The effects of pH on the rheology of beta-lactoglobulin/kappa-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 oscill atory measurements over pH 4-7. For single beta-lactoglobulin, it was found that the gelation temperature and the storage modulus of the gels were str ongly pH-dependent. However, the rheological behaviour was almost the same; the beta-lactoglobulin gels were formed upon first heating from 45 to 90 d egrees C, reinforced on cooling to 20 degrees C, and then weakened during r eheating steps to 70 degrees C. Single kappa-carrageenan systems showed als o the same behaviour over pH 4-7; gelation occurred at temperatures below 2 5 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 be haviour: one over pH 5-7 and the other at pH 4.0. Over pH 5-7, the behaviou r of beta-lactoglobulin and kappa-carrageenan, upon gel formation and melti ng, 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 showe d a peculiar behaviour, similar to that of the pure protein gel. The presen ce of kappa-carrageenan is only evidenced by the significant enhancement of the protein gel strengths. At this pH, below the protein pI, the electrost atic attractive forces between beta-lactoglobulin and kappa-carrageenan are important. Therefore, these two biopolymers associated and then formed a m ixed gel, with one continuous network structures, similar to that of comple x coacervate networks. (C) 2000 Elsevier Science Ltd. All rights reserved.