Chemical characteristics and gelling properties of agar from two Philippine <i>Gracilaria</i> spp. (Gracilariales, Rhodophyta)

Abstract

The chemical structure of agars extracted from Philippine <i>Gracilaria arcuata</i> and <i>G. tenuistipitata</i> were determined by NMR and infrared spectroscopy. Agar with alternating 3-linked 6-O-methyl-β-D-galactopyranosyl and 4-linked 3,6-anhydro-2- O-methyl-α-L-galactopyranosyl units was isolated from <i>G. arcuata</i>, while the agar from <i>G. tenuistipitata</i> possesses the regular agarobiose repeating unit with partial methylation at the 6-position of the D-galactosyl residues. Both agars exhibit sulphate substitution at varying positions in the polymer. Chemical analyses reveal higher 3,6-anhydrogalactose and lower sulphate contents in alkali-modified than in native agar from both samples. Also, alkali modification enhanced agar gel strength and syneresis. Native <i>G. arcuata</i> agar produces a viscous solution (2000 cP at 75 °C) with a high gelling point (>60 °C) that forms a soft gel even after alkali modification (gel strength: <300 g cm−2). On the other hand, the agar from <i>G. tenuistipitata</i> exhibits gel qualities typical of most Gracilaria agars.