Phase separation in aqueous gelatin-κ-carrageenan systems

At low ionic strength (mu = 0.002), pH = 5.0 (close to the isoionic point (iip = 4.8)) and total biopolymers' concentration (TBC) less than 0.2% w/w, gelatin is compatible with kappa-carrageenan (kappa CG). When TBC is higher than 0.2% w/w, gelatin forms water-insoluble complexes with kappa CG and phase separation is observed independently of pH (3.0-9.0) and temperature, and is characterised by a relatively small degree of co-precipitation, which increases when TBC in the initial mixture increases. At pH = 5.0, the yield of complex precipitated is maximal at a 3.0 molar ratio of gelatin and kappa CG in the initial mixture. The complexes are formed and stabilised via electrostatic interactions, rather than through hydrogen bonds or hydrophobic interactions and an "all or none" type complex formation mechanism takes place. At mu = 0.1-0.2 (NaCl) pH greater than or equal to iip, and 313 K, neither co-precipitation nor incompatibility were observed in an overwide range of total biopolymers' concentration (0.01-25% w/w). At mu = 0.35, pH = 5.0 and 313 K, gelatin exhibits incompatibility with kappa CG when the concentration of kappa CG in the initial mixture is higher than 0.65% w/w. The phase diagram of the system, in the above mentioned conditions, indicates a possible specific interaction of the two biopolymers even at a very high ionic strength. (C) 1999 Elsevier Science Ltd. All rights reserved.