Rheology and structure of mixed kappa-carrageenan/iota-carrageenan gels

The rheology of mixed kappa-carrageenan/iota-carrageenan gels was investigated in the presence of 75 or 150 mM K+. The addition of iota-carrageenan to kappa-carrageenan led to an initial weak increase of the Young's modulus. A maximum occurred at concentrations ratios close to 1:1, followed by a weak decrease at higher iota-carrageenan concentrations. When kappa- and iota-carrageenan were mixed at a constant total polysaccharide concentration, a plateau of the Young's modulus and a clear local minimum in the fracture strain in both compression and extension were identified at ratios between 2:1 and 4:3 (iota:kappa). At all other ratios, the expected strong increase of the Young's modulus and decrease of the fracture strain were observed with an increasing kappa-carrageenan fraction. A weak maximum of the fracture stress in compression and extension was observed at ratios between 1:1 and 1:2 (iota:kappa). The structural and rheological findings support segregative phase-separation of mixed carrageenan gels in the presence of K+. Phase separation could be promoted through curing of the gel network of iota-carrageenan at temperatures above the coil-to-helix transition of kappa-carrageenan. (C) 2014 Elsevier Ltd. All rights reserved.