylated on 3-linked β-D-galactopyranosyl, 2-O-methylated on 4-linked 3,6-anhydro-α-L -galactopyranosyl and 4-O-methyl-α-L- galactopyranosyl units attached to the C6 of 3-linked D-galactopyranosyl units were isolated from G. fisheri. The large parts of 6-O-methylated on 3-linked β-D-galactopyranosyl units and partial methylation on C2 of 4-linked 3,6-anhydro-α-L - galactopyranosyl units were observed in the agar extracted from G. edulis which corresponded with higher gelling temperature (»60 C°). In contrast, the Japanese agar extracted from Gracilaria sp. showed a typical pattern of agarobiose with partial methylation at C6 of the D-galactopyranosyl units. All agars extracted exhibited sulfate substitution at different positions in the polymers. High sulfate contents were obtained in native agar of G. fisheri (4.56%) and G. edulis (7.54%) that mainly branched at C-4 of the D-galactopyranosyl unit. The presence of this unit was responsible for poor gelling ability of the agar polymers. Alkali treatment was effective both in removing alkali-labile sulfate and increasing the gel strength in Gracilaria sp. (334.5 ± 14.1 g/cm