Galactomannan gelation: A thermal and rheological investigation analyzed using the cascade model

High-temperature water soluble fractions of locust bean gum with average ga lactose contents of 15.2% and 16% w/w displayed conventional gelling behavi or. Gel properties were dependent on galactose content, gelling temperature and concentration. The gels appeared elastic, failed at up to 150% strain, and exhibited true gellike character with long-lived cross-links. Rheologi cal meltdown occurred over a broad temperature range, and the gels melted a t temperatures between 100 and 110 degrees C. Cascade analysis of the stora ge modulus vs concentration data gave critical gelling concentrations, C-o = 0.14-0.24% w/w after 3 months gelation, but this analysis was unable to u niquely determine the number of potential cross-linking forming sites per m olecule, i.e., the functionality, f. A value of f = 5, as well as average v alues of the standard molar enthalpy, Delta H degrees = -99.6 kJ/mol and st andard molar entropy, Delta S degrees = -0.201 kJ/(mol K), for cross-link f ormation, was subsequently determined through an extension of the cascade f ormalism to describe modulus vs temperature meltdown data. That these value s did not reflect equilibrium melting was shown by a detailed comparison of the analyses of concentration and temperature dependencies of the storage modulus using the cascade model and also by an increase in the number and l ength of cross-links after 8 months gelation time and by rheological anneal ing at slower heating rates. An attempt by simulation to assess errors intr oduced through the assumption of equilibrium gel melting indicated that the true functionality, f, was closer to 20 and that the idealized critical ge lling concentrations (were the gels able to reach equilibrium) would be muc h smaller than the 0.14-0.24% w/w values obtained after 3 months. The simul ation also showed that the real values of Delta H degrees and Delta S degre es were likely to be close to the estimates obtained using the equilibrium theory and nonequilibrium data obtained after 3 months.