Seasonal variations in the yield, gelling properties, and chemical composition of agars from <i>Gracilaria eucheumoides</i> and <i>Gelidiella acerosa</i> (Rhodophyta) from the Philippines

Abstract

The yield, physical, and chemical properties of agars from two Philippine red seaweeds, <i>Gracilaria eucheumoides</i> and <i>Gelidiella acerosa</i>, were investigated on a bimonthly basis. The yield of agar from <i>Gracilaria eucheumoides</i> was at a maximum during the early rainy season (May, 29%) and at a minimum during the summer month of March (20%). In <i>Gelidiella acerosa</i>, a peak in agar yield was also recorded in May (21%), with generally higher yields recorded during the rainy than in the dry season. Agar gel strengths fluctuated from 225 to 430 g cm−2 and from 160 to 820 g cm−2 for <i>Gracilaria eucheumoides</i> and <i>Gelidiella acerosa</i>, respectively, and both agars exhibited strongest gels in July. Significant seasonal variations were observed in the gelling and melting temperatures of agar from <i>Gracilaria eucheumoides</i>, but not from <i>Gelidiella acerosa</i>. Sulphate content only varied slightly in agar samples from <i>Gracilaria eucheumoides</i>, while a higher sulphate content was found in <i>Gelidiella acerosa</i> agar during the dry season. Moreover, the sulphate content in <i>G. acerosa</i> agar fluctuated inversely with the 3,6-anhydrogalactose content. A FT-IR analysis showed a fairly constant spectrum for temporal <i>Gracilaria eucheumoides</i> agar while peaks attributed to S–O vibrations intensified in <i>Gelidiella acerosa</i> samples which were recorded to contain high sulphate residues and possess low gel strengths. Diagnosis of the FT-IR spectra in the 1000–400 cm−1 frequency range was also conducted in comparison with agarose and <i>Gracilaria chilensis</i> agar.