Effects of sucrose addition on the rheology and microstructure of kappa-carrageenan gel

The thermoreversible gelation behaviour of aqueous kappa-carrageenan solution with addition of sucrose (up to 30 wt%) was investigated via small and large deformation oscillatory rheology. The structures of these kappa-carrageenan/sucrose gels were characterised by Fourier transform infrared (FTIR) spectroscopy, small angle X-ray scattering (SAXS) and field emission scanning electron microscopy (FESEM). Incorporation of 30 wt% sucrose shifted both the gelation temperature (from 36.8 degrees C to 52.8 degrees C) and melting temperature (from 51.2 degrees C to 67.3 degrees C) to a higher level and made gel network stronger. The critical relaxation exponent n, critical gel strength Sg and normalised S-g obtained from Winter-Chambon equation varied as the concentration of sucrose increased. More specifically, with an increased addition of sucrose, n decreased while both S-g and normalised S-g increased. The shift of FTIR band of G4S in kappa-carrageenan and loss of the vibration band of free hydroxyl group in sucrose indicated the interactions between sucrose and kcarrageenan molecules. SAXS and FESEM results supported the rheology findings and revealed that denser and thicker carrageenan fibril structures formed when addition of sucrose increased. Moreover, upon 30 wt% sucrose, the average cross-sectional radius of gyration R-c of the carrageenan chains increased from 1.17 to 1.47 nm. In summary, a scheme was proposed to demonstrate how sucrose addition promoted the gelation of k-carrageenan by facilitating the formation of denser and thicker fibril helical junctions. (C) 2017 Elsevier Ltd. All rights reserved.