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A negatively charged polysaccharide, iota-carrageenan (CI) has been mixed with a gelatin (G) sample having a pI of about 4.5. At pH 7, the net charge of the gelatin is negative. However, associative interactions may be formed locally when both biopolymers are in random coil conformation. In order to understand the evolution of these interactions during gelation, a methylene blue (MB) spectrophotometric method has been used. Methylene blue molecules interact with carrageenan sulphated groups giving rise to a modification of the optical density spectrum. As the CI/MB spectrum changes at decreasing temperatures, the effect of the carrageenan concentration on the CI/MB spectra has been studied at 60, 50, 40, 30 and 20 °C. The corresponding carrageenan concentration of saturation has been found. Its value does not change with temperature showing that the stoichiometry of binding is unchanged. Therefore, the change in spectrum with temperature has been attributed to an increase in carrageenan chains rigidity. Different concentrations of gelatin (in the range 0.1–0.75 wt%) have been added to the CI/MB solution. The optical density spectrum of CI/MB/G solution versus decreasing temperature is measured and compared with the one obtained for the CI/MB solution. When carrageenan chains are in ordered conformation, the modifications of the spectra cannot be simply interpreted by a competition between gelatin and MB for the carrageenan sulphated groups. From these results, the hypothesis of an increase in carrageenan chains rigidity, induced by local associative interactions between carrageenan chains and ordered gelatin chains is discussed.

The negatively charged polysaccharide, iota-carrageenan has been mixed with two types of gelatin (PS and LH) which differ by their pI, about 9 and 4.5, respectively. At pH 7, gelatin PS chains have a net positive charge and gelatin LH chains have a net negative charge. Phase diagrams in non-gelling conditions (60°C) and pH 7 have been established in both 25 and 100 mM NaCl. The iota-carrageenan/gelatin PS phase diagrams are typical of systems involving attractive interactions leading to the formation of complexes: both polymers are mostly in the lower phase and increasing salt concentration leads to a reduction of the phase separated domain. The iota-carrageenan/gelatin LH mixture shows a very different behavior. No macroscopic bulk phase separation is observed. However, an increase in the turbidity measured by spectrophotometry is observed for certain compositions which would indicate microscopic phase separation. From the limit between the turbid and transparent concentration domains, the binodal may be drawn. Considering the negative net charge of gelatin LH chains at pH 7, repulsive interactions were expected to govern the phase separation. However, in this case too, by increasing salt concentration the phase-separated domain is reduced, where an increase would have been expected for thermodynamic incompatibility. By following the evolution of the absorption spectrum of a methylene blue/iota-carrageenan/gelatin LH mixture as a function of the salt concentration, it has been shown that gelatin LH chains associate with carrageenan ones in low salt conditions up to 200 mM NaCl.