15C-16 |
Monitoring of cooking operations by ultrasonic sensors |
H. SIGFUSSON1, M. D. Goff2, and J. N. Coupland2. (1) Oklahoma Food & Agricultural Products Research & Technology Center, Oklahoma State Univ., Rm. 106, Stillwater, OK 74078, (2) Dept. of Food Science, Pennsylvania State Univ., 126 Borland Lab., University Park, PA 16802 Food safety standards require that foods reach a defined minimum internal temperature during cooking. The effectiveness of the operation can be determined experimentally using thermocouples, or theoretically from heat transfer equations. This is often impractical for process control, demonstrating a need for non-invasive methods for end-point determination during cooking. The ultrasonic speed of sound is a strong function of temperature. The changing speed of sound in food samples during chilling has been used to calculate the temperature at the sample thermal core. The objectives were to use ultrasound to non-invasively monitor the core temperature of foods during a cooking operation. Refrigerated food samples (approximately 10°C) were placed in a sample cell. One face of the sample cell was then brought into contact with water at 90.0 ± 0.01°C while the other faces were insulated. The sample core temperature (±0.1°C) was monitored using a thermocouple and the time of flight (TOF) of an ultrasonic (2.25 MHz) pulse parallel to the direction of heat gain was measured simultaneously. The ultrasonic velocity in the food samples decreased as the temperature increased with time, until a core temperature of approximately 70°C was obtained, after which point there was no further change in velocity for the remainder of the cooking operation. The effects of temperature gradients on measured ultrasonic velocity could be accurately modeled by first calculating the temperature distribution across the slab, second calculating local isothermal ultrasonic velocities at each point in the sample, and finally integrating these velocities with respect to position and dividing the result into the total sample thickness. The strong dependence of ultrasonic velocity on temperature makes this technique appear a good candidate for a non-invasive thermometer. However, the unfortunate temperature-velocity relation for water means this is unlikely to be applicable for many food safety concerns.
Session 15C, Food Chemistry: Food composition and analysis
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