| Abstract
| - The gelation of locust bean gum in concentrated sucrose solutionshas been studied as afunction of temperature, LBG and sucrose concentrations. Acritical gelling concentration of approximately1% w/w LBG in 60% w/w sucrose was measured. The gelation rateinitially increased with decreasingtemperature until a maximum in gelation rate was found close to −5°C. Along with the large temperaturehysteresis between the temperature for maximum gelation rate and themelting temperature (>44 °C),it is proposed that the cross-linking was controlled by nucleation andgrowth processes rather thanreversible pairwise cross-linking. The slow evolution of LBGcross-links was detected at lower frequenciesthrough an increase in the storage modulus, while at higher frequenciesa decrease in the storage moduluswas observed. This decrease at higher frequencies was associatedwith a reduction in chain entanglementsduring the initial stages of gelation. In addition to cross-linkformation through association of 1,4-linkedβ-d-mannan regions which were sterically uninhibited bygalactose, chain localization may also havebeen entropically driven due to a reduction in solvent quality as thetemperature was decreased towardthe gelling temperature. LBG exhibited incompatible behaviour athigh sucrose concentrations as arheological inversion was detected for sucrose concentrations greaterthan 50% w/w sucrose (at 1.5%w/w LBG), where the polymer rheology changed to that resembling aconcentrated sucrose solution. Arheological assessment of the influence of temperature upon theviscoelastic properties of a concentratedLBG/sucrose solution was also performed using the time/temperaturesuperposition principle. Theevaluated WLF constants c1 and c2 were 7.5 and50 K for Tg = 255 K, similar to those reportedfor sucrose,glucose, and maltodextrin solutions and several inorganic and organicliquids. These constants did notretain their original meaning, however, owing to the heterogeneousnature of 1.5% w/w LBG, 60% w/wsucrose solution, as reflected by the rheologically derived high glasstransition temperature.
|
