On-line moisture and density measurement of foods with microwave sensors
1997
King, R.J.
New developments and applications are presented for continuous, on-line determination of moisture content and dry density of food products with microwave sensors. The focus of this work is on applications to harvested foods (various grains and nuts), dried fruits and extruded foods. Two unique types of sensors and their ancillary electronics for process control and/or product quality measurement are introduced. For both, measurement principles are based on the interaction of electromagnetic fields with the dielectric and power dissipative properties of matter, particularly of water. These properties are diagnostics of the partial water and dry densities. The first type of sensor is a contacting open reflection resonator which can be interfaced in various ways to the material being measured, e.g., sensors can tee flat or curved for flush mounting in a shaker, hopper, glide sled, chute or pipe. The second type is noncontacting, wherein a microwave beam is transmitted through the test material. Through measurement of the transmitted signal attenuation (absorption) and phase delay, the moisture and dry basis weights (mass/area) are determined by empirical algorithms. Either sensor type can be used for continuous, on-line measurements or as stand-alone benchtop instruments. Choice of sensor type depends on the test material form (liquid, particulate or solid), the ranges of material moisture content and thickness, and logistical constraints associated with installation. Generally, accuracies of better than 0.5% moisture content and 2% absolute density are achieved up to 20% moisture. Resolution of small changes in moisture is about 0.1%, depending on the application and the degree of on-line data averaging.
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