Development of a functional model for tomato paste rheology : a thesis presented in partial fulfilment of the requirements for the degree of Master of Food Technology at Massey University, Manawatu, New Zealand
Tomato paste is a seasonal product, processed for retail or packed aseptically in bulk to use
as a raw ingredient for manufacturing many other tomato based products such as sauces,
ketchup, and soups. Paste is added to formulated food products to provide flavour, tomato
solids, and viscosity. Viscosity is mostly imparted by the insoluble solids but there is a
contribution from the soluble solids in the tomato paste. Because the composition and
physical nature of tomato solids varies with processing methods, tomato variety and
maturity, the functional properties of tomato pastes can also be highly variable. The
objective of this study was to develop methodologies that could be used to characterise
tomato paste batches in such a way that the functionality of the paste is predictable. Ideally
rheological functionality should be predictable from compositional information and
characterisation should require a minimum of measurement effort.
This work explored how paste composition impacted on paste rheology and found that
much of the variation in flow properties of tomato concentrates can be explained by
appropriate characterisation of the water insoluble and soluble solids levels in the paste.
Serum contributes to the flow behaviour of tomato paste due to the presence of soluble
solids in the serum. In particular, it was found that it was primarily sugars that cause this
effect, potentially by enhancing the pectin-pectin interactions in the WIS components of
the paste. In this work it was found that there were measurable differences in serum
viscosity between pastes, however good overall model predictions could be achieved
without considering the serum phase beyond the soluble solids concentration.
The Herschel-Bulkley model was found to be the most appropriate model to describe the
flow behaviour of tomato paste. Herschel Bulkley parameters could then be linked to the
insoluble and soluble solids levels in the paste. For some pastes the model could be fitted
with just one paste specific parameter plus four other generally fitted constants (which
apply to any paste). When applied to other pastes however, at least one of the other
parameters was also required to be paste specific. These parameters relate the yield stress
and the flow behaviour index to the water insoluble solids content. Because these two
parameters need to be fitted for individual pastes, it is thought that they are influenced by
the particle size and shape and/or their composition of the WIS fraction. For example
elongated particles will orientate within a flow field with varying shear rates, thereby
influencing the flow behaviour index.
There is potential to fit the two key paste specific parameters for a paste from a single flow
curve. This could provide an industry implementable method to characterise tomato paste
batches. Such a characterisation method would be useful for predicting flow behaviour
under different processing conditions and how dilution during product formulation will
affect viscosity. Future work should be carried out to extend this work to those aims.