Chemical, sensory and rheological properties of some commercial German and Egyptian tomato ketchups

The rheological behaviour of eight tomato ketchups measured in the shear rate range from 0.1 to 100 1/s and with oscillatory tests was studied over a wide range of temperatures ( 0-50 degreesC) using a Physica UDS 200 rheometer. The results indicated that these tomato ketchups behave as non-Newtonian fluids semi-solid and have a definite yield stress. The relationship between eta(eff) and temperature of all the tomato ketchup brands under investigation was examined. Significantly higher correlation was found between heff and temperature. The heff values decrease with an increase in temperature. Oscillatory test data revealed weak gel-like ( dispersion structure) behaviour of the ketchup: the magnitudes of G' were higher than those of G{''}, and both increased with oscillatory frequency. The effect of temperature on the viscosity can be described by means of an Arrhenius-type equation. The flow activation energy for viscous flow depends on the chemical composition; the flow activation energy increases with the total solids contents. Chemical, physical and sensory tests for tomato ketchups were made.