EVALUATION OF IN-LINE SENSORS FOR SELECTED PROPERTIES MEASUREMENTS INCONTINUOUS FOOD-PROCESSING

The present investigation deals with the evaluation of Jive in-line se nsors-refractometer; viscometer; density meter; pH meter and power met er-to measure soluble solids, viscosity density pH and power respectiv ely in the continuous processing of different food products. The food products used were: apple sauce, banana puree, banana concentrate and a yogurt beverage. The food products were processed in a pilot scale s etup consisting of feed tank, pumps, in-line sensors, data logger and other accessories. The variables covered in the present investigation were: temperature (10-60 degrees C), flow rate (18.92-113.56 l/min) an d soluble solids or Brix (5-30%, w/w). These sensors were calibrated o ff-line by an Abbe refractometer (for soluble solids), pycnometer (for density) and pH meter (for pH). Off-line measurements were also taken for viscosity by Brookfield viscometer and Bostwick viscometer: Total solids were measured by a modified vacuum oven method. An in-line Mas elli refractometer gave values of soluble solids within an accuracy of +/-0.5%. The in-line density meter gave consistently higher values by about 4%. In-line viscosity values could not be compared with the off -line measurements because of the order of magnitude difference in the shear rate during stationary and flow conditions of the food products . pH values obtained by the in-line sensor were very close to the off- line values (maximum deviation: +/-1%). Power values could not be comp ared, since there was no method available to measure it off-line. The sensors gave satisfactory results under different operating conditions in the range of variables mentioned. Their performance was by prolong ed use during continuous operations of about 8 h. (C) 1997 Elsevier Sc ience Ltd.