Seasonal variations in the composition and gel quality of agar from Gracilaria edulis in the Philippines

Abstract

Agar from Gracilaria edulis collected in the Philippines is composed primarily of a repeating disaccharide unit of 4-linked 3,6-anhydro-α-L-galactopyranose (LA) and 3-linked 6-O-methyl-β-D-galactopyranose (G6M). Chemical and spectroscopic analyses further revealed partial (19–27 mol %) methylation at O-2 of the LA residues. Sulfate was attached to O-4 of G6M and/or G, with approximately 20% of these residues bearing the substituent. No pyruvate substituent was detected. A seasonal assessment of the composition, rheological and physical properties of Gracilaria edulis agar was undertaken. Constituent sugar analysis revealed a variable degree of methylation (80–91 mol % on O-6 of the G and 19–27 mol % on O-2 of the LA residues); the highest amounts of mono-O-methylated residues (G6M and LA2M) were found in the samples collected in the dry season while the lowest was obtained during the rainy season. The sample collected during the onset of the rainy season contained the lowest amount of sulfate (2.3%) while the cold season sample contained the highest sulfate content (3.2%). Seaweed samples collected during the onset of the rainy season exhibited the highest agar yield, gel strength, deformation, cohesiveness, and melting temperature. Gracilaria edulis agar possesses gel characteristics useful in the food industry.