Rheological properties of mixed starch-κ-carrageenan gels in relation to enzymatic digestibility

The rheological properties of mixed Hylon VII and Ca-forms of κ-carrageenan gels were studied at a total polysaccharide concentration of 3%. Before mixing, Hylon VII was heated to 150°C which allowed degradation of the granule structure. κ-Carrageenan was preheated at 70°C and the mixing occurred at 90°C. The gelation of κ-carrageenan and amylose occurred at the same temperature (about 20°C). Microscopic examination of the gels showed a phase-separated type of structure. When the concentration of κ-carrageenan was 0.3 or 1.0% and that of HYLON VII, 2.7 and 2.0%, respectively, two reversible networks were formed. Although the gel-formation occurred at the same temperature, a two-step melting behavior was observed for the mixtures. The melting temperatures of the individual HYLON VII and κ-carrageenan maintained their position in the composite gels suggesting gelation of the two components separately in their own phases. Microscopic examination showed that when the ratio of carrageenan to Hylon VII increased, the size of amylose-rich domains in the dispersed phase also increased. The in vitro α-amylolysis of starch was independent on the storage modulus of the gel in the range 185–15,000 Pa, whereas in the range 15,000–84,200 Pa a good correlation (−0.94) between G′ and in vitro α-amylolysis of single or mixed HYLON VII was found.