Agars from Three Fijian Gracilaria Species

Abstract The polysaccharides present in three species of Gracilaria from a single site in Fiji, G. maramae South, G. edulis (Gmelin) Silva and G. arcuata Zanardini v. snackeyi Weber van Bosse have been characterised as substituted agars by chemical, physical and 13C-NMR spectroscopic analyses, primarily after alkali treatment. The agar from Gracilaria edulis is unmethylated but 13% of the 3-linked β-D-galactopyranosyl units bear 4,6-O-pyruvate acetal substituents. The resonance for the quaternary carbon of the pyruvate acetal was observed in the 13C-NMR spectrum of this agar. The agar from Gracilaria maramae is partially methylated, 45% on O-6 of the 3-linked β-D-galactopyranosyl units and 10% on O-2 of the 4-linked 3,6-anhydro-α-L-galactopyranosyl units. In addition, constituent sugar and glycosyl linkage analyses revealed 7 mol% of 4-linked L-galactopyranosyl units not clearly observed by 13C-NMR spectroscopy. The agar from Gracilaria arcuata v. snackeyi is much more highly methylated, 83% on O-6 of the β-D-galactopyranosyl and 88% on O-2 of the 3,6-anhydro-α-L-galactopyranosyl units. The alkali-modified agars were obtained in acceptable yields and their gels had good clarity and low colour but gel strengths (300–350 g cm−2 for 1% gels) at the low end of the range favoured by agar processors. The Gracilaria arcuata v. snackeyi agar has a higher melting point (ca. 100 °C) than conventional agars, which may be a useful new property.