The present study has demonstrated the significant effect
of added soy protein on the rheological properties of tomato
juice. Two opposing phenomena in tomato juice with soy
were assumed to be involved when shearing was applied: a
structure breakdown followed by aggregation at different
shear rates. Most likely, changes to the protein were
responsible for the different time-dependent behaviors.
The transition from a thixotropic to a rheopectic behavior
may be accounted for by the formation of aggregates.
These aggregates may have resulted from rearrangements of
proteins during shearing that subsequently become more
available for interactions with high DE pectins and all other
particles present in the tomato juice.
Dynamic tests confirmed the gel-like behavior of the
two products. The addition of soy to tomato juice increased
G0 and G00: The higher gel strength of the system may have
reduced the serum separation and the water holding capacity
of the system as suggested by others, without altering its
shear thinning behavior. The two products depict
characteristics of ‘physical gels’ governed by non-covalent
interactions which are dynamically formed and broken.
The application of a modified Cox and Merz rule indicated
the complexity of the colloidal products.
Further studies are underway to enhance our
understanding on a molecular level of the interactions
between tomato juice and soy protein.