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

Gelation and melting of single and mixed systems of 10% β-lactoglobulin and 1% κ-carrageenan have been investigated by small-deformation oscillatory measurements over pH 4–7. For single β-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 β-lactoglobulin gels were formed upon first heating from 45 to 90°C, reinforced on cooling to 20°C, and then weakened during reheating steps to 70°C. Single κ-carrageenan systems showed also the same behaviour over pH 4–7; gelation occurred at temperatures below 25°C and the melting took place on reheating above 40°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 β-lactoglobulin and κ-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 κ-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 β-lactoglobulin and κ-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.