Rheological behaviour of low-methoxyl pectin gels over an extended frequency window

The rheological behaviour of biopolymer gels is commonly investigated by means of small amplitude oscillatory measurements. This consolidated approach was used in the present paper to characterize the influence of ionic strength and temperature on the gelation of commercial low-methoxyl pectin (LMP) (DE = 22.5%). The main results showed that the sol/gel transition is very sensitive to the ionic strength of the medium, while the viscoelastic properties of the gel structure, developed over 8 h cure test, were retained up to 60 °C. The soft material under study displays the typical rheological behaviour of a solid-like material, in good agreement with results previously reported in the literature. It is known that the best time or frequency window useful to explore in dynamic mode the viscolelastic behaviour of complex materials such as a gel would range over several decades of frequency. The possibility to extend the frequency domain of the dynamic tests in the region of the lower frequencies (10−5–102 rad s−1) was therefore investigated by matching information coming from dynamic and transient rheological tests and converting data from time domain into frequency domain throughout the discrete retardation spectrum of the material, as described by Kaschta [Kaschta, J., & Schwarzl, F. R. (1994). Calculation of discrete retardation spectra from creep data - I. Method. Rheologica Acta, 33, 517-529.]. Results show that around 10−5 rad s−1 the mechanical spectrum of LMP gel approaches the beginning of the terminal region, where the rheological behaviour is mainly dominated by viscous flow, thus indicating a liquid-like character of a material and thus accounting for the large proportion of solvent (higher than 98%) contained in the network.