Phase separation in aqueous gelatin-kappa-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 p hase separation is observed independently of pH (3.0-9.0) and temperature, and is characterised by a relatively small degree of co-precipitation, whic h increases when TBC in the initial mixture increases. At pH = 5.0, the yie ld of complex precipitated is maximal at a 3.0 molar ratio of gelatin and k appa CG in the initial mixture. The complexes are formed and stabilised via electrostatic interactions, rather than through hydrogen bonds or hydropho bic interactions and an "all or none" type complex formation mechanism take s 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 co ncentration of kappa CG in the initial mixture is higher than 0.65% w/w. Th e 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.