Abstract: We studied the effect of pH on the rheological and structural properties of low methoxyl pectin/sodium caseinate composite system. At different acidic pH values (3.0, 3.5, 4.0, 4.5, 5.0, 5.5), static shear and dynamic viscoelastic rheological properties, zeta potential, particle size, turbidity, and microstructure were measured. The results showed that the rheological properties of the composite system changed in two stages. Fluid index n and tanδ increased whereas consistency coefficient K decreased with the increase of pH from 3.0 to 4.0. As the pH increased from 4.5 to 5.5, n and tanδ decreased rapidly and then increased to a level higher than the initial one while K increased steeply and then decreased. Zeta potential decreased with increasing pH; it was positive at pH 3.0 to 4.0 but was negative at pH 4.5 to 5.5. Particle size and turbidity increased to a maximum at pH 4.5 and then decreased with the increase of pH. As the pH value decreased, the microstructure of the composite system changed from a disordered state into a dense one. At pH 3.0–4.0, the structure of the casein surface was dominated by low methoxyl pectin, and the network structure of low methoxyl pectin was more obvious at pH 4.5–5.5. Therefore, changes in pH can cause changes in the rheological and structural properties of the composite system, thereby affect the stability of the system.

Key words: low methoxyl pectin, sodium caseinate, pH, rheological properties, microstructure