EXPERIMENTAL STUDY ON RHEOLOGICAL PROPERTIES OF NATURAL VEGETABLE JUICES

Tins paper describes the results of an experimental study on theological behavior of the vegetable juices in order to have a suitable process design, operation and control. In the food industry, juices are the most important liquid derivatives of fruits and vegetables, and their flow characteristics have been an extensive topic of study in recent years. The aim of this work was to determine flow curves, the viscosity curve and other theological properties of the vegetable juices as a function of temperature and concentration and to obtain simple equations to correlate experimental data. The vegetable juices used in this study were raw juices. The extraction process was performed using a unit powered by centrifugal force to yield the juice. Rheological properties were determined for red beet, carrot and cabbage juices, which was produced from beet fruit at 11.2 +/- 0.5 degrees Brix, carrot at 10.2 +/- 0.5 degrees Brix, respectively, cabbage at 6.5 +/- 0.5 degrees Brix soluble solid content. Density measurements were carried out by a pycnometer method and are usually in range of 984 - 1.0334 g cm(-3). In order to obtain different concentrations, concentrated juice was diluted with distilled water. Rheological measurements were carried out using a Haake Viscotester model VT550, a Searle type rheometer. Based on the obtained result it was shown, that fresh vegetable juice has a non-Newtonian behavior. In the tested ranges, the samples, after a period storage will behave as pseudo plastic fluids and power-law model was satisfactorily fitted to the experimental data. Non-linear regression was performed to fit experimental data obtaining a good fit. Both temperature and concentration showed a strong influence on theological properties of natural vegetable juices.