Rheological effects of soy protein addition to tomato juice

The rheological properties of tomato juice containing 1% soy protein were compared to standard tomato juice. Shear thinning behavior was observed by steady-shear flow experiments for both tomato juices with and without soy. Addition of soy protein significantly affected the rheological time-dependent behavior of tomato juice. In the case of plain juice, a slight thixotropic behavior was observed. The tomato juice with added soy exhibited a thixotropic behavior at low shear rates followed by a transition to rheopectic behavior at higher shear rates. Dynamic tests indicated a physical gel behavior for both samples. The addition of soy protein increased the dynamic modulus values, suggesting an enhanced aggregation between pectins and soy protein leading to increased stability of the suspension, without major qualitative effects on the gel-like behavior. The application of a modified Cox and Merz rule indicated the complexity of the colloidal products. Rheological measurements suggest that rearrangements of the soy protein, when subjected to different shear rates, influenced the colloidal interactions with tomato particles and reinforced the network structure and the stability of the overall tomato system.