Ultrasonic monitoring of unsteady-state cooling of food products

The cooling (50°C to 2°C) of blocks of gelatin, chicken, salmon, beef, and yogurt was monitored using ultrasonic time of flight (TOF) measurements. Heat loss was from one side and the core temperature was monitored with a thermocouple and modeled as unsteady-state heat transfer in an infinite slab. The TOF of an ultrasonic (2.25 MHz) pulse parallel to the direction of heat loss was measured simultaneously. The ultrasonic TOF increased at a decreasing rate during the cooling process, reaching a constant value when the rate of change of temperature with time was zero. The ultrasonic TOF measurement showed good agreement with a theoretical prediction of both TOF and core temperature based on a time integral of speed of sound calculated as a function of time and position. Ultrasonics offers a non-invasive means to monitor product temperature online during a cooling process.