Phase transition of locust bean gum-, tara gum- and guar gum-water systems

摘要

Phase transition behaviour of neutral galactomannans, i.e., locust bean gum (LBG), tara gum (Tara-G) and guar gum (GG)-water systems is investigated. In this study, water content {W_c = (gram of water)/(gram of dry sample)} of these systems was varied from 0.2 to 3.6 g g~(-1). In the DSC heating curves, glass transition (T_g), cold crystallization (T_(cc)) and melting (T_m) were observed in all three samples. In addition, liquid crystal transition (T~*) was observed in GG-water systems at a temperature higher than T_m. Using T_g, T_(cc), T_m and T~*, phase diagrams of each system were established. From the melting enthalpy of ice in the systems, three types of water, non-freezing water (W_(nf)), freezing bound water (W_(fb)) and free water were calculated. The maximum amount of W_(nf) was observed at W_c = 0.7 g g~(-1), where T_g showed the lowest temperature. The amount of W_(nf) in LBG and GG is higher than that of Tara-G, whereas the highest amount of W_(fb) is found in GG. T~* was only observed in GG-water systems. It is concluded that frequency of the side chains in the repeating unit of the main chains of these three galactomannan affects the frozen structure of the glassy state in the presence of water.