Rheological characterisation of kappa-carrageenan/locust bean gum mixtures

The ratio of mannuronic acid to guluronic acid (M/G) in alginic acids from blade, stipe and holdfast of Lessonia trabeculata collected in three locations of the Chilean coast was determined by total hydrolysis with formic acid. It was found that the reaction of alginic acids with 90% formic acid for 6 h at 100°C, followed by treatment with 1.5 N formic acid for 2 h at 100°C produced the total hydrolysis. The values of M/G ratio, determined by HPLC, were slightly lower than those obtained by the traditional hydrolysis method employing 80% sulphuric acid. No systematic tissue-to-tissue variations in the M/G ratio were observed. FT-IR spectroscopy, especially in the second-derivative mode, was applied for characterizing alginates and partial hydrolysis fractions. The alginate fraction enriched in polymannuronic acid presented two bands at ~893 and ~822 cm-1 while the polyguluronic acid enriched fractions showed four characteristic absorption bands at around 947, 903, 813 and 780 cm-1.