Viscosity of galactomannans during high temperature processing:: influence of degradation and solubilisation

The rheological properties of guar gum (GG) and locust bean gum (LBG), in response to high temperature treatments, were measured using a rheometer equipped with a high pressure cell. This has allowed the viscosity to be assessed at temperatures above 100 degrees C and as the polymer suspension is heated from 20 to 121 degrees C and then cooled back to ambient temperature to simulate a food sterilisation cycle. Activation energies for depolymerisation estimated from viscosity changes with time at a series of constant temperatures were estimated as 63 kJ/mol for GG and 98, 104, 110 kJ/mol for three different samples of LEG. A model was developed to interpret the viscosity change through the simulated sterilisation cycle. This took into account the degradation of the polysaccharide and the change in viscosity due to thermal motion. Estimations of molecular weight changes during the heating process suggest that GG is more susceptible to thermal degradation than LEG. It is suggested that this is due to the greater ability of the latter to associate in solution. (C) 1999 Elsevier Science Ltd. All rights reserved.