Evaluation of in-line sensors for selected properties measurements in continuous food processing

Abstract

The present investigation deals with the evaluation of five in-line sensors — refractometer, viscometer, density meter, pH meter and power meter — to measure soluble solids, viscosity, density, pH and power respectively 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 setup consisting of feed tank, pumps, in-line sensors, data logger and other accessories. The variables covered in the present investigation were: temperature (10–60 °C), flow rate (18.92–113.56 l/min) and soluble solids or Brix (5–30%, w/w). These sensors were calibrated off-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 Maselli 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 compared, 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 not affected by prolonged use during continuous operations of about 8 h.