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R. SAGGIN, Food Science Department, Pennsylvania State University, 126 Borland Lab, University Park, PA 16802 and J. N. Coupland.

JUSTIFICATION: Ultrasound can be used as an analytical technique to provide information about the physicochemical properties of foods. Ultrasonic waves are mechanical vibrations (>20 kHz) commonly progressing as compressions and rarefactions in the direction of transmission (longitudinal waves) but can also involve deformation normal to the direction of transmission (shear waves). The ultrasonic determination of shear properties of food fluids would be extremely valuable as a non-contact, on-line probe of rheology. Measurement of the reflectance coefficient offers an intriguing means to this end.

OBJECTIVE: To use shear and longitudinal ultrasonic reflectance to characterize the viscosity and solids content of lipids.

METHOD: Samples of high viscosity calibration oils and a compound coating were cooled from 50-5^oC. The reflectance intensity of a shear and longitudinal ultrasonic pulse from the oil surface was measured and normalized to that from a calibration material (water). The viscosities of the oils were measured using a concentric cylinder viscometer and the solids content of the fats estimated by differential scanning calorimetry.

RESULTS: At all temperatures the longitudinal reflection coefficient was linearly related to viscosity but the line for each oil was different. However, for shear (ultrasound) a single master curve (r²=0.94) relating reflectance measurements to viscosity was developed for all oil compositions and temperatures. With both ultrasonic modes, there was a large increase in normalized reflectance for the compound coating (16% and 7% for longitudinal and shear respectively) during crystallization and below the solidification point reflection coefficient was independent of temperature.

SIGNIFICANCE: Shear reflectance is a sensitive and reliable measurement of oil viscosity. Both shear and longitudinal reflectance can be used to measure solid fat content. Ultrasonic reflectance sensors are non-invasive and require no modification of container geometry so are ideal for retrofitting to existing process equipment.