A non-invasive temperature measurement system comprises an ultrasound transducer configured to emit an ultrasound stimulus pulse toward a product package. An ultrasound receiver is configured to generate a reflected ultrasound waveform from electrical signals that represent physical characteristics of a plurality of reflected ultrasound pulses from a plurality of surfaces of the product package. A first reflected ultrasound pulse is from a first side of the product package closest to the transducer and a second reflected ultrasound pulse is from a second side of product package farthest from the transducer. A signal processor processes the reflected ultrasound waveform to determine a time lag between the first reflected ultrasound pulse and the second reflected ultrasound pulse. The time lag is then correlated to a temperature of a product in the product package. The ultrasound stimulus pulse does not induce nucleation of ice in a supercooled fluid.