Effect of apple fibre addition and temperature on the rheological properties of apple juice and compensation study

The addition of dietary fibre has become a trend in the food sector and, as it changes some physical properties like flow parameters, knowing these changes is essential for food process design. In this work, the influence of both the temperature (5-60 degrees C) and the fibre content (0-16%, w/w) on the flow behaviour of apple juice were studied. The apple juice presented thixotropic behaviour above a fibre content of 10% for all the range of temperatures tested. The thixotropy followed the kinetic model of Figoni and Shoemaker. Flow behaviour was found to vary depending on the fibre content: the Newtonian model for 0-8%, the power law model (with pseudoplastic behaviour) for 10-12% and the Herschel-Bulkley model for 14-16%. The temperature effect was found to agree with the Arrhenius - equation. The iso-consistency temperatures were 79.6 and -135.9 degrees C for the Newtonian and non-Newtonian models, respectively. The compensation for the Newtonian juices was found to be no more significant than the propagation of the experimental errors. Nonetheless, the compensation for the non-Newtonian juices was found to be real, the control being entropic for all the working temperatures.