Viscoelastic relaxation spectra of some native starch gels

Oscillatory mechanical tests are commonly used for characterizing the rheological properties of starch gels. Although widely used, simple phenomenological models (e.g., Maxwell and Kelvin-Voigt) do not suffice for describing the viscoelastic dynamics of starch gels in the face of shear stress applications. Given the complex nature of starch gels, it is expected that mechanical dynamics are expressed over a wide range of time scales. From a phenomenological standpoint, this work considers the relaxation spectrum as a tool for characterizing viscoelastic properties of starch gels. To this end, 15% w/w gels made of four native starches (wheat, potato, corn and banana) were prepared and dynamic oscillatory measurements were performed at various strain magnitudes. The relaxation spectrum was estimated by fitting experimental data by the application of Tikhonov regularization procedure. The number and relative intensity of the peaks obtained from the interpretation of the relaxation spectrum were suitable to distinguish some features of the viscoelastic properties of the starch gels. The presence of a dominant peak at about 0.1 s was observed for all gels, whose resolution is enhanced for high strain magnitudes. This indicates that strong flow patterns increase the homogenization degree of starch gels. Overall, the results showed that the relaxation spectra can be considered a signature of the viscoelastic properties of specific starch gels.