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

Abstract

The thermoreversible gelation behaviour of aqueous κ-carrageenan solution with addition of sucrose (up to 30 wt%) was investigated via small and large deformation oscillatory rheology. The structures of these κ-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 °C to 52.8 °C) and melting temperature (from 51.2 °C to 67.3 °C) to a higher level and made gel network stronger. The critical relaxation exponent n, critical gel strength Sg and normalised Sg obtained from Winter-Chambon equation varied as the concentration of sucrose increased. More specifically, with an increased addition of sucrose, n decreased while both Sg and normalised Sg increased. The shift of FTIR band of G4S in κ-carrageenan and loss of the vibration band of free hydroxyl group in sucrose indicated the interactions between sucrose and κ-carrageenan 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 Rc 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 κ-carrageenan by facilitating the formation of denser and thicker fibril helical junctions.