Characterisation of rheology and microstructures of kappa-carrageenan in ethanol-water mixtures

The effects of ethanol (up to 20 wt%) on the rheological properties and structural characteristics of kappa-carrageenan gel were investigated by Field Emission Scanning Electron Microscopy (FESEM) and Small Angle X-ray Scattering (SAXS). Both the sol-gel and gel-sol transition temperatures shifted to higher degree (from 36.8 +/- 0.5 to 52.5 +/- 1.4 degrees C and from 51.2 +/- 0.6 to 67.0 +/- 0.5 degrees C, respectively) upon 20 wt% ethanol addition (P < 0.05). The critical relaxation exponent n and the critical gel strength S-g obtained from Winter-Chambon criterion decreased and increased, respectively as the ethanol concentration increased. The kappa-carrageenan gel was formed due to the formation of fibrillar networks, and the fibrillar density increased upon ethanol addition via FESEM. Moreover, upon 20 wt% ethanol addition, the average radius of gyration of kappa-carrageenan strand increased from 1.18 +/- 0.03 of control to 1.55 +/- 0.02 nm by SAXS. A mechanism underlying the effect of ethanol on the kappa-carrageenan gelation was proposed based on coil to double helix transition followed by the helix aggregation.