CONCENTRATION-DEPENDENCE OF THE CRITICAL VISCOELASTIC PROPERTIES OF GELATIN AT THE GEL POINT

Gelatin gel properties have been studied through the evolution of the storage [G'(omega)] and the loss [G''(omega)] moduli during gelation or melting near the gel point at several concentrations. The linear viscoelastic properties at the percolation threshold follow a power-law G'(omega)alphaG''(omega)alphaomega(DELTA) and correspond to the behavior described by a rheological constitutive equation known as the Gel Equation. The critical point is characterized by the relation: tan delta = G''/G' = cst = tan (DELTA . pi/2) and it may be precisely located using the variations of tan delta versus the gelation or melting parameter (time or temperature) at several frequencies. The effect of concentration and of time-temperature gel history on its variations has been studied. On gelation, critical temperatures at each concentration were extrapolated to infinite gel times. On melting, critical temperatures were determined by heating step by step after a controlled period of aging. Phase diagrams [ T = f(C)] were obtained for gelation and melting and the corresponding enthalpies were calculated using the Ferry-Eldridge relation. A detailed study of the variations of DELTA with concentration and with gel history was carried out. The values of DELTA which were generally in the 0.60-0.72 range but could be as low as 0.20-0.30 in some experimental conditions, were compared with published and theoretical values.