Correlation between in-line measurements of tomato ketchup shear viscosity and extensional viscosity

• For this case, the extensional viscosity curve correlates well with that of shear viscosity.
• The tomato ketchup Trouton ratio measured by HCF is surprisingly high, approximately 46.
• The shear viscosity in-line measurements at low flow rate are close to those carried out off-line.
• The yield stress in-line measurements at low pumping rates were close to those obtained using a stress-controlled rheometer.
• Bostwick measurements gave only one apparent shear viscosity, which roughly corresponded to the other measurements.

The viscosity and shear thinning behavior are essential characteristics of tomato ketchup. A real-time monitoring of those characteristics during processing is important to obtain a good quality of the final product and to reduce production waste. This work investigates the measurement of rheological in-line flow properties of tomato ketchup, using a real-time technique that combines ultrasound velocity profiling (UVP) and pressure difference (PD) assessment. In-line data were compared to those obtained off-line using a rotational viscometer. There was a poor correlation with the Bostwick measurement, whereas the flow curves calculated from flow velocimetry data were very similar to those measured off-line. The extensional viscosity of ketchup was determined through the measurement of Hyperbolic Contraction Flow; the curve followed a trend similar to that for the shear viscosity over the deformation rate investigated.