ScienceAsia 32 Supplement 1 (2006): 011-017 |doi: 10.2306/scienceasia1513-1874.2006.32(s1).011

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

Jantana Praiboon,^a* Anong Chirapart,^b Yoshihiko Akakabe,^c Orapin Bhumibhamon^d and Tadahiko Kajiwara^c

 
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 ¹H, ¹³C NMR and FT-IR spectroscopy. Agar with partial 6-O-methylated on 3-linked -D-galactopyranosyl, 2-O-methylated on 4-linked 3,6-anhydro--L -galactopyranosyl and 4-O-methyl--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 -D-galactopyranosyl units and partial methylation on C2 of 4-linked 3,6-anhydro--L - galactopyranosyl units were observed in the agar extracted from G. edulis which corresponded with higher gelling temperature (>> 60 C^o). 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/cm²) whereas only a slight effect was noted on G. fisheri (228.27 48.18 g/cm²) and G. edulis (239.95 28.35 g/cm²). Further investigation may need to determine the constituent sugars and an alternative utilization of the Thai gracilaroids.

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a Department of Biotechnology, Faculty of Agro-industry, Kasetsart University, Bangkok 10900, Thailand.
b Department of Fishery Biology, Faculty of Fishery, Kasetsart University, Bangkok 10900, Thailand.
c Department of Biological Chemistry, Faculty of Agriculture, Yamaguchi University, Yamaguchi 753-8515, Japan.
d Schools of Agroindustry, Mae Fah Luang University, Chiang Rai 57100, Thailand.

* Corresponding author, E-mail: juntana_p@yahoo.com