Title : Large amplitude oscillatory rheology and phase transition study of binary gelatin/alginate gels
Gelatin hydrogels are extensively employed in the food industry for the purposes of thickening, textural engineering, and stabilization, mainly due to their viscoelasticity and thermoreversibility. The present study extends the investigation on the rheological behavior of binary aqueous mixtures of gelatin and alginate. The biomaterial concentration was up to 5 wt.% whereas the alginate to gelatin ratio was up to one. Using custom-made isolated prototype modules, discrete and thin gel layers were created at 5 °C. The gelation and the melting point of the gels were studied under small amplitude oscillatory measurements, by applying a rate of 1 °C/min. We found that both phase transitions were shifted to higher temperatures for the binary mixtures. Isothermal measurements of the storage modulus (G’) against the gelation time were also performed, at 10, 15, and 20 °C. The isothermal curves were sharply increased at lower temperatures, reflecting the increased formation rate of the gelatin triple helixes. The kinetic gelation model of Djabourov et al. could describe all isothermal gelation data of binary solutions. Large amplitude oscillatory shear (LAOS) measurements were conducted on fresh gels, prepared on the rheometer plate, as well as on individual gel specimens at 5 °C. Strain sweeps up to 1000% strain were performed at 5, 10, and 20 °C. Although the sweep patterns were qualitatively similar, the waveform analysis showed that the lower temperature results in an earlier onset of the nonlinear behavior, at about 10% strain. Additionally, nonlinear stress analysis of the Lissajous plots was performed. It was found that the fresh gels exhibited strain hardening behavior whereas the layers were intact and showed strain softening. Furthermore, alginate microbeads with diameters from 30 to 200 µm were generated using microfluidics. First insights into the LAOS behavior of filled gelatin gels will be presented.