A study of gelatin gelation by Fourier transform mechanical spectroscopy

This paper reports the results of experiments which were designed to test the hypothesis that the recording of anomalous values of rheological exponents in the gelation of gelatin may, under some circumstances, be due to inappropriate rheometry. Results obtained from stress relaxation tests and oscillatory shear frequency sweeps are compared with the time-resolved results of oscillatory shear measurements based on Fourier Transform Mechanical Spectroscopy (FTMS) which is used to identify the Gel Point. The experiments reported herein involve gelatin concentrations and temperatures which result in gel times in the range ca. 102-104 s, corresponding to the range of fast and slow gelation conditions reported elsewhere. Within this range, under conditions of appropriately low sample mutation number in our FTMS experiments, the values of the relaxation exponent α obtained at the Gel Point are found to be essentially independent of temperature and concentration, with a value of α ∼ 0.69. The results reported herein establish the rheometrical conditions under which rapid gelation produces anomalously low values of α and we conclude that the appearance of low α values, in our experiments, can be attributed to inappropriate rheometrical procedures.