The effects of pH on the rheology of β-lactoglobulin/κ-carrageenan mixed gels

Gelation and melting of single and mixed systems of 10% beta-lactoglobulin and 1% kappa-carrageenan have been investigated by small-deformation oscillatory measurements over pH 4-7. For single beta-lactoglobulin, it was found that the gelation temperature and the storage modulus of the gels were strongly pH-dependent. However, the rheological behaviour was almost the same; the beta-lactoglobulin gels were formed upon first heating from 45 to 90 degrees C, reinforced on cooling to 20 degrees C, and then weakened during reheating steps to 70 degrees C. Single kappa-carrageenan systems showed also the same behaviour over pH 4-7; gelation occurred at temperatures below 25 degrees C and the melting took place on reheating above 40 degrees C. In contrast, over this pH range, mixed systems showed two distinct types of behaviour: one over pH 5-7 and the other at pH 4.0. Over pH 5-7, the behaviour of beta-lactoglobulin and kappa-carrageenan, upon gel formation and melting, can easily be identified in that of the mixed systems, thus suggesting that phase-separated gels were formed. While at pH 4.0, the mixed gel showed a peculiar behaviour, similar to that of the pure protein gel. The presence of kappa-carrageenan is only evidenced by the significant enhancement of the protein gel strengths. At this pH, below the protein pI, the electrostatic attractive forces between beta-lactoglobulin and kappa-carrageenan are important. Therefore, these two biopolymers associated and then formed a mixed gel, with one continuous network structures, similar to that of complex coacervate networks. (C) 2000 Elsevier Science Ltd. All rights reserved.