GELATION BEHAVIOR OF CONCENTRATED LOCUST BEAN GUM SOLUTIONS

The gelation of locust bean gum in concentrated sucrose solutions has been studied as a function of temperature, LBG and sucrose concentrati ons. A critical gelling concentration of approximately 1% w/w LBG in 6 0% w/w sucrose was measured. The gelation rate initially increased wit h decreasing temperature until a maximum in gelation rate was found cl ose to -5 degrees C. Along with the large temperature hysteresis betwe en the temperature for maximum gelation rate and the melting temperatu re (>44 degrees C), it is proposed that the cross-linking was controll ed by nucleation and growth processes rather than reversible pairwise cross-linking. The slow evolution of LBG cross-links was detected at l ower frequencies through an increase in the storage modulus, while at higher frequencies a decrease in the storage modulus was observed. Thi s decrease at higher frequencies was associated with a reduction in ch ain entanglements during the initial stages of gelation. In addition t o cross-link formation through association of 1,4-linked p-D-mannan re gions which were sterically uninhibited by galactose, chain localizati on may also have been entropically driven due to a reduction in solven t quality as the temperature was decreased toward the gelling temperat ure. LBG exhibited incompatible behaviour at high sucrose concentratio ns as a rheological inversion was detected for sucrose concentrations greater than 50% w/w sucrose (at 1.5% w/w LBG), where the polymer rheo logy changed to that resembling a concentrated sucrose solution. A rhe ological assessment of the influence of temperature upon the viscoelas tic properties of a concentrated LBG/sucrose solution was also perform ed using the time/temperature superposition principle. The evaluated W LF constants c(1) and c(2) were 7.5 and 50 K for T-g=255 K, similar to those reported for sucrose, glucose, and maltodextrin solutions and s everal inorganic and organic liquids. These constants did not retain t heir original meaning, however, owing to the heterogeneous nature of 1 .5% w/w LBG, 60% w/w sucrose solution, as reflected by the theological ly derived high glass transition temperature.